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A measurement of the wind speed on a brown dwarf
Abstract
High winds in an extrasolar atmosphere
Brown dwarfs are objects intermediate in mass between large planets and small stars, and their atmospheres share many characteristics with gas giant planets. Wind speeds in Solar System gas giant atmospheres can be derived by comparing the planet's rotational periods in the infrared (tracing the upper atmosphere) and radio (tied to the interior). Allers et al. observed a nearby brown dwarf, 2MASS J10475385+2124234, and determined its infrared and radio periods. They derived an average wind speed of ∼650 meters per second in a west-to-east direction. This technique should also work for exoplanets.
Science , this issue p. 169
... They also found two smaller peaks MNRAS 525, 150-163 (2023) at shorter periods, and suggested causes including heterogeneous cloud co v erage. Allers et al. ( 2020 ) measured the first true rotation period of a brown dwarf from its radio emission. This was different to the rotational period measured from its infrared emission, by an amount corresponding to an eastward atmospheric motion of 600 ± 300 ms −1 . ...
... In this section, we plot simulated observations of our suite of tests, showing their phase curves and the resulting periodograms. Studies such as Allers et al. ( 2020 ) and Apai et al. ( 2017 ) have used observations like these to estimate rotation periods, sizes of surface features, and wind speeds. ...
... This section shows the Lomb-Scargle periodograms corresponding to the phase curves in Section 5.1 (Lomb 1976 ;Scargle 1982 ). We could have used a Fourier power spectrum instead of a Lomb-Scargle periodogram because we have uniformly sampled data, but chose to use the latter as it is the method used in observational analyses (Apai et al. 2017 ;Allers et al. 2020 ). ...
Observations of time-varying thermal emission from brown dwarfs suggest that they have large-scale atmospheric circulation. The magnitude of this variability ranges from a few percent to tens of percent, implying a range of sizes of atmospheric perturbations. Periodograms of phase curves of the thermal emission reveal a range of peaks with different periods and widths, suggesting different atmospheric flow speeds and directions. This implies a variety of atmospheric circulations in the different brown dwarfs observed to date, but there is no general theoretical understanding of the circulation regimes these objects can support, and the resulting sizes and velocities of their atmospheric features. We therefore use an idealised two-dimensional shallow-water model of a brown dwarf atmosphere to understand their potential large-scale circulation regimes. We non-dimensionalise the model to reduce the number of input parameters to two non-dimensional numbers: the thermal Rossby number and the non-dimensional radiative timescale. This allows us to define a parameter space that bounds the entire range of brown dwarf behaviour possible in our model. We analyse the resulting height, velocity, and potential vorticity fields in this parameter space, and simulate observed phase curve and periodograms for comparison with real observations. We use our results to qualitatively define four circulation regimes, which we hope will be useful for interpreting observations and for guiding simulations with more detailed physical models.
... Saumon et al. 2006;Stephens et al. 2009;Leggett et al. 2016Leggett et al. , 2019Miles et al. 2020), especially in stratified atmospheres where convection does not play a direct role in mixing. Other techniques detecting or constraining the presence of global circulation of BDs include Doppler imaging (Crossfield et al. 2014), simultaneous tracking of near-IR and radio variability (Allers et al. 2020), and precise near-IR polarization measurements (Millar-Blanchaer et al. 2020). ...
... Doppler broadening of spectral lines (Reiners & Basri 2008) and rotation periods inferred from light-curve variability (e.g. Artigau et al. 2009;Radigan et al. 2012;Metchev et al. 2015;Apai et al. 2017;Allers et al. 2020) indicate typical rotation period of less than 2 h to slightly more than 10 h for field BDs. Planetarymass, free-floating giant planets and directly imaged EGPs likely rotate rapidly as well (Snellen et al. 2014;Zhou et al. 2016;Bryan et al. 2018). ...
... The β effect plays a central role in driving zonal banding and jets in Jovian and Saturnian atmospheres (see reviews by, for example, Vasavada & Showman 2005;Showman et al. 2018). This would help us to clarify whether BDs and directly imaged EGPs exhibit zonal jets like Jupiter and Saturn as indicated by long-term light-curve monitoring (Apai et al. 2017) and simultaneous tracking of radio and near-IR flux variability (Allers et al. 2020). Global models are also necessary to assess whether circulation driven by cloud radiative feedback can quantitatively explain the observed light-curve variability of BDs. ...
Observations of brown dwarfs (BDs), free-floating planetary-mass objects, and directly imaged extrasolar giant planets (EGPs) exhibit rich evidence of large-scale weather. Cloud radiative feedback has been proposed as a potential mechanism driving the vigorous atmospheric circulation on BDs and directly imaged EGPs, and yet it has not been demonstrated in three-dimensional dynamical models at relevant conditions. Here, we present a series of atmospheric circulation models that self-consistently couple dynamics with idealized cloud formation and its radiative effects. We demonstrate that vigorous atmospheric circulation can be triggered and self-maintained by cloud radiative feedback. Typical isobaric temperature variation could reach over 100 K and horizontally averaged wind speed could be several hundreds of . The circulation is dominated by cloud-forming and clear-sky vortices that evolve over time-scales from several to tens of hours. The typical horizontal length-scale of dominant vortices is closed to the Rossby deformation radius, showing a linear dependence on the inverse of rotation rate. Stronger rotation tends to weaken vertical transport of vapour and clouds, leading to overall thinner clouds. Domain-mean outgoing radiative flux exhibits variability over time-scales of tens of hours due to the statistical evolution of storms. Different bottom boundary conditions in the models could lead to qualitatively different circulation near the observable layer. The circulation driven by cloud radiative feedback represents a robust mechanism generating significant surface inhomogeneity as well as irregular flux time variability. Our results have important implications for near-infrared (IR) colours of dusty BDs and EGPs, including the scatter in the near-IR colour–magnitude diagram and the viewing-geometry-dependent near-IR colours.
... (v) Simultaneous tracking of near-IR and radio variability, in which the former measures the period at which atmospheric features rotate in and out of the view and the latter likely reflects the rotation period of the interior, could track possible differential rotation between the atmosphere and the interior. Recently, Allers et al. (2020) applied this technique to a nearby T6 brown dwarf and showed that the period of the near-IR variability is slightly shorter than that of the radio emission, suggesting that the dominant atmospheric features travel eastward relative to the interior with a zonal speed of a few hundred m s −1 . ...
... The differential propagation of equatorially trapped waves induces short-term evolution of simulated lightcurves, analogous to the wave beating effects described in Apai et al. (2017). Eastward propagating equatorial Kelvin waves are sometimes dominant, causing a slightly shorter rotation period of the atmospheric features relative to the underlying planetary rotation period, which agrees well with observational results by Allers et al. (2020). When the bottom dissipation is weak, strong and broad zonal jets develop and modify wave propagation and lightcurve variability. ...
... Both dynamical features are driven by the cloud radiative feedback, providing an essential physical mechanism to explain several types of time evolution of lightcurve variability (see a summary and analysis in Apai et al. 2017). Eastward propagating Kelvin waves with phase speeds of a few hundred m s −1 are sometimes dominant in our simulations, and the existence of these waves in atmospheres of BDs may explain the shorter rotation period of the atmosphere than that of the interior observed for a nearby BD (Allers et al. 2020). Our models predict that different equatorial waves that may travel in different zonal velocities could influence the lightcurves differently at different times. ...
Brown dwarfs, planetary-mass objects and directly imaged giant planets exhibit significant observational evidence for active atmospheric circulation, raising critical questions about mechanisms driving the circulation, its fundamental nature and time variability. Our previous work has demonstrated the crucial role of cloud radiative feedback on driving a vigorous atmospheric circulation using local models that assume a Cartesian geometry and constant Coriolis parameters. In this study, we extend the models to a global geometry and explore properties of the global dynamics. We show that, under relatively strong dissipation in the bottom layers of the model, horizontally isotropic vortices are prevalent at mid-to-high latitudes while large-scale zonally propagating waves are dominant at low latitudes near the observable layers. The equatorial waves have both eastward and westward phase speeds, and the eastward components with typical velocities of a few hundred m s−1 usually dominate the equatorial time variability. Lightcurves of the global simulations show variability with amplitudes from 0.5 percent to a few percent depending on the rotation period and viewing angle. The time evolution of simulated lightcurves is critically affected by the equatorial waves, showing wave beating effects and differences in the lightcurve periodicity to the intrinsic rotation period. The vertical extent of clouds is the largest at the equator and decreases poleward due to the increasing influence of rotation with increasing latitude. Under weaker dissipation in the bottom layers, strong and broad zonal jets develop and modify wave propagation and lightcurve variability. Our modeling results help to qualitatively explain several features of observations of brown dwarfs and directly imaged giant planets, including puzzling time evolution of lightcurves, a slightly shorter period of variability in IR than in radio wavelengths, and the viewing angle dependence of variability amplitude and IR colors.
... Filippazzo et al. (2015) provided physical parameters for this object: T eff = 880 ± 76 K, ( ) = g log 4.96 0.49 dex, age around 0.5-10 Gyr, M = 41.61 ± 26.03 M Jup , and R = 4.96 ± 0.49 R Jup . Allers et al. (2020) used the Infrared Array Camera on the Spitzer Space Telescope, on its [4.5] channel, for two days, for 7 hr and 14 hr each. They found sinusoidal variability of 0.5% and adopted a period of 1.741 ± 0.007 hr. ...
... In addition, Allers et al. (2020) measured the wind speed for this object, in radio observations taken with the Karl G. Jansky Very Large Array, which generates an (unknown) atmospheric J1416+1348 is a resolved L+T subdwarf system, separated by 9ʺ (Burgasser et al. 2010). The primary dwarf is ULAS J141623.94+134836.3 (J1416+1348A) with an L7 spectral type. ...
The majority of brown dwarfs show some level of photometric or spectrophotometric variability in different wavelength ranges. This variability allow us to trace the 3D atmospheric structures of variable brown dwarfs and directly-imaged exoplanets with radiative-transfer models and mapping codes. Nevertheless, to date, we do not have an informed method to preselect the brown dwarfs that might show a higher variability amplitude for a thorough variability study. In this work, we designed and tested near-infrared spectral indices to preselect the most likely variable mid- and late-T dwarfs, which overlap in effective temperatures with directly-imaged exoplanets. We used time-resolved near-infrared Hubble Space Telescope Wide Field Camera 3 spectra of a T6.5 dwarf, 2MASS J22282889–431026, to design our novel spectral indices. We tested these spectral indices on 26 T5.5–T7.5 near-infrared SpeX/IRTF spectra, and we provided eight new mid- and late-T variable candidates. We estimated the variability fraction of our sample as 38 − 30 + 4 %, which agrees with the variability fractions provided by Metchev for mid- to late-T dwarfs. In addition, two of the three previously known variables in our sample of SpeX spectra are flagged as variable candidates by our indices. Similarly, all seven known nonvariables in our sample are flagged as nonvariable objects by our indices. These results suggest that our spectral indices might be used to find variable mid- and late-T brown dwarf variables. These indices may be crucial in the future to select cool directly-imaged exoplanets for variability studies.
... Consequently, making the not unreasonable assumption that the majority of dwarfs with kG fields have dipolar field orientations approximately aligned with their axes of rotation, those dwarfs that are likely to be detected with active aurorae are expected to have axes that are highly inclined with respect to our line of sight, and they should emit regular beamed coherent radio emission synchronised to the dwarf's rotational period. A good example is the case of the nearby field T6.5 dwarf 2MASS J10475385+2124234, whose radio derived period was determined to be a consequence of co-aligned ECM emission (Williams & Berger 2015), and whose subsequent rotational period as derived from Spitzer demonstrated evidence for the first direct measurement of zonal atmospheric winds around a brown dwarf (Allers et al. 2020), and established a template for studying exoplanetary atmospheres. ...
... Having extracted the differential photometric time-series of BRI 0021 for each run, we used Lomb-Scargle (LS) periodogram analysis (Lomb 1976;Scargle 1982Scargle , 1989, phase dispersion minimization (PDM) (Stellingwerf 1978) and a Markov Chain Monte Carlo (MCMC) sinusoid model fitting method (Allers et al. 2020), to determine the presence of any underlying periodic signals. For the former we chose a range of LS power peaks above the noise-floor ( > 5 σ significance), and for the PDM analyses, we performed 10 5 Monte-Carlo simulations per assay in order to evaluate the statistical significance of the potential outcomes. ...
We report I-band photometric observations of the radio-detected M9.5 dwarf BRI 0021-0214, obtained with the Galway Ultra Fast Imager (GUFI) on the 1.8m Vatican Advanced Technology Telescope VATT at Mt. Graham International Observatory, Arizona. In total, 19 hours of observations over a 73 day baseline were obtained. BRI 0021-0214 was shown to exhibit modulated emission with a period of 3.052 ± 0.004 hours with a mean amplitude variability of 0.0044 mag. When combined with rotational velocity data obtained from previous work, our newly discovered rotation period gives an inclination angle of 51.7 degrees for the rotation axis of BRI 0021-0214 relative to our line of sight. Previous studies have reported that the most plausible cause for optical variability from this dwarf is a consequence of suspended co-rotating dust clouds in its atmosphere. However reports of enhanced Hα and intermittent coherent radio emission suggest the possibility of auroral activity in its magnetosphere. Further, more coordinated multiwavlength observations of this dwarf could fully resolve the nature of this elusive rapid-rotator object’s observational properties.
... developed byVos (2019). An example of using this code appears inAllers et al. (2020). They used an opensource implementation of the affine-invariant MCMC ensemble sample(Goodman & Weare, 2010), and the emcee package (Foreman-Mackey et al., 2013). ...
... We explored the 4-dimension parameter space to model the lightcurve using 500 walkers with 20000 steps and discarding an initial burn-in sample of 1000 steps.Figure 3.16 shows the posterior probability distributions of the amplitude, period, phase and the fitting model parameters. For observational data taken of BRI 0021 obtained on Oct 10 2017, using the same methodology as originally applied byAllers et al. (2020). Each parameter is constrained, and the MCMC algorithm determines a period of 3.046 ± 0.008 hrs. ...
Brown dwarfs and low-mass stars together comprise the group of galactic objects known as ultracool dwarfs. A number have been detected as radio sources – in some cases, emitting periodic radio pulses synchronised to the dwarf's known rotational period. Detections of optical variability in ultracool dwarfs have been primarily attributed to stellar rotation, with the modulations observed a consequence of either magnetic spots on the surface, the presence of atmospheric dust, or auroral emission. A combination of these mechanisms may be required to explain some dwarf lightcurves. Rotational estimates of ultracool dwarfs are typically obtained spectroscopically; however accurate estimates of true rotation velocities require knowledge of the dwarf's rotational inclination axes. Direct measurement of a rotational signature in photometric data however provides an unambiguous rotational period, and this information can be used to constrain dwarf inclination geometries. In this thesis, we report on over ~ 160 hrs monitoring in I-band of multiple epochs from four ultracool dwarfs, spanning M tight binary dwarf 2MASS J1314203+132001A and L tight binary dwarf 2MASS J0746425+200032AB, the M9.5 dwarf BRI 0021-0214, and the L3.5 dwarf 2MASS J00361617+18211. This photometric campaign was carried out using the Galway Ultra-Fast Imager (GUFI) on the 1.8m Vatican Advanced Technology Telescope (VATT), on Mt. Graham, Arizona. All selected dwarfs exhibit periodic optical variability, where periods of both secondary components for our binary samples were newly discovered. This thesis discusses the use of two photometric analysis tools with the explicit aim of improving the quality of ground-based photometric measurements. Each data set was used to test the performance of the two systems. We find the LuckyPhot technique has obvious benefits to high precision photometry by reducing photometric errors, where the mean RMS error was reduced by ~ 47% with respect to the errors produced by the more standard GUFI pipeline method. This thesis also outlines a novel tool, Light Curve Fitter, which we apply to the binaries to investigate the presence of periodic photometric modulation in both binary members: refining the dominant member variability parameters, and searching for an elusive period of the weaker member. Light Curve Fitter is a python-based program, capable of detecting superposition of two sinusoidal waves to untangle the weaker components variability signature from that of the dominant source variability. We identify a newly discovered optical variability in the primary and secondary components of ultracool dwarf binary 2MASS J1314203+132001AB and 2MASS J0746425+200032AB, respectively. The optical data presented for both systems shows strongly correlated emissions in terms of phase and temporal variability. We have also shown the A and B variability signals of both dwarf binaries 2MASS J0746425+200032AB and 2MASS J1314203+132001AB, respectively, to be extremely consistent and stable over multiple epochs. This stability had seen in both radio and spectroscopic data, and the mechanism driving these processes in different parts of the electromagnetic spectrum could perhaps be fundamentally linked. We also investigate the orbital coplanarity of both binary dwarfs. Here the ability to deconvolve the inclination angle from the spectroscopic radial velocities, using direct estimates of the dwarf rotational periods, allows us to constrain the spin-orbit coupling of the binary system. In the case of the L dwarf binary 2MASS J0746425+200032AB, we calculate the equatorial inclination angle of the binary rotation axes are in alignment with the orbital plane of the system to within 10 degrees, consistent with solar-type binary formation mechanisms. For the M7 dwarf binary 2MASS J1314203+132001AB, due to missing parameters for the primary component, we investigate a tentative alignment of the spin-orbital axes of the A component. We find that the equatorial inclination angle of the secondary member spin axes is largely consistent with being aligned perpendicularly to the orbital plane. Finally, we find the rotation axes of the two single dwarfs are not perpendicular to our line of sight.
... It is also useful to place the observed wind speed range of 4.5 km/s in the context of the recent study by Allers et al. (2020), who derived the wind speed in a brown dwarf relative to its interior by comparing its near-infrared rotational period to its radiobased rotational period. This study found an eastward wind speed of 650±310 m/s for a rapidly-rotating (P=1.77±0.04 ...
... Finally, we note that detailed and high-precision observations of the effective periods of the jet-dominated regions, such as those presented here, may be complemented by radio-wavelength period measurements that probe the rotation rate of the interior of the brown dwarfs. In a recent study (Allers et al. 2020) demonstrated the potential of comparing radio periods to near-infrared rotation periods and, from their difference, deduced an eastward wind speed of 650±310 m/s. The TESS observations presented here do not probe the rotation rate of the interior and, thus, lack an absolute reference frame for wind speed measurements. ...
Brown dwarfs were recently found to display rotational modulations, commonly attributed to cloud cover of varying thickness, possibly modulated by planetary-scale waves. However, the long-term, continuous, high-precision monitoring data to test this hypothesis for more objects is lacking. By applying our novel photometric approach to TESS data, we extract a high-precision lightcurve of the closest brown dwarfs, which form the binary system Luhman 16AB. Our observations, that cover about 100 rotations of Luhman 16B, display continuous lightcurve evolution. The periodogram analysis shows that the rotational period of the component that dominates the lightcurve is 5.28 h. We also find evidence for periods of 2.5 h, 6.94 h, and 90.8 h. We show that the 2.5 h and 5.28 h periods emerge from Luhman 16B and that they consist of multiple, slightly shifted peaks, revealing the presence of high-speed jets and zonal circulation in this object. We find that the lightcurve evolution is well fit by the planetary-scale waves model, further supporting this interpretation. We argue that the 6.94 h peak is likely the rotation period of Luhman 16A. By comparing the rotational periods to observed v sin(i) measurements, we show that the two brown dwarfs are viewed at angles close to their equatorial planes. We also describe a long-period (P~91 h) evolution in the lightcurve, which we propose emerges from the vortex-dominated polar regions. Our study paves the way toward direct comparisons of the predictions of global circulation models to observations via periodogram analysis.
... Of particular promise are combined millimeter-(Atacama Large Millimeter/submillimeter Array [ALMA]) and centimeter-wave (VLA) observations, which have provided among the most detailed radio map yet of any star, other than the Sun [37, 38]. Combining VLA and near-infrared wavelength observations can measure wind speeds on brown dwarfs [39,40], likely resulting from zonal wind patterns, and potentially analogous flows in (very) low-mass stars. Astrometric observations conducted with the VLBA can reveal the presence of lower-mass companions [e.g., 41,42,43,44,45,46], which provide a more complete description of stellar systems and can assist with discriminating Version for community comment. ...
The next-generation Very Large Array (ngVLA) is intended to be the premier centimeter-wavelength facility for astronomy and astrophysics, building on the substantial scientific legacies of the Karl G. Jansky Very Large Array (VLA) and the Very Long Baseline Array (VLBA). The ngVLA would open a new window on the Universe through ultra-sensitive imaging of thermal line and continuum emission to milliarcsecond resolution, while delivering unprecedented broad-band continuum imaging and polarimetry of non-thermal emission. The ngVLA would provide a critical electromagnetic complement to a suite of particle detectors and gravitational-wave observatories, as well as space- and ground-based telescopes operating from infrared to gamma-ray wavelengths, hence enabling multi-messenger and multi-band astronomy and astrophysics. Current construction plans call for the ngVLA to leverage some of the physical infrastructure of both the VLA and the VLBA, potentially drawing on overlapping personnel and information infrastructure. Multiple options can be envisioned for a VLA+VLBA to ngVLA transition. In order to assess risks and benefits of possible transition plans, the ngVLA project established the VLA+VLBA to ngVLA Transition Advisory Group (TAG). The primary deliverable from the TAG is a ``VLA+VLBA to ngVLA Transition Option Concepts'' report (this report) that includes a prioritized list of transition options.
... The shape of both ΔT and ΔP as a function of wavelength broadly matches the shape of the observed variability reported by Apai et al. (2013). lead to the onset of the electron cyclotron maser instability that produces detectable radio emission (e.g., Hallinan et al. 2015;Kao et al. 2016Kao et al. , 2018Pineda et al. 2017;Allers et al. 2020;Richey-Yowell et al. 2020). These energetic electron beams may also have a significant effect on the atmosphere. ...
We present an atmospheric retrieval analysis of a pair of highly variable, ∼200 Myr old, early T type planetary-mass exoplanet analogs SIMP J01365662+0933473 and 2MASS J21392676+0220226 using the Brewster retrieval framework. Our analysis, which makes use of archival 1–15 μ m spectra, finds almost identical atmospheres for both objects. For both targets, we find that the data is best described by a patchy, high-altitude forsterite (Mg 2 SiO 4 ) cloud above a deeper, optically thick iron (Fe) cloud. Our model constrains the cloud properties well, including the cloud locations and cloud particle sizes. We find that the patchy forsterite slab cloud inferred from our retrieval may be responsible for the spectral behavior of the observed variability. Our retrieved cloud structure is consistent with the atmospheric structure previously inferred from spectroscopic variability measurements, but clarifies this picture significantly. We find consistent C/O ratios for both objects, which supports their formation within the same molecular cloud in the Carina-Near moving group. Finally, we note some differences in the constrained abundances of H 2 O and CO, which may be caused by data quality and/or astrophysical processes such as auroral activity and their differing rotation rates. The results presented in this work provide a promising preview of the detail with which we will characterize extrasolar atmospheres with JWST, which will yield higher-quality spectra across a wider wavelength range.
... The size of the retrieved spots constrains wind speeds in brown-dwarf atmospheres based on Rhines' length argument (Rhines 1975;Showman & Guillot 2002). Combining periods measured at infrared (IR) and radio wavelengths, which trace the stratospheric and magnetospheric rotation rates, Allers et al. (2020) directly measured the wind speed of a brown dwarf. Amplitudes and phase offsets of rotational modulations often vary with the wavelength (e.g., Buenzli et al. 2012;Lew et al. 2016;Yang et al. 2016;Biller et al. 2018), and the observed wavelength dependence of the modulations supports a picture in which heterogeneous clouds are the primary source of variability in L/T transition dwarfs (e.g., Apai et al. 2013;Buenzli et al. 2014b;. ...
The photometric and spectral variability of brown dwarfs probes heterogeneous temperature and cloud distributions and traces the atmospheric circulation patterns. We present a new 42 hr Hubble Space Telescope (HST) Wide Field Camera 3 G141 spectral time series of VHS 1256-1257 b, a late L-type planetary-mass companion that has been shown to have one of the highest variability amplitudes among substellar objects. The light curve is rapidly evolving and best fit by a combination of three sine waves with different periods and a linear trend. The amplitudes of the sine waves and the linear slope vary with the wavelength, and the corresponding spectral variability patterns match the predictions by models invoking either heterogeneous clouds or thermal profile anomalies. Combining these observations with previous HST monitoring data, we find that the peak-to-valley flux difference is 33% ± 2% with an even higher amplitude reaching 38% in the J band, the highest amplitude ever observed in a substellar object. The observed light curve can be explained by maps that are composed of zonal waves, spots, or a mixture of the two. Distinguishing the origin of rapid light curve evolution requires additional long-term monitoring. Our findings underscore the essential role of atmospheric dynamics in shaping brown-dwarf atmospheres and highlight VHS 1256-1257 b as one of the most favorable targets for studying the atmospheres, clouds, and atmospheric circulation of planets and brown dwarfs.
... The size of the retrieved spots constrains wind speeds in brown dwarf atmospheres based on the Rhines' length argument (Rhines 1975;Showman & Guillot 2002). Combining periods measured at infrared and radio wavelengths, which trace the stratospheric and magnetospheric rotation rates, Allers et al. (2020) directly measured the wind speed of a brown dwarf. Amplitudes and phase offsets of rotational modulations often vary with wavelength (e.g., Buenzli et al. 2012;Lew et al. 2016;Yang et al. 2016;Biller et al. 2018), and the observed wavelength-dependence of the modulations supports a picture in which heterogeneous clouds are the primary source of variability in L/T transition dwarfs (e.g., Apai et al. 2013;Buenzli et al. 2014b;. ...
Photometric and spectral variability of brown dwarfs probes heterogeneous temperature and cloud distribution and traces the atmospheric circulation patterns. We present a new 42-hr Hubble Space Telescope (HST) Wide Field Camera 3 G141 spectral time series of VHS 12561257 b, a late L-type planetary-mass companion that has been shown to have one of the highest variability amplitudes among substellar objects. The light curve is rapidly evolving and best-fit by a combination of three sine waves with different periods and a linear trend. The amplitudes of the sine waves and the linear slope vary with wavelength, and the corresponding spectral variability patterns match the predictions by models invoking either heterogeneous clouds or thermal profile anomalies. Combining these observations with previous HST monitoring data, we find that the peak-to-valley flux difference is % with an even higher amplitude reaching 38% in the J band, the highest amplitude ever observed in a substellar object. The observed light curve can be explained by maps that are composed of zonal waves, spots, or a mixture of the two. Distinguishing the origin of rapid light curve evolution requires additional long-term monitoring. Our findings underscore the essential role of atmospheric dynamics in shaping brown dwarf atmospheres and highlight VHS 12561257 b as one of the most favorable targets for studying atmospheres, clouds, and atmospheric circulation of planets and brown dwarfs.
... Models of the global circulation of brown dwarfs Tan & Showman 2021b;Tan 2022) predict significant variability due to a combination of gravity waves generated by interaction with the convective interior and cloud-radiative feedback. Such variability has been observed in a range of brown dwarfs to date (e.g., Gelino et al. 2002;Artigau et al. 2009;Radigan et al. 2012;Biller et al. 2013;Crossfield et al. 2014;Faherty et al. 2014;Karalidi et al. 2016;Lew et al. 2016;Apai et al. 2017;Allers et al. 2020;Vos et al. 2022), providing evidence that cloud patchiness and large-scale wave motions induce the observed variability. Additionally, the observational characterization of highly irradiated brown dwarfs orbiting white dwarfs (e.g., Casewell et al. 2018Casewell et al. , 2020Lew et al. 2022;Zhou et al. 2022) has recently motivated numerical models of their atmospheric circulation Sainsbury-Martinez et al. 2021). ...
The atmospheres of ultra-hot Jupiters have been characterized in detail through recent phase curve and low- and high-resolution emission and transmission spectroscopic observations. Previous numerical studies have analyzed the effect of the localized recombination of hydrogen on the atmospheric dynamics and heat transport of ultra-hot Jupiters, finding that hydrogen dissociation and recombination lead to a reduction in the day-to-night contrasts of ultra-hot Jupiters relative to previous expectations. In this work, we add to previous efforts by also considering the localized condensation of clouds in the atmospheres of ultra-hot Jupiters, their resulting transport by the atmospheric circulation, and the radiative feedback of clouds on the atmospheric dynamics. To do so, we include radiatively active cloud tracers into the existing MITgcm framework for simulating the atmospheric dynamics of ultra-hot Jupiters. We take cloud condensate properties appropriate for the high-temperature condensate corundum from CARMA cloud microphysics models. We conduct a suite of general circulation model (GCM) simulations with varying cloud microphysical and radiative properties, and we find that partial cloud coverage is a ubiquitous outcome of our simulations. This patchy cloud distribution is inherently set by atmospheric dynamics in addition to equilibrium cloud condensation, and causes a cloud greenhouse effect that warms the atmosphere below the cloud deck. Nightside clouds are further sequestered at depth due to a dynamically induced high-altitude thermal inversion. We post-process our GCMs with the Monte Carlo radiative transfer code gCMCRT and find that the patchy clouds on ultra-hot Jupiters do not significantly impact transmission spectra but can affect their phase-dependent emission spectra.
... Light curves in IR and radio wavelengths of a nearby T dwarf re vealed e vidence of f ast eastw ard traveling features with a speed up to ∼600 m s −1 (Allers et al. 2020 ). Models in Tan & Showman ( 2021b ) showed that zonal waves triggered by cloud radiative feedbacks may explain this net eastward signal. ...
Observations of brown dwarfs and relatively isolated young extrasolar giant planets have provided unprecedented details to probe atmospheric dynamics in a new regime. Questions about mechanisms governing the global circulation and its fundamental nature remain to be completely addressed. Previous studies have shown that small-scale, randomly varying thermal perturbations resulting from interactions between convection and the overlying stratified layers can drive zonal jet streams, waves and turbulence. In this work, we improve upon our previous work by using a general circulation model coupled with a two-stream grey radiative transfer scheme to represent more realistic heating and cooling rates. We examine the formation of zonal jets and their time evolution, and vertical mixing of passive tracers including clouds and chemical species. Under relatively weak radiative and frictional dissipation, robust zonal jets with speeds up to a few hundred m s−1 are typical outcomes. The off-equatorial jets tend to be pressure-independent while the equatorial jets exhibit significant vertical wind shear. On the other hand, models with strong dissipation inhibit the jet formation and leave isotropic turbulence in off-equatorial regions. Quasi-periodic oscillations of the equatorial flow with periods ranging from tens of days to months are prevalent at relatively low atmospheric temperatures. Sub-micron cloud particles can be easily transported to several scale heights above the condensation level, while larger particles form thinner layers. Cloud decks are significantly inhomogeneous near their cloud tops. Chemical tracers with chemical timescales >105 s can be driven out of equilibrium. The equivalent vertical diffusion coefficients, Kzz, for the global-mean tracer transport are diagnosed from our models and are typically on the order of 1 ∼ 102 m2 s−1. Finally, we derive an analytic estimation of Kzz for different types of tracers under relevant conditions.
... Gagné et al. 2021, in preparation). Apai et al. 2017;Biller et al. 2018;Vos et al. 2018Allers et al. 2020;Zhou et al. 2020). Science images were obtained in staring mode, using 12 s exposures for a total monitoring duration of 20 hr. ...
We present a survey for photometric variability in young, low-mass brown dwarfs with the Spitzer Space Telescope. The 23 objects in our sample show robust signatures of youth and share properties with directly imaged exoplanets. We present three new young objects: 2MASS J03492367+0635078, 2MASS J09512690−8023553, and 2MASS J07180871−6415310. We detect variability in 13 young objects, and find that young brown dwarfs are highly likely to display variability across the L2–T4 spectral type range. In contrast, the field dwarf variability occurrence rate drops for spectral types >L9. We examine the variability amplitudes of young objects and find an enhancement in maximum amplitudes compared to field dwarfs. We speculate that the observed range of amplitudes within a spectral type may be influenced by secondary effects such as viewing inclination and/or rotation period. We combine our new rotation periods with the literature to investigate the effects of mass on angular momentum evolution. While high-mass brown dwarfs (>30 M Jup ) spin up over time, the same trend is not apparent for lower-mass objects (<30 M Jup ), likely due to the small number of measured periods for old, low-mass objects. The rotation periods of companion brown dwarfs and planetary-mass objects are consistent with those of isolated objects with similar ages and masses, suggesting similar angular momentum histories. Within the AB Doradus group, we find a high-variability occurrence rate and evidence for common angular momentum evolution. The results are encouraging for future variability searches in directly imaged exoplanets with facilities such as the James Webb Space Telescope and 30 m telescopes.
... We used the Infrared Array Camera (IRAC; Fazio et al. 2004) to observe our targets in the Channel 1 (3.6 µm) band as part of the Cycle 14 Program: "The Young and the Restless: Revealing the Turbulent, Cloudy Nature of Young Brown Dwarfs and Exoplanets" (PID: 14128, PI: J Faherty). The observations were designed following the recommendations for obtaining high precision photometry from the Spitzer Science Center, and following the many examples of brown dwarf vari- ability monitoring in the literature (Metchev et al. 2015;Apai et al. 2017;Biller et al. 2018;Vos et al. 2018;Vos et al. 2020a;Allers et al. 2020;Zhou et al. 2020). Science images were obtained in staring mode, using 12 s exposures for a total monitoring duration of 20 hr. ...
We present a survey for photometric variability in young, low-mass brown dwarfs with the Spitzer Space Telescope. The 23 objects in our sample show robust signatures of youth and share properties with directly-imaged exoplanets. We present three new young objects: 2MASS J03492367+0635078, 2MASS J09512690 8023553 and 2MASS J071808716415310. We detect variability in 13 young objects, and find that young brown dwarfs are highly likely to display variability across the L2--T4 spectral type range. In contrast, the field dwarf variability occurrence rate drops for spectral types L9. We examine the variability amplitudes of young objects and find an enhancement in maximum amplitudes compared to field dwarfs. We speculate that the observed range of amplitudes within a spectral type may be influenced by secondary effects such as viewing inclination and/or rotation period. We combine our new rotation periods with the literature to investigate the effects of mass on angular momentum evolution. While high mass brown dwarfs () spin up over time, the same trend is not apparent for lower mass objects (), likely due to the small number of measured periods for old, low-mass objects. The rotation periods of companion brown dwarfs and planetary-mass objects are consistent with those of isolated objects with similar ages and masses, suggesting similar angular momentum histories. Within the AB Doradus group, we find a high variability occurrence rate and evidence for common angular momentum evolution. The results are encouraging for future variability searches in directly-imaged exoplanets with facilities such as the James Webb Space Telescope and 30-meter telescopes.
... Observation-based estimates of the zonal wind speed for planetary bodies outside the solar system exist, for instance, on brown dwarfs, which are hypothesized to have polar vortex-dominated regimes (Apai et al., 2021). By comparing the period of the IR emissions from the upper atmosphere, with the period of the radio emissions from the planet interior, Allers et al. (2020) estimated that their chosen brown dwarf had strong westerlies of 650 ± 310 ms −1 , although subsequent modeling studies have not been able to capture the magnitude of these winds (Tan & Showman, 2021). ...
Among the great diversity of atmospheric circulation patterns observed throughout the solar system, polar vortices stand out as a nearly ubiquitous planetary‐scale phenomenon. In recent years, there have been significant advances in the observation of planetary polar vortices, culminating in the fascinating discovery of Jupiter's polar vortex clusters during the Juno mission. Alongside these observational advances has been a major effort to understand polar vortex dynamics using theory, idealized and comprehensive numerical models, and laboratory experiments. Here, we review our current knowledge of planetary polar vortices, highlighting both the diversity of their structures, as well as fundamental dynamical similarities. We propose a new convention of vortex classification, which adequately captures all those observed in our solar system, and demonstrates the key role of polar vortices in the global circulation, transport, and climate of all planets. We discuss where knowledge gaps remain, and the observational, experimental, and theoretical advances needed to address them. In particular, as the diversity of both solar system and exoplanetary data increases exponentially, there is now a unique opportunity to unify our understanding of polar vortices under a single dynamical framework.
... If the periodic variations of the planet remain stable over many cycles, it may be possible to derive an accurate model for intrinsic variations and isolate any transit signals. For example, the light curves of the brown dwarfs WISEP J190648.47+401106.8 and 2MASS J10475385+2124234 remained stable over many rotations (Gizis et al. 2015;Allers et al. 2020). However, other brown dwarfs have light curves that evolve on rotational timescales, most notably in several L/T transition brown dwarfs such as 2MASS J13243553+6358281, SIMP J01365662+0933473, and 2MASS J21392216+0220185 (Apai et al. 2017). ...
All-sky imaging surveys have identified several dozen isolated planetary-mass objects (IPMOs) far away from any star. Here we examine the prospects for detecting transiting moons around these objects. We expect transiting moons to be common, occurring around 10%–15% of IPMOs, given that close-orbiting moons have a high geometric transit probability and are expected to be a common outcome of giant planet formation. The IPMOs offer an advantage over other directly imaged planets in that high-contrast imaging is not necessary to detect the photometric transit signal. For at least 30 (>50%) of the currently known IPMOs, observations of a single transit with the James Webb Space Telescope would have low enough forecast noise levels to allow for the detection of an Io- or Titan-like moon. The intrinsic variability of the IPMOs will be an obstacle. Using archival time-series photometry of IPMOs with the Spitzer Space Telescope as a proof of concept, we found evidence for a fading event of 2MASS J1119–1137 AB that might have been caused by intrinsic variability but is also consistent with a single transit of a habitable-zone 1.7 R_⊕ exomoon. Although the interpretation of this particular event is inconclusive, the characteristics of the data and the candidate signal suggest that Earth-sized habitable-zone exomoons around IPMOs are detectable with existing instrumentation.
... The global advective timescale is estimated with τ adv = R p /u wind with R p the radius of the object and u wind the typical wind velocities in the atmosphere. The wind velocity for 2M 1047 is taken from the recent wind measurement in Allers et al. (2020). All the values used to get these estimations A30, page 7 of 13 A&A 653, A30 (2021) Fig. 11. ...
Aims. Clouds are expected to form in a broad range of conditions in the atmosphere of exoplanets given the variety of possible condensible species. This diversity, however, might lead to very different small-scale dynamics depending on radiative transfer in various thermal conditions. Here, we aim to provide some insight into these dynamical regimes.
Methods. We performed an analytical linear stability analysis of a compositional discontinuity with a heating source term that depends on a given composition. We also performed idealized two-dimensional simulations of an opacity discontinuity in a stratified medium, using the ARK code. We used a two-stream gray model for radiative transfer and explored the brown-dwarf and Earth-like regimes.
Results. We revealed the existence of a radiative Rayleigh-Taylor instability (RRTI, hereafter, which is a particular case of diabatic Rayleigh-Taylor instability) when an opacity discontinuity is present in a stratified medium. This instability is similar in nature to diabatic convection and relies only on buoyancy with radiative transfer heating and cooling. When the temperature is decreasing with height in the atmosphere, a lower-opacity medium on top of a higher-opacity medium is shown to be dynamically unstable, whereas a higher-opacity medium on top of a lower-opacity medium is stable. This stability-instability behavior is reversed if the temperature is increasing with height.
Conclusions. The existence of a RRTI could have important implications for the stability of the cloud cover with regard to a wide range of planetary atmospheres. In our Solar System, it could help explain the formation of mammatus cloud in Earth atmospheres and the existence of the Venus cloud deck. Likewise, it suggests that stable and large-scale cloud covers could be ubiquitous in strongly irradiated exoplanets, but might be more patchy in low-irradiated or isolated objects such as brown dwarfs and directly imaged exoplanets.
... If the periodic variations of the planet remain stable over many cycles, it may be possible to derive an accurate model for intrinsic variations and isolate any transit signals. For example, the light curves of the brown dwarfs WISEP J190648.47+401106.8 and 2MASS J10475385+2124234 remained stable over many rotations (Gizis et al. 2015;Allers et al. 2020). However, other brown dwarfs have light curves that evolve on rotational timescales, most notably in several L/T transition brown dwarfs such as 2MASS J13243553+6358281, SIMP J01365662+0933473, and 2MASS J21392216+0220185 (Apai et al. 2017). ...
All-sky imaging surveys have identified several dozen isolated planetary-mass objects (IPMOs), far away from any star. Here, we examine the prospects for detecting transiting moons around these objects. We expect transiting moons to be common, occurring around 10-15% of IPMOs, given that close-orbiting moons have a high geometric transit probability and are expected to be a common outcome of giant planet formation. IPMOs offer an advantage over other directly imaged planets in that high-contrast imaging is not necessary to detect the photometric transit signal. For at least 30 (>50%) of the currently known IPMOs, observations of a single transit with the James Webb Space Telescope would have low enough forecasted noise levels to allow for the detection of an Io-like or Titan-like moon. Intrinsic variability of the IPMOs will be an obstacle. Using archival time-series photometry of IPMOs with the Spitzer Space Telescope as a proof-of-concept, we found evidence for a fading event of 2MASS J1119-1137 AB that might have been caused by intrinsic variability, but is also consistent with a single transit of a habitable-zone 1.7 exomoon. Although the interpretation of this particular event is inconclusive, the characteristics of the data and the candidate signal suggest that Earth-sized habitable-zone exomoons around IPMOs are detectable with existing instrumentation.
... The wind velocity is determined by the pulse. Wind speed measurements, particularly in the case of tornado warnings and high-speed wind exposure, are important for determining climate hazards (Allers et al. 2020). ...
... Thermal phase curves and low-resolution spectra, while not directly measuring the Doppler shifts of atmospheric gas, have been used to infer the presence of heat redistribution by winds and jets (e.g., Knutson et al. 2012;Kataria et al. 2015;Wong et al. 2016;Rogers 2017;Wong et al. 2020;von Essen et al. 2020). Wind speeds have also been inferred for brown dwarfs using IR and radio variability (Apai et al. 2017;Allers et al. 2020). In addition to the relative dearth of observed velocity signatures, there is also a critical lack of GCMs which take into account the most rarefied bound atmospheric layers at pressures p < 10 −3 bar, i.e. the thermosphere, that are typically sampled with transmission spectroscopy of atomic ions. ...
While steady empirical progress has been made in understanding the structure and composition of hot planet atmospheres, direct measurements of velocity signatures, including winds, rotation, and jets, have lagged behind. Quantifying atmospheric dynamics of hot planets is critical to a complete understanding of their atmospheres and such measurements may even illuminate other planetary properties, such as magnetic field strengths. In this manuscript we present the first detection of the Balmer lines H and H in the atmosphere of the ultra-hot Jupiter WASP-33 b. Using atmospheric models which include the effects of atmospheric dynamics, we show that the shape of the average Balmer line transmission spectrum is consistent with rotational velocities in the planet's thermosphere of km s. We also measure a low-significance blue-shift of km s in the transmission spectrum which is naturally explained by a global wind across the planet's terminator. In a separate analysis the time-resolved velocity centroids of individual transmission spectra show unambiguous evidence of rotation, with a best-fit velocity of km s, consistent with the value of derived from the shape of the average Balmer line transmission spectrum. Our observations demonstrate the power of high signal-to-noise, time-resolved transmission spectra to measure the effects of velocity structures in exoplanet atmospheres. The large rotational and wind velocities we measure highlight the need for more detailed 3D global climate simulations of the rarefied upper-atmospheres of ultra-hot gas giants.
Planetary-mass objects and brown dwarfs at the transition ( T eff ∼ 1300 K) from relatively red L dwarfs to bluer mid-T dwarfs show enhanced spectrophotometric variability. Multiepoch observations support atmospheric planetary-scale (Kelvin or Rossby) waves as the primary source of this variability; however, large spots associated with the precipitation of silicate and metal clouds have also been theorized and suggested by Doppler imaging. We applied both wave and spotted models to fit near-infrared (NIR), multiband ( Y / J / H / K ) photometry of SIMP J013656.5+093347 (hereafter SIMP0136) collected at the Canada–France–Hawaii Telescope using the Wide-field InfraRed Camera. SIMP0136 is a planetary-mass object (12.7 ± 1.0 M J ) at the L/T transition (T2 ± 0.5) known to exhibit light-curve evolution over multiple rotational periods. We measure the maximum peak-to-peak variability of 6.17% ± 0.46%, 6.45% ± 0.33%, 6.51% ± 0.42%, and 4.33% ± 0.38% in the Y , J , H , and K bands, respectively, and find evidence that wave models are preferred for all four NIR bands. Furthermore, we determine that the spot size necessary to reproduce the observed variations is larger than the Rossby deformation radius and Rhines scale, which is unphysical. Through the correlation between light curves produced by the waves and associated color variability, we find evidence of planetary-scale, wave-induced cloud modulation and breakup, similar to Jupiter’s atmosphere and supported by general circulation models. We also detect a 93.°8 ± 7.°4 (12.7 σ ) phase shift between the H − K and J − H color time series, providing evidence for complex vertical cloud structure in SIMP0136's atmosphere.
We present a photometric variability survey of young planetary-mass objects using the New Technology Telescope in the JS and KS bands. Surface gravity plays an important role in the atmospheric structure of brown dwarfs, as young low gravity L dwarfs have a higher variability rate than field L dwarfs. In this study, we extend variability studies to young T-type planetary-mass objects and investigate the effects of surface gravity on the variability of L and T dwarfs across a large sample. We conduct continuous monitoring for 18 objects with spectral types from L5 to T8 and detect four new variables and two variable candidates. Combining with previous variability surveys of field and young L and T objects, we find that young objects tend to be more variable than field objects within peak-to-peak variability amplitude ranges of 0.5%–10% and period ranges of 1.5–20 hr. For the first time, we constrain the variability rate of young T dwarfs to be % compared to % for field T dwarfs. Both field and young samples have higher variability rates at the L/T transition than outside the L/T transition. The differences in the variability rates between field and young samples are about 1σ and therefore larger sample sizes are needed to confirm and refine the results. Besides the L/T transition, young L dwarfs with strong variability tend to assemble in a narrow spectral type range of L6–L7.5. This work supports the critical role of surface gravity on the atmospheric structure from L to T spectral types.
As a toy model, we envision the origin of life as occurring at thermodynamic equilibrium via a large number of small systems, at least one of which reaches the threshold for life. The argument is developed in the context of the hypothesized lipid world, and its amphiphilic vesicles presumably formed from interplanetary dust particles. With enough vesicles, the probability of at least one vesicle overcoming all the activation energy barriers at equilibrium is one. An analogy is made to a fish ladder, often successfully traversed by only a small fraction of fish. We introduce the idea of a geographic catchment area as a means of concentrating vesicles and perhaps other nonvolatile components in an endoreic lake. The fish ladder model can apply to many other scenarios involving encapsulation. We pose it as a probabilistically plausible null model against which other hypotheses for the origin of life could be tested and invite others to narrow its estimated parameters. As a shortest estimate, we arrive at 3.6 million years for amphiphiles to accumulate to the critical vesicle concentration, well within the Hadean, followed by Darwinian evolution starting within 10 ‐8 seconds. The idea may someday be testable via massive sterile experiments. Extension of these ideas to more complex, explicit proposals for the origin of life requires determination of forward and backward activation energies, energy sources, and environmental cycles.
There are many Archaea with flat, polygonal shapes, resembling shaped droplets in form. We hypothesized that the transition from abiogenesis to a living cell started with shaped droplets. We begin with proteins that have a transmembrane domain, such as the S layer, which is a common feature of Archaea and Bacteria. We show how the anchor transmembrane domain may have been hydrophobically “selected” from random mixed chirality peptides by the cell membrane itself when it was thinner, possibly produced from short meteorite amphiphiles. These peptides may have been the first proteins, formed mostly from three “core” amino acids that we deduce from t‐RNA phylogeny: alanine, glycine, and isoleucine. They may have formed by PCR‐like wet/dry cycles along with RNA, which may have protected them from hydrolysis as they grew, causing the membrane to thicken to its present thickness, a positive feedback, abiotic process. The possibly daily flattening and swelling of vesicles as the temperature changed at night, with 10 11 day/night cycles available, might have aided the PCR‐like activity. Thus, the peptide/protein world may have been simultaneous with the RNA world. The juxtaposition of peptides with RNA may have led to the genetic code. Interactions between membrane‐selected peptides and amphiphiles may have led to chirality of both.
Only a handful of late T brown dwarfs have been monitored for spectrophotometric variability, leaving incomplete the study of the atmospheric cloud structures of the coldest brown dwarfs, which share temperatures with some cold, directly imaged exoplanets. 2MASS J00501994–332240 is a T7.0 rapidly rotating, field brown dwarf that showed low-level photometric variability in data obtained with the Spitzer Space Telescope. We monitored 2MASS J00501994–332240 during ∼2.6 hr with MOSFIRE, installed at the Keck I telescope, with the aim of constraining its near-infrared spectrophotometric variability. We measured fluctuations with a peak-to-peak amplitude of 1.48% ± 0.75% in the J -band photometric light curve, an amplitude of 0.62% ± 0.18% in the J -band spectrophotometric light curve, an amplitude of 1.26% ± 0.93% in the H -band light curve, and an amplitude of 5.33% ± 2.02% in the CH 4 − H 2 O band light curve. Nevertheless, the Bayesian information criterion does not detect significant variability in any of the light curves. Thus, given the detection limitations due to the MOSFIRE sensitivity, we can only claim tentative low-level variability for 2M0050–3322 in the best-case scenario. The amplitudes of the peak-to-peak fluctuations measured for 2MASS J00501994–332240 agree with the variability amplitude predictions of general circulation models for a T7.0 brown dwarf for an edge-on object. Radiative transfer models predict that the Na 2 S and KCl clouds condense at pressures lower than that traced by the CH 4 –H 2 O band, which might explain the higher peak-to-peak fluctuations measured for this light curve. Finally, we provide a visual recreation of the map provided by general circulation models and the vertical structure of 2MASS J00501994–332240 provided by radiative transfer models.
Only a handful of late-T brown dwarfs have been monitored for spectro-photometric variability, leaving incomplete the study of the atmospheric cloud structures of the coldest brown dwarfs, that share temperatures with some cold, directly-imaged exoplanets. 2MASSJ00501994-332240 is a T7.0 rapidly rotating, field brown dwarf that showed low-level photometric variability in data obtained with the Spitzer Space telescope. We monitored 2MASSJ00501994-332240 during ~2.6 hr with MOSFIRE, installed at the Keck I telescope, with the aim of constraining its near-infrared spectro-photometric variability. We measured fluctuations with a peak-to-peak amplitude of 1.48+\-0.75% in the J-band photometric light curve, an amplitude of 0.62+/-0.18% in the J-band spectro-photometric light curve, and an amplitude of 1.26+/-0.93% in the H-band light curve, and an amplitude of 5.33+/-2.02% in the CH_4-H_2O band light curve. Nevertheless, the Bayesian Information Criterion does not detect significant variability in any of the light curves. Thus, given the detection limitations due to the MOSFIRE sensitivity, we can only claim tentative low-level variability for 2M0050-3322 in the best-case scenario. The amplitudes of the peak-to-peak fluctuations measured for 2MASSJ00501994-332240 agree with the variability amplitude predictions of General Circulation Models for a T7.0 brown dwarf for an edge on object. Radiative-transfer models predict that the Na_2S and KCl clouds condense at pressures lower than that traced by the CH_4-H_2O band, which might explain the higher peak-to-peak fluctuations measured for this light curve. Finally, we provide a visual recreation of the map provided by General Circulation Models and the vertical structure of 2MASSJ00501994-332240 provided by radiative-transfer models.
The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has completed its commissioning and begun the Commensal Radio Astronomy FasT Survey (CRAFTS), a multi-year survey to cover 60\% of the sky, in 2020. We present predictions for the number of radio-flaring ultracool dwarfs (UCDs) that are likely to be detected by CRAFTS. Based on the observed flaring UCDs from a number of targeted radio surveys in the literature, we derive a lower limit of the detection rate to be 3\%. Assuming a flat radio spectrum with = -1.0 for UCD flares, we construct a flare luminosity function (here ). CRAFTS is found to be sensitive enough for flares from UCDs up to 180 pc. Considering the Galactic thin disk, we carry out a 3D Monte Carlo simulation of the UCD population, which is then fed to mock CRAFTS observations. We estimate that 170 flaring UCDs would be detected through transient searches in circular polarization. Though only marginally sensitive to the scale height of UCDs, the results are very sensitive to the assumed spectral index . For from 0 to -2.5, the number of expected detections increases dramatically from 20 to 3460. We also contemplate the strategies for following up candidates of flaring UCDs, and discuss the implications of survey results for improving our knowledge of UCD behavior at L band and dynamos.
We present the discovery of rapid photometric variability in three ultra-cool dwarfs from long-duration monitoring with the Spitzer Space Telescope. The T7, L3.5, and L8 dwarfs have the shortest photometric periods known to date: hr, hr, and hr, respectively. We confirm the rapid rotation through moderate-resolution infrared spectroscopy, which reveals projected rotational velocities between 79 and 104 km s ⁻¹ . We compare the near-infrared spectra to photospheric models to determine the objects’ fundamental parameters and radial velocities. We find that the equatorial rotational velocities for all three objects are ≳100 km s ⁻¹ . The three L and T dwarfs reported here are the most rapidly spinning and likely the most oblate field ultra-cool dwarfs known to date. Correspondingly, all three are excellent candidates for seeking auroral radio emission and net optical/infrared polarization. As of this writing, 78 L-, T-, and Y-dwarf rotation periods have now been measured. The clustering of the shortest rotation periods near 1 hr suggests that brown dwarfs are unlikely to spin much faster.
We present the discovery of rapid photometric variability in three ultra-cool dwarfs from long-duration monitoring with the Spitzer Space Telescope. The T7, L3.5, and L8 dwarfs have the shortest photometric periods known to date: h, h, and h, respectively. We confirm the rapid rotation through moderate-resolution infrared spectroscopy that reveals projected rotational velocities between 79 and 104 km s. We compare the near-infrared spectra to photospheric models to determine the objects' fundamental parameters and radial velocities (RVs). We find that the equatorial rotational velocities for all three objects are 100 km s. The three L and T dwarfs reported here are the most rapidly spinning and likely the most oblate field ultra-cool dwarfs known to date. Correspondingly, all three are excellent candidates for seeking auroral radio emission and net optical/infrared polarization. As of this writing, 78 L-, T-, and Y-dwarf rotation periods have now been measured. The clustering of the shortest rotation periods near 1 h suggests that brown dwarfs are unlikely to spin much faster.
While steady empirical progress has been made in understanding the structure and composition of hot-planet atmospheres, direct measurements of velocity signatures, including winds, rotation, and jets, have lagged behind. Quantifying atmospheric dynamics of hot planets is critical for a complete understanding of their atmospheres, and such measurements may even illuminate other planetary properties, such as magnetic field strengths. In this manuscript we present the first detection of the Balmer lines Hα and Hβ in the atmosphere of the ultra-hot Jupiter WASP-33 b. Using atmospheric models that include the effects of atmospheric dynamics, we show that the shape of the average Balmer line transmission spectrum is consistent with rotational velocities in the planet’s thermosphere of vrot = 10.1-⁺1.00.8 km s⁻¹. We also measure a low-significance day- to nightside velocity shift of -4.6-⁺3.43.4 km s⁻¹ in the transmission spectrum, which is naturally explained by a global wind across the planet’s terminator. In a separate analysis the time-resolved velocity centroids of individual transmission spectra show unambiguous evidence of rotation, with a best-fit velocity of 10.0-⁺2.02.4 km s⁻¹, consistent with the value of vrot derived from the shape of the average Balmer line transmission spectrum. Our observations and analysis confirm the power of time-resolved transmission spectra with a high signal-to-noise ratio to measure the velocity structures in exoplanet atmospheres. The high rotational and wind velocities we measure highlight the need for more detailed 3D global climate simulations of the rarefied upper atmospheres of ultra-hot gas giants. © 2021. The American Astronomical Society. All rights reserved.
Brown dwarfs were recently found to display rotational modulations, commonly attributed to cloud cover of varying thickness, possibly modulated by planetary-scale waves. However, the long-term, continuous, highprecision monitoring data to test this hypothesis for more objects is lacking. By applying our novel photometric approach to Transiting Exoplanet Survey Satellite data, we extract a high-precision lightcurve of the closest brown dwarfs, which form the binary system Luhman 16 AB. Our observations, which cover about 100 rotations of Luhman 16 B, display continuous lightcurve evolution. The periodogram analysis shows that the rotational period of the component that dominates the lightcurve is 5.28 hr. We also find evidence for periods of 2.5, 6.94, and 90.8 hr. We show that the 2.5 and 5.28 hr periods emerge from Luhman 16 B and that they consist of multiple, slightly shifted peaks, revealing the presence of high-speed jets and zonal circulation in this object. We find that the lightcurve evolution is well fit by the planetary-scale waves model, further supporting this interpretation. We argue that the 6.94 hr peak is likely the rotation period of Luhman 16 A. By comparing the rotational periods to observed v sin(i) measurements, we show that the two brown dwarfs are viewed at angles close to their equatorial planes. We also describe a long-period (P ∼ 91 hr) evolution in the lightcurve, which we propose emerges from the vortexdominated polar regions. Our study paves the way toward direct comparisons of the predictions of global circulation models to observations via periodogram analysis.
Groundbased and spacecraft telescopic observations, combined with an intensive modeling effort, have greatly enhanced our understanding of hot giant planets and brown dwarfs over the past ten years. Although these objects are all fluid, hydrogen worlds with stratified atmospheres overlying convective interiors, they exhibit an impressive diversity of atmospheric behavior. Hot Jupiters are strongly irradiated, and a wealth of observations constrain the day-night temperature differences, circulation, and cloudiness. The intense stellar irradiation, presumed tidal locking and modest rotation leads to a novel regime of strong day-night radiative forcing. Circulation models predict large day-night temperature differences, global-scale eddies, patchy clouds, and, in most cases, a fast eastward jet at the equator—equatorial superrotation. The warm Jupiters lie farther from their stars and are not generally tidally locked, so they may exhibit a wide range of rotation rates, obliquities, and orbital eccentricities, which, along with the weaker irradiation, leads to circulation patterns and observable signatures predicted to differ substantially from hot Jupiters. Brown dwarfs are typically isolated, rapidly rotating worlds; they radiate enormous energy fluxes into space and convect vigorously in their interiors. Their atmospheres exhibit patchiness in clouds and temperature on regional to global scales—the result of modulation by large-scale atmospheric circulation. Despite the lack of irradiation, such circulations can be driven by interaction of the interior convection with the overlying atmosphere, as well as self-organization of patchiness due to cloud-dynamical-radiative feedbacks. Finally, irradiated brown dwarfs help to bridge the gap between these classes of objects, experiencing intense external irradiation as well as vigorous interior convection. Collectively, these diverse objects span over six orders of magnitude in intrinsic heat flux and incident stellar flux, and two orders of magnitude in rotation rate—thereby placing strong constraints on how the circulation of giant planets (broadly defined) depend on these parameters. A hierarchy of modeling approaches have yielded major new insights into the dynamics governing these phenomena.
Aims. The rotational spectral modulation (spectro-photometric variability) of brown dwarfs is usually interpreted as a sign indicating the presence of inhomogeneous cloud covers in the atmosphere. This paper is aimed at exploring the role of temperature fluctuations in these spectral modulations. These fluctuations could naturally arise in a convective atmosphere impacted by such diabatic processes as complex chemistry, namely, a mechanism recently proposed to explain the L/T transition: CO/CH 4 radiative convection.
Methods. After exploring the observed spectral-flux ratios between different objects along the cooling sequence, we used the 1D radiative-convective code ATMO, with ad hoc modifications of the temperature gradient, to model the rotational spectral modulation of 2MASS 1821, 2MASS 0136, and PSO 318.5-22. We also explored the impact of CH 4 abundance fluctuations on the spectral modulation of 2MASS 0136.
Results. The spectral-flux ratio of different objects along the cooling sequence and the rotational spectral modulation within individual objects at the L/T transition have similar characteristics. This strongly suggests that the main parameter varying along the cooling sequence, namely, temperature, might play a key role in the rotational spectral modulations at the L/T transition. Modeling the spectral bright-to-faint ratio of the modulation of 2MASS 1821, 2MASS 0136, and PSO 318.5-22 shows that most spectral characteristics can be reproduced by temperature variations alone. Furthermore, the approximately anti-correlated variability between different wavelengths can be easily interpreted as a change in the temperature gradient in the atmosphere, which is a consequence we expect from CO/CH 4 radiative convection as an explanation of the L/T transition. The deviation from an exact anti-correlation could then be interpreted as a phase shift similar to the hot-spot shift at different bandpasses in the atmospheres of hot Jupiters.
Conclusions. Our results suggest that the rotational spectral modulation from cloud opacity and temperature variations are degenerate. If the nearly anti-correlated signal between different wavelengths is, indeed, a strong sign of a change in the temperature gradient, the detection of direct cloud spectral signatures, for instance, the silicate absorption feature at 10 μ m, would help to confirm the presence of clouds and their contribution to spectral modulations (which does not exclude temperature variations or other mechanisms that may also be at play). Future studies considering the differences in the spectral modulation of objects with and without the silicate absorption feature may give us some insight into how to distinguish cloud-opacity fluctuations from temperature fluctuations.
A planetary atmosphere is the outer gas layer of a planet. Besides its scientific significance among the first and most accessible planetary layers observed from space, it is closely connected with planetary formation and evolution, surface and interior processes, and habitability of planets. Current theories of planetary atmospheres were primarily obtained through the studies of eight large planets, Pluto and three large moons (Io, Titan, and Triton) in the Solar System. Outside the Solar System, more than four thousand extrasolar planets (exoplanets) and two thousand brown dwarfs have been confirmed in our Galaxy, and their population is rapidly growing. The rich information from these exotic bodies offers a database to test, in a statistical sense, the fundamental theories of planetary climates. Here we review the current knowledge on atmospheres of exoplanets and brown dwarfs from recent observations and theories. This review highlights important regimes and statistical trends in an ensemble of atmospheres as an initial step towards fully characterizing diverse substellar atmospheres, that illustrates the underlying principles and critical problems. Insights are obtained through analysis of the dependence of atmospheric characteristics on basic planetary parameters. Dominant processes that influence atmospheric stability, energy transport, temperature, composition and flow pattern are discussed and elaborated with simple scaling laws. We dedicate this review to Dr. Adam P. Showman (1968-2020) in recognition of his fundamental contribution to the understanding of atmospheric dynamics on giant planets, exoplanets and brown dwarfs. © 2020 National Astronomical Observatories, CAS and IOP Publishing Ltd..
2020. The American Astronomical Society. All rights reserved. We present Spitzer Space Telescope variability monitoring observations of three low-gravity L dwarfs with previous detections of variability in the near-IR: 2MASS J0045+16, 2MASS J0501-00, and 2MASS J1425-36. We detect significant periodic variability in two of our targets, 2MASS J0045+16 and 2MASS J0501-00. We do not detect variability in 2MASS J1425-36. Combining our new rotation periods with rotational velocities, we calculate inclination angles of 22° ± 1°, 60+13°-8 and 52+19°-131 for 2MASS J0045+16, 2MASS J0501-00, and 2MASS J1425-36, respectively. Our three new objects are consistent with the tentative relations between inclination, amplitude, and color anomaly previously reported. Objects with the highest variability amplitudes are inclined equator on, while the maximum observed amplitude decreases as the inclination angle decreases. We also find a correlation between the inclination angle and (J-K)2MASS color anomaly for the sample of objects with measured inclinations. Compiling the entire sample of brown dwarfs with Spitzer variability detections, we find no enhancement in amplitude for young, early-L dwarfs compared to the field dwarf population. We find a possible enhancement in amplitude of low-gravity late-L dwarfs at 4.5 μm. We do not find a correlation between amplitude ratio and spectral type for field dwarfs or for the young population. Finally, we compile the rotation periods of a large sample of brown dwarfs with ages 1 Myr-1 Gyr and compare the rotation rates predicted by evolutionary models assuming angular momentum conservation. We find that the rotation rates of the current sample of brown dwarfs fall within the expected range set by evolutionary models and breakup limits.
Evidence of star–planet interactions in the form of planet-modulated chromospheric emission has been noted for a number of hot Jupiters. Magnetic star–planet interactions involve the release of energy stored in the stellar and planetary magnetic fields. These signals thus offer indirect detections of exoplanetary magnetic fields. Here, we report the derivation of the magnetic field strengths of four hot Jupiter systems, using the power observed in calcium ii K emission modulated by magnetic star–planet interactions. By approximating the fractional energy released in the calcium ii K line, we find that the surface magnetic field values for the hot Jupiters in our sample range from 20 G to 120 G, around 10–100 times larger than the values predicted by dynamo scaling laws for planets with rotation periods of around 2–4 days. However, these values are in agreement with scaling laws relating the magnetic field strength to the internal heat flux in giant planets. Large planetary magnetic field strengths may produce observable electron cyclotron maser radio emission by preventing the maser from being quenched by the planet’s ionosphere. Intensive radio monitoring of hot Jupiter systems will help to confirm these field values and inform the generation mechanism of magnetic fields in this important class of exoplanets.
Probing the depths of Jupiter
The Juno mission set out to probe the hidden properties of Jupiter, such as its gravitational field, the depth of its atmospheric jets and its composition beneath the clouds. A collection of papers in this week's issue report some of the mission's key findings. Jupiter's gravitational field varies from pole to pole, but the cause of this asymmetry is unknown. Rotating planets that are squashed at the poles like Jupiter can have a gravity field that is characterized by a solid-body component, plus components that arise from motions in the atmosphere. Luciano Iess and colleagues use Juno's Doppler tracking data to determine Jupiter's gravity harmonics. They find that the north–south asymmetry arises from atmospheric and interior wind flows. To determine the depths of these flows, Yohai Kaspi and colleagues analyse the odd gravitational harmonics and find that the J3, J5, J7 and J9 harmonics are consistent with the jets extending deep into the atmosphere, perhaps as far as 3,000 kilometres. They conclude that the mass of Jupiter's dynamical atmosphere is about one per cent of Jupiter's total mass. The composition of Jupiter beneath its turbulent atmosphere remains a mystery. If different parts of a spinning object rotate at different rates, then the object probably has a fluid composition. Tristan Guillot and colleagues study the even gravitational harmonics and find that, below a depth of about 3,000 kilometres, Jupiter is rotating almost as a solid body. The atmospheric zonal flows extend downwards by more than 2,000 kilometres, but not beyond 3,500 kilometres, as is also the case with the jets.
Probing the depths of Jupiter
The Juno mission set out to probe the hidden properties of Jupiter, such as its gravitational field, the depth of its atmospheric jets and its composition beneath the clouds. A collection of papers in this week's issue report some of the mission's key findings. Jupiter's gravitational field varies from pole to pole, but the cause of this asymmetry is unknown. Rotating planets that are squashed at the poles like Jupiter can have a gravity field that is characterized by a solid-body component, plus components that arise from motions in the atmosphere. Luciano Iess and colleagues use Juno's Doppler tracking data to determine Jupiter's gravity harmonics. They find that the north–south asymmetry arises from atmospheric and interior wind flows. To determine the depths of these flows, Yohai Kaspi and colleagues analyse the odd gravitational harmonics and find that the J3, J5, J7 and J9 harmonics are consistent with the jets extending deep into the atmosphere, perhaps as far as 3,000 kilometres. They conclude that the mass of Jupiter's dynamical atmosphere is about one per cent of Jupiter's total mass. The composition of Jupiter beneath its turbulent atmosphere remains a mystery. If different parts of a spinning object rotate at different rates, then the object probably has a fluid composition. Tristan Guillot and colleagues study the even gravitational harmonics and find that, below a depth of about 3,000 kilometres, Jupiter is rotating almost as a solid body. The atmospheric zonal flows extend downwards by more than 2,000 kilometres, but not beyond 3,500 kilometres, as is also the case with the jets.
We present the detection of [m] photometric variability in two young, L/T transition brown dwarfs, WISE J004701.06+680352.1 (W0047) and 2MASS J2244316+204343 (2M2244) using the \textit{Spitzer Space Telescope}. We find a period of hr and a peak-to-peak amplitude of for W0047, and a period of hr and amplitude of for 2M2244. This period is significantly longer than that measured previously during a shorter observation. {We additionally detect significant J-band variability in 2M2244 using the Wide-Field Camera on UKIRT. } We determine the radial and rotational velocities of both objects using Keck NIRSPEC data. We find a radial velocity of km s for 2M2244, and confirm it as a bona fide member of the AB Doradus moving group. We find rotational velocities of km sand km s for W0047 and 2M2244, respectively. With inclination angles of and , W0047 and 2M2244 are viewed roughly equator-on. Their remarkably similar colours, spectra and inclinations are consistent with the possibility that viewing angle may influence atmospheric appearance. We additionally present \textit{Spitzer} monitoring of the young, T5.5 object SDSS111010+011613 (SDSS1110) where we detect no variability. For periods hr, we place an upper limit of on the peak-to-peak variability amplitude of SDSS1110.
We present Spitzer/Infrared Array Camera Ch1 and Ch2 monitoring of six brown dwarfs during eight different epochs over the course of 20 months. For four brown dwarfs, we also obtained simulataneous Hubble Space Telescope (HST)/WFC3 G141 grism spectra during two epochs and derived light curves in five narrowband filters. Probing different pressure levels in the atmospheres, the multiwavelength light curves of our six targets all exhibit variations, and the shape of the light curves evolves over the timescale of a rotation period, ranging from 1.4 to 13 hr. We compare the shapes of the light curves and estimate the phase shifts between the light curves observed at different wavelengths by comparing the phase of the primary Fourier components. We use state-of-the-art atmosphere models to determine the flux contribution of different pressure layers to the observed flux in each filter. We find that the light curves that probe higher pressures are similar and in phase, but are offset and often different from the light curves that probe lower pressures. The phase differences between the two groups of light curves suggest that the modulations seen at lower and higher pressures may be introduced by different cloud layers. © 2016. The American Astronomical Society. All rights reserved.
Deducing the cloud cover and its temporal evolution from the observed
planetary spectra and phase curves can give us major insight into the
atmospheric dynamics. In this paper, we present Aeolus, a Markov-Chain Monte
Carlo code that maps the structure of brown dwarf and other ultracool
atmospheres. We validated Aeolus on a set of unique Jupiter Hubble Space
Telescope (HST) light curves. Aeolus accurately retrieves the properties of the
major features of the jovian atmosphere such as the Great Red Spot and a major
5um hot spot. Aeolus is the first mapping code validated on actual observations
of a giant planet over a full rotational period. For this study, we applied
Aeolus to J and H-bands HST light curves of 2MASSJ21392676+0220226 and
2MASSJ0136565+093347. Aeolus retrieves three spots at the top-of-the-atmosphere
(per observational wavelength) of these two brown dwarfs, with a surface
coverage of 21+-3% and 20.3+-1.5% respectively. The Jupiter HST light curves
will be publicly available via ADS/VIZIR.
We present results from the "Weather on Other Worlds" Spitzer Exploration
Science program to investigate photometric variability in L and T dwarfs,
usually attributed to patchy clouds. We surveyed 44 L3-T8 dwarfs, spanning a
range of colors and surface gravities. We find that 14/23 (61%; 95%
confidence interval: 41%-78%) of our single L3-L9.5 dwarfs are variable with
peak-to-peak amplitudes between 0.2% and 1.5%, and 5/16 (31%; 95% confidence
interval: 14%-56%) of our single T0-T8 dwarfs are variable with amplitudes
between 0.8% and 4.6%. After correcting for sensitivity, we find that 80% (95%
confidence interval: 53%-100%) of L dwarfs vary by >0.2%, and 36% (95%
confidence interval: 19%-52%) of T dwarfs vary by >0.4%. Given viewing geometry
considerations, we conclude that photospheric heterogeneities causing >0.2%
3-5-micron flux variations are present on virtually all L dwarfs, and probably
on most T dwarfs. A third of L dwarf variables show irregular light curves,
indicating that L dwarfs may have multiple spots that evolve over a single
rotation. Also, approximately a third of the periodicities are on time scales
>10 h, suggesting that slowly-rotating brown dwarfs may be common. We observe
an increase in the maximum amplitudes over the entire spectral type range,
revealing a potential for greater temperature contrasts in T dwarfs than in L
dwarfs. We find a tentative association (92% confidence) between low surface
gravity and high-amplitude variability among L3-L5.5 dwarfs. Although we can
not confirm whether lower gravity is also correlated with a higher incidence of
variables, the result is promising for the characterization of directly imaged
young extrasolar planets through variability.
Jupiter’s dynamo is modelled using the anelastic convection-driven dynamo equations. The reference state model is taken from French et al. (2012), which used density functional theory to compute the equation of state and the electrical conductivity in Jupiter’s interior. Jupiter’s magnetic field is approximately dipolar, but self-consistent dipolar dynamo models are rather rare when the large variation in density and the effective internal heating are taken into account. Jupiter-like dipolar magnetic fields were found here at small Prandtl number, Pr=0.1Pr=0.1. Strong differential rotation in the dynamo region tends to destroy a dominant dipolar component, but when the convection is sufficiently supercritical it generates a strong magnetic field, and the differential rotation in the electrically conducting region is suppressed by the Lorentz force. This allows a magnetic field to develop which is dominated by a steady dipolar component. This suggests that the strong zonal winds seen at Jupiter’s surface cannot penetrate significantly into the dynamo region, which starts approximately 7000 km below the surface.
The spin of a planet arises from the accretion of angular momentum during its formation, but the details of this process are still unclear. In the Solar System, the equatorial rotation velocities and, consequently, spin angular momenta of most of the planets increase with planetary mass; the exceptions to this trend are Mercury and Venus, which, since formation, have significantly spun down because of tidal interactions. Here we report near-infrared spectroscopic observations, at a resolving power of 100,000, of the young extrasolar gas giant planet β Pictoris b (refs 7, 8). The absorption signal from carbon monoxide in the planet's thermal spectrum is found to be blueshifted with respect to that from the parent star by approximately 15 kilometres per second, consistent with a circular orbit. The combined line profile exhibits a rotational broadening of about 25 kilometres per second, meaning that β Pictoris b spins significantly faster than any planet in the Solar System, in line with the extrapolation of the known trend in spin velocity with planet mass.
emcee is an extensible, pure-Python implementation of Goodman &
Weare's Affine Invariant Markov chain Monte Carlo (MCMC) Ensemble
sampler. It's designed for Bayesian parameter estimation. The algorithm
behind emcee has several advantages over traditional MCMC sampling
methods and has excellent performance as measured by the autocorrelation
time (or function calls per independent sample). One advantage of the
algorithm is that it requires hand-tuning of only 1 or 2 parameters
compared to ˜ N^2 for a traditional algorithm in an N-dimensional
parameter space. Exploiting the parallelism of the ensemble method,
emcee permits any user to take advantage of multiple CPU cores without
extra effort.
The Infrared Array Camera (IRAC) is now the only science instrument in
operation on the Spitzer Space Telescope. The 3.6 and 4.5 µm
channels are temperature-stabilized at ~28.7K, and the sensitivity of
IRAC is nearly identical to what it was in the cryogenic mission. The
instrument point response function (PRF) is a set of values from which
one can determine the point spread function (PSF) for a source at any
position in the field, and is dependent on the optical characteristics
of the telescope and instrument as well as the detector sampling and
pixel response. These data are necessary when performing PSF-fitting
photometry of sources, for deconvolving an IRAC image, subtracting out a
bright source in a field, or for estimating the flux of a source that
saturates the detector. Since the telescope and instrument are operating
at a higher temperature in the post-cryogenic mission, we re-derive the
PRFs for IRAC from measurements obtained after the warm mission
temperature set point and detector biases were finalized and compare
them to the 3.6 and 4.5 µm PRFs determined during the cryogenic
mission to assess any changes.
Photometric monitoring from warm Spitzer reveals that the L3 dwarf DENIS-P J1058.7-1548 varies sinusoidally in brightness with a period of 4.25^{+0.26}_{-0.16} hr and an amplitude of 0.388% ± 0.043% (peak-to-valley) in the 3.6 μm band, confirming the reality of a 4.31 ± 0.31 hr periodicity detected in J-band photometry from the SOAR telescope. The J-band variations are a factor of 2.17 ± 0.35 larger in amplitude than those at 3.6 μm, while 4.5 μm Spitzer observations yield a 4.5 μm/3.6 μm amplitude ratio of only 0.23 ± 0.15, consistent with zero 4.5 μm variability. This wide range in amplitudes indicates rotationally modulated variability due to magnetic phenomena and/or inhomogeneous cloud cover. Weak Hα emission indicates some magnetic activity, but it is difficult to explain the observed amplitudes by magnetic phenomena unless they are combined with cloud inhomogeneities (which might have a magnetic cause). However, inhomogeneous cloud cover alone can explain all our observations, and our data align with theory in requiring that the regions with the thickest clouds also have the lowest effective temperature. Combined with published vsin (i) results, our rotation period yields a 95% confidence lower limit of R * >= 0.111 R sun, suggesting upper limits of 320 Myr and 0.055 M sun on the age and mass. These limits should be regarded cautiously because of ~3σ inconsistencies with other data; however, a lower limit of 45° on the inclination is more secure. DENIS-P J1058.7-1548 is only the first of nearly two dozen low-amplitude variables discovered and analyzed by the Weather on Other Worlds project.
A technique is described that is used to improve the detection of radio-frequency interference in astronomical radio observatories. It is applied on a two-dimensional interference mask after regular detection in the time-frequency domain with existing techniques. The scale-invariant rank (SIR) operator is defined, which is a one-dimensional mathematical morphology technique that can be used to find adjacent intervals in the time or frequency domain that are likely to be affected by RFI. The technique might also be applicable in other areas in which morphological scale-invariant behaviour is desired, such as source detection. A new algorithm is described, that is shown to perform quite well, has linear time complexity and is fast enough to be applied in modern high resolution observatories. It is used in the default pipeline of the LOFAR observatory.
We present the results of an intense photometric monitoring in the
near-infrared (~0.9 microns) with the TRAPPIST robotic telescope of the newly
discovered binary brown dwarf WISE J104915.57-531906.1, the third closest
system to the Sun at a distance of only 2 pc. Our twelve nights of photometric
time-series reveal a quasi-periodic (P = 4.87+-0.01 h) variability with a
maximal peak-peak amplitude of ~11% and strong night-to-night evolution. We
attribute this variability to the rotational modulation of fast-evolving
weather patterns in the atmosphere of the coolest component (~T1-type) of the
binary, in agreement with the cloud fragmentation mechanism proposed to drive
the spectroscopic morphologies of brown dwarfs at the L/T transition. No
periodic signal is detected for the hottest component (~L8-type). For both
brown dwarfs, our data allow us to firmly discard any unique transit during our
observations for planets >= 2 Rearth. For orbital periods smaller than ~9.5 h,
transiting planets are excluded down to an Earth-size.
The Infrared Array Camera (IRAC) is one of three focal plane instruments on the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broadband images at 3.6, 4.5, 5.8, and 8.0 μm. Two nearly adjacent 52 × 52 fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 μm; 4.5 and 8 μm). All four detector arrays in the camera are 256 × 256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.
We present a novel interpretation of the non-coplanar baselines effect
in synthesis radio telescopes as being due to differential Fresnel
diffraction in the neighborhood of the array antennas, and a new
algorithm to deal with this effect. Our new algorithm, which we call ``w
projection'', has markedly superior performance compared to existing
algorithms. At roughly equivalent levels of accuracy, w-projection can
be up to an order of magnitude faster than the corresponding facet-based
algorithms.
The Spitzer Space Telescope Infrared Array Camera (IRAC) basic
calibrated data reduction pipeline is designed to take a single raw
frame from a single IRAC detector and produce a flux-calibrated image
that has had all well-understood instrumental signatures removed. We
discuss several modifications to the pipeline developed in the last two
years in response to the Spitzer warm mission. Due to the different
instrument characteristics in the warm mission, we have significantly
changed pipeline procedures for masking residual images and mitigating
column pulldown. In addition, the muxbleed correction was turned off,
because it is not present in the warm data. Parameters relevant to
linearity correction, bad pixels, and the photometric calibration have
been updated and are continually monitored.
The Infrared Array Camera (IRAC) on the Spitzer Space Telescope has been used to measure < 10 −4 temporal variations in point sources (such as transiting extrasolar planets) at 3.6 and 4.5 µm. Due to the under-sampled nature of the PSF, the warm IRAC arrays show variations of as much as 8% in sensitivity as the center of the PSF moves across a pixel due to normal spacecraft pointing wobble and drift. These intra-pixel gain variations are the largest source of correlated noise in IRAC photometry. Usually this effect is removed by fitting a model to the science data themselves (self-calibration), which could result in the removal of astrophysically interesting signals. We describe a new technique for significantly reducing the gain variations and improving photometric precision in a given observation, without using the data to be corrected. This comprises: (1) an adaptive centroiding and repositioning method ("Peak-Up") that uses the Spitzer Pointing Control Reference Sensor (PCRS) to repeatedly position a target to within 0.1 IRAC pixels of an area of minimal gain variation; and (2) the high-precision, high-resolution measurement of the pixel gain structure using non-variable stars. We show that the technique currently allows the reduction of correlated noise by almost an order of magnitude over raw data, which is comparable to the improvement due to self-calibration. We discuss other possible sources of correlated noise, and proposals for reducing their impact on photometric precision.
A number of ultra-cool dwarfs emit circularly polarised radio waves generated
by the electron cyclotron maser instability. In the solar system such radio is
emitted from regions of strong auroral magnetic field-aligned currents. We thus
apply ideas developed for Jupiter's magnetosphere, being a well-studied
rotationally-dominated analogue in our solar system, to the case of
fast-rotating UCDs. We explain the properties of the radio emission from UCDs
by showing that it would arise from the electric currents resulting from an
angular velocity shear in the fast-rotating magnetic field and plasma, i.e. by
an extremely powerful analogue of the process which causes Jupiter's auroras.
Such a velocity gradient indicates that these bodies interact significantly
with their space environment, resulting in intense auroral emissions. These
results strongly suggest that auroras occur on bodies outside our solar system.
Brown dwarfs and directly imaged giant planets exhibit significant evidence for active atmospheric circulation, which induces a large-scale patchiness in the cloud structure that evolves significantly over time, as evidenced by infrared light curves and Doppler maps. These observations raise critical questions about the fundamental nature of the circulation, its time variability, and its overall relationship to the circulation on Jupiter and Saturn. Jupiter and Saturn themselves exhibit numerous robust zonal (east–west) jet streams at the cloud level; moreover, both planets exhibit long-term stratospheric oscillations involving perturbations of zonal wind and temperature that propagate downward over time on timescales of ∼4 yr (Jupiter) and ∼15 yr (Saturn). These oscillations, dubbed the quasi-quadrennial oscillation (QQO) for Jupiter and the semiannual oscillation (SAO) on Saturn, are thought to be analogous to the quasi-biennial oscillation (QBO) on Earth, which is driven by upward propagation of equatorial waves from the troposphere. To investigate these issues, we here present global, three-dimensional, high-resolution numerical simulations of the flow in the stratified atmosphere—overlying the convective interior—of brown dwarfs and Jupiter-like planets. The effect of interior convection is parameterized by inducing small-scale, randomly varying perturbations in the radiative–convective boundary at the base of the model. Radiative damping is represented using an idealized Newtonian cooling scheme. In the simulations, the convective perturbations generate atmospheric waves and turbulence that interact with the rotation to produce numerous zonal jets. Moreover, the equatorial stratosphere exhibits stacked eastward and westward jets that migrate downward over time, exactly as occurs in the terrestrial QBO, Jovian QQO, and Saturnian SAO. This is the first demonstration of a QBO-like phenomenon in 3D numerical simulations of a giant planet.
Measurements of photometric variability at different wavelengths provide insights into the vertical cloud structure of brown dwarfs and planetary-mass objects. In seven Hubble Space Telescope consecutive orbits, spanning ~10 hr of observing time, we obtained time-resolved spectroscopy of the planetary-mass T8 dwarf Ross 458 C using the near-infrared Wide Field Camera 3. We found spectrophotometric variability with a peak-to-peak signal of 2.62 ± 0.02% (in the 1.10–1.60 μm white light curve). Using three different methods, we estimated a rotational period of 6.75 ± 1.58 hr for the white light curve, and similar periods for narrow J- and H-band light curves. Sine wave fits to the narrow J- and H-band light curves suggest a tentative phase shift between the light curves with wavelength when we allow different periods between both light curves. If confirmed, this phase shift may be similar to the phase shift detected earlier for the T6.5 spectral type 2MASS J22282889–310262. We find that, in contrast with 2M2228, the variability of Ross 458C shows evidence for a color trend within the narrow J-band, but gray variations in the narrow H-band. The spectral time-resolved variability of Ross 458C might be potentially due to heterogeneous sulfide clouds in the atmosphere of the object. Our discovery extends the study of spectral modulations of condensate clouds to the coolest T-dwarf, planetary-mass companions.
Directly imaged planetary-mass companions offer unique opportunities in atmospheric studies of exoplanets. They share characteristics of both brown dwarfs and transiting exoplanets, and therefore are critical for connecting atmospheric characterizations for these objects. Rotational phase mapping is a powerful technique to constrain the condensate cloud properties in ultra-cool atmospheres. Applying this technique to directly imaged planetary-mass companions will be extremely valuable for constraining cloud models in low mass and surface-gravity atmospheres and for determining the rotation rate and angular momentum of substellar companions. Here, we present Hubble Space Telescope Wide Field Camera 3 near-infrared time-resolved photometry for three planetary-mass companions, AB Pic B, 2M0122B, and 2M1207b. Using two-roll differential imaging and hybrid point-spread function modeling, we achieve sub-percent photometric precision for all three observations. We find tentative modulations (<2σ) for AB Pic B and 2M0122B, but cannot reach conclusive results on 2M1207b due to strong systematics. The relatively low significance of the modulation measurements cannot rule out the hypothesis that these planetary-mass companions have the same vertical cloud structures as brown dwarfs. Our rotation rate measurements, combined with archival period measurements of planetary-mass companions and brown dwarfs, do not support a universal mass-rotation relation. The high precision of our observations and the high occurrence rates of variable low-surface-gravity objects encourage high-contrast time-resolved observations with the James Webb Space Telescope. © 2019. The American Astronomical Society. All rights reserved.
Rotational modulations are observed on brown dwarfs and directly imaged exoplanets, but the underlying mechanism is not well understood. Here we analyze Jupiter's rotational light curves at 12 wavelengths from the ultraviolet (UV) to the mid-infrared (mid-IR). The peak-to-peak amplitudes of Jupiter's light curves range from subpercent to 4% at most wavelengths, but the amplitude exceeds 20% at 5 μm, a wavelength sensing Jupiter's deep troposphere. Jupiter's rotational modulations are primarily caused by discrete patterns in the cloudless belts instead of the cloudy zones. The light-curve amplitude is controlled by the sizes and brightness contrasts of the Great Red Spot (GRS), expansions of the North Equatorial Belt (NEB), patchy clouds in the North Temperate Belt (NTB), and a train of hot spots in the NEB. In reflection, the contrast is controlled by upper tropospheric and stratospheric hazes, clouds, and chromophores in the clouds. In thermal emission, the small rotational variability is caused by the spatial distribution of temperature and opacities of gas and aerosols; the large variation is caused by the NH 3 cloud holes and thin-thick clouds. The methane-band light curves exhibit opposite-shape behavior compared with the UV and visible wavelengths, caused by a wavelength-dependent brightness change of the GRS. Light-curve evolution is induced by periodic events in the belts and longitudinal drifting of the GRS and patchy clouds in the NTB. This study suggests several interesting mechanisms related to distributions of temperature, gas, hazes, and clouds for understanding the observed rotational modulations on brown dwarfs and exoplanets. © 2019. The American Astronomical Society. All rights reserved.
We have used NSF's Karl G. Jansky Very Large Array (VLA) to observe a sample of five known radio-emitting late L and T dwarfs ranging in age from ~0.2 - 3.4Gyr. We observed each target for seven hours, extending to higher frequencies than previously attempted and establishing proportionally higher limits on maximum surface magnetic field strengths. Detections of circularly polarized pulses at 8 - 12~GHz yield measurements of 3.2 - 4.1 kG localized magnetic fields on four of our targets, including the archetypal cloud variable and likely planetary-mass object T2.5 dwarf SIMP J01365663+0933473. We additionally detect a pulse at 15 - 16.5 GHz for the T6.5 dwarf 2MASS 10475385+2124234, corresponding to a localized 5.6 kG field strength. For the same object, we tentatively detect a 16.5 - 18 GHz pulse, corresponding to a localized 6.2 kG field strength. We measure rotation periods between ~1.47 - 2.28 hr for 2MASS J10430758+2225236, 2MASS J12373919+6526148, and SDSS J04234858-0414035, supporting (i) an emerging consensus that rapid rotation may be important for producing strong dipole fields in convective dynamos and/or (ii) rapid rotation is a key ingredient for driving the current systems powering auroral radio emission. We observe evidence of variable structure in the frequency-dependent time series of our targets on timescales shorter than a rotation period, suggesting a higher degree of variability in the current systems near the surfaces of brown dwarfs. Finally, we find that age, mass, and temperature together cannot account for the strong magnetic fields produced by our targets.
We present simultaneous HST WFC3 + Spitzer IRAC variability monitoring for the highly-variable young (20 Myr) planetary-mass object PSO J318.5-22. Our simultaneous HST + Spitzer observations covered 2 rotation periods with Spitzer and most of a rotation period with HST. We derive a period of 8.60.1 hours from the Spitzer lightcurve. Combining this period with the measured for this object, we find an inclination of 56.2. We measure peak-to-trough variability amplitudes of 3.40.1 for Spitzer Channel 2 and 4.4 - 5.8 (typical 68 confidence errors of 0.3) in the near-IR bands (1.07-1.67 m) covered by the WFC3 G141 prism -- the mid-IR variability amplitude for PSO J318.5-22 one of the highest variability amplitudes measured in the mid-IR for any brown dwarf or planetary mass object. Additionally, we detect phase offsets ranging from 200--210 (typical error of 4) between synthesized near-IR lightcurves and the Spitzer mid-IR lightcurve, likely indicating depth-dependent longitudinal atmospheric structure in this atmosphere. The detection of similar variability amplitudes in wide spectral bands relative to absorption features suggests that the driver of the variability may be inhomogeneous clouds (perhaps a patchy haze layer over thick clouds), as opposed to hot spots or compositional inhomogeneities at the top-of-atmosphere level.
Beating bands in substellar atmospheres
Brown dwarfs are objects with masses that are between those of large planets and small stars. They share many features with gas giant planets, particularly conditions in their atmospheres. Apai et al. analyzed how the infrared brightness of three brown dwarfs changes over time. Several perplexing features can be explained if bands of clouds rotating within their atmospheres generate beat patterns. Such bands are seen in optical images of Jupiter but best match infrared images of Neptune. The results shed light on the atmospheric physics of brown dwarfs and gas giant planets around the Sun and other stars.
Science , this issue p. 683
In this paper we study the full sample of known Spitzer [3.6 μm] and J-band variable brown dwarfs. We calculate the rotational velocities, , of 16 variable brown dwarfs using archival Keck NIRSPEC data and compute the inclination angles of 19 variable brown dwarfs. The results obtained show that all objects in the sample with mid-IR variability detections are inclined at an angle , while all objects in the sample displaying J-band variability have an inclination angle . J-band variability appears to be more affected by inclination than Spitzer [3.6 μm] variability, and is strongly attenuated at lower inclinations. Since J-band observations probe deeper into the atmosphere than mid-IR observations, this effect may be due to the increased atmospheric path length of J-band flux at lower inclinations. We find a statistically significant correlation between the color anomaly and inclination of our sample, where field objects viewed equator-on appear redder than objects viewed at lower inclinations. Considering the full sample of known variable L, T, and Y spectral type objects in the literature, we find that the variability properties of the two bands display notably different trends that are due to both intrinsic differences between bands and the sensitivity of ground-based versus space-based searches. However, in both bands we find that variability amplitude may reach a maximum at ~7–9 hr periods. Finally, we find a strong correlation between color anomaly and variability amplitude for both the J-band and mid-IR variability detections, where redder objects display higher variability amplitudes.
We present five epochs of WFC3 HST Jupiter observations taken between 2009–2016 and extract global zonal wind profiles for each epoch. Jupiter’s zonal wind field is globally stable throughout these years, but significant variations in certain latitude regions persist. We find that the largest uncertainties in the wind field are due to vortices or hot-spots, and show residual maps which identify the strongest vortex flows. The strongest year-to-year variation in the zonal wind profiles is the 24°N jet peak. Numerous plume outbreaks have been observed in the Northern Temperate Belt and are associated with decreases in the zonal velocity and brightness. We show that the 24°N jet peak velocity and brightness decreased in 2012 and again in late 2016, following outbreaks during these years. Our February 2016 zonal wind profile was the last highly spatially resolved measurement prior to Junos first science observations. The final 2016 data were taken in conjunction with Juno’s perijove 3 pass on 11 December, 2016, and show the zonal wind profile following the plume outbreak at 24°N in October 2016.
The Lomb-Scargle periodogram is a well-known algorithm for detecting and characterizing periodic signals in unevenly-sampled data. This paper presents a conceptual introduction to the Lomb-Scargle periodogram and important practical considerations for its use. Rather than a rigorous mathematical treatment, the goal of this paper is to build intuition about what assumptions are implicit in the use of the Lomb-Scargle periodogram and related estimators of periodicity, so as to motivate important practical considerations required in its proper application and interpretation.
We present the first detection of photometric variability of a spectroscopically-confirmed Y dwarf. The Infrared Array Camera on board the Spitzer Space Telescope was used to obtain times series photometry at 3.6 and 4.5 microns over a twenty four hour period at two different epochs separated by 149 days. Variability is evident at 4.5 um in the first epoch and at 3.6 and 4.5 um in the second epoch which suggests that the underlying cause or causes of this variability change on the timescales of months. The second-epoch [3.6] and [4.5] light curves are nearly sinusoidal in form, in phase, have periods of roughly 8.5 hours, and have semi-amplitudes of 3.5%. We find that a simple geometric spot model with a single bright spot reproduces these observations well. We also compare our measured semi-amplitudes of the second epoch light curves to predictions of the static, one-dimensional, partly cloudy and hot spot models of Morley and collaborators and find that neither set of models can reproduce the observed [3.6] and[4.5] semi-amplitudes simultaneously. More advanced two- or three-dimensional models that include time-dependent phenomena like vertical mixing, cloud formation, and thermal relaxation are therefore sorely needed in order to properly interpret our observations.
We measure wind velocities on opposite sides of the hot Jupiter HD189733b
by modeling sodium absorption in high-resolution HARPS transmission spectra.
Our model implicitly accounts for the Rossiter-McLaughlin effect, which we show
can explain the high wind velocities suggested by previous studies. Our results
reveal a strong eastward motion of the atmosphere of HD189733b, with a
redshift of kms on the leading limb of the
planet and a blueshift of kms on the trailing
limb. These velocities can be understood as a combination of tidally locked
planetary rotation and an eastward equatorial jet; closely matching the
predictions of atmospheric circulation models. Our results show that the sodium
absorption of HD189733b is intrinsically velocity broadened and so previous
studies of the average transmission spectrum are likely to have overestimated
the role of pressure and thermal broadening.
We have observed 6 late-L and T dwarfs with the Karl G. Jansky Very Large
Array (VLA) to investigate the presence of highly circularly polarized radio
emission, associated with large-scale auroral currents. Previous surveys
encompassing ~60 L6 or later targets in this spectral range have yielded only
one detection. Our sample includes the previously detected T6.5 dwarf 2MASS
10475385+2124234 as well as 5 new targets selected for the presence of H-alpha
emission or optical/infrared photometric variability, which are possible
manifestations of auroral activity. We detect 2MASS 10475385+2124234, as well
as 4 of the 5 targets in our biased sample, including the strong IR variable
SIMP J01365662+0933473 and bright H-alpha emitter 2MASS 12373919+6526148,
reinforcing the possibility that activity at these disparate wavelengths is
related. The radio emission frequency corresponds to a precise determination of
the lower-bound magnetic field strength near the surface of each dwarf and this
new sample provides robust constraints on dynamo theory in the low mass brown
dwarf regime. Magnetic fields >2.5 kG are confirmed for 5/6 targets. Our
results provide tentative evidence that the dynamo operating in this mass
regime may be inconsistent with predicted values from a recently proposed
model. Further observations at higher radio frequencies are essential for
verifying this assertion.
We combine optical, near-infrared and mid-infrared spectra and photometry to
construct expanded spectral energy distributions (SEDs) for 145 field age
(\textgreater 500 Myr) and 53 young (lower age estimate \textless 500 Myr)
ultracool dwarfs (M6-T9). This range of spectral types includes very low mass
stars, brown dwarfs, and planetary mass objects, providing fundamental
parameters across both the hydrogen and deuterium burning minimum masses for
the largest sample assembled to date. A subsample of 29 objects have well
constrained ages as probable members of a nearby young moving group (NYMG). We
use 182 parallaxes and 16 kinematic distances to determine precise bolometric
luminosities () and radius estimates from evolutionary models
give semi-empirical effective temperatures () for the full range
of young and field age late-M, L and T dwarfs. We construct age-sensitive
relationships of luminosity, temperature and absolute magnitude as functions of
spectral type and absolute magnitude to disentangle the effects of degenerate
physical parameters such as , surface gravity, and clouds on
spectral morphology. We report bolometric corrections in J for both field age
and young objects and find differences of up to a magnitude for late-L dwarfs.
Our correction in Ks shows a larger dispersion but not necessarily a
different relationship for young and field age sequences. We also characterize
the NIR-MIR reddening of low gravity L dwarfs and identify a systematically
cooler of up to 300K from field age objects of the same spectral
type and 400K cooler from field age objects of the same magnitude.
The LOw Frequency ARray (LOFAR) will observe at 20-200 MHz. At those frequencies, large ionospheric phase variations considerably distort the observed brightness distribution. Fortunately, the image may be stabilized for long integrations by using bright radio sources in the sky. The downside is that LOFAR fields will be very crowded, which presents calibration challenges of its own. This is expecially true for the bright and extended sources that enter via the relatively high sidelobes of the LOFAR station beams. An extra complication is that these bearnshapes vary rather strongly in frequency and time. Altogether, LOFAR will require much more processing than existing radio telescopes, and has only just become possible with the new generation of computers. Even so, new processing techniques like 'peeling' had to be developed to speed things up by several orders of magnitude.
Context. Observations suggest that exoplanets such as HD 189733b form clouds in their atmospheres which have a strong feedback onto their thermodynamical and chemical structure, and overall appearance.
Aims. Inspired by mineral cloud modelling efforts for brown dwarf atmospheres, we present the first spatially varying kinetic cloud model structures for HD 189733b.
Methods. We apply a 2-model approach using results from a 3D global radiation-hydrodynamic simulation of the atmosphere as input for a detailed, kinetic cloud formation model. Sampling the 3D global atmosphere structure with 1D trajectories allows us to model the spatially varying cloud structure on HD 189733b. The resulting cloud properties enable the calculation of the scattering and absorption properties of the clouds.
Results. We present local and global cloud structure and property maps for HD 189733b. The calculated cloud properties show variations in composition, size and number density of cloud particles which are strongest between the dayside and nightside. Cloud particles are mainly composed of a mix of materials with silicates being the main component. Cloud properties, and hence the local gas composition, change dramatically where temperature inversions occur locally. The cloud opacity is dominated by absorption in the upper atmosphere and scattering at higher pressures in the model. The calculated 8 μ m single scattering albedo of the cloud particles are consistent with Spitzer bright regions. The cloud particles scattering properties suggest that they would sparkle/reflect a midnight blue colour at optical wavelengths.
Periodic radio bursts from very low mass stars and brown dwarfs
simultaneously probe their magnetic and rotational properties. The brown dwarf
2MASSI J1047539+212423 (2M 1047+21) is currently the only T dwarf (T6.5)
detected at radio wavelengths. Previous observations of this source with the
Arecibo observatory revealed intermittent, 100%-polarized radio pulses similar
to those detected from other brown dwarfs, but were unable to constrain a pulse
periodicity; previous VLA observations detected quiescent emission a factor of
~100 times fainter than the Arecibo pulses but no additional events. Here we
present 14 hours of Very Large Array observations of this object that reveal a
series of pulses at ~6 GHz with highly variable profiles, showing that the
pulsing behavior evolves on time scales that are both long and short compared
to the rotation period. We measure a periodicity of ~1.77 hr and identify it
with the rotation period. This is just the sixth rotation period measurement in
a late T dwarf, and the first obtained in the radio. We detect a pulse at 10
GHz as well, suggesting that the magnetic field strength of 2M 1047+21 reaches
at least 3.6 kG. Although this object is the coolest and most rapidly-rotating
radio-detected brown dwarf to date, its properties appear continuous with those
of other such objects, suggesting that the generation of strong magnetic fields
and radio emission may continue to even cooler objects. Further studies of this
kind will help to clarify the relationships between mass, age, rotation, and
magnetic activity at and beyond the end of the main sequence, where both
theories and observational data are currently scarce.
A video of a short talk discussing this work is at
http://youtu.be/OqCtFxcBB2s, and an experimental Web-native version of the
paper may be viewed at http://purl.org/net/pkgwpub/goreham.
We report the results of a J band search for cloud-related variability in
the atmospheres of 62 L4-T9 dwarfs using the Du Pont 2.5-m telescope at Las
Campanas Observatory and the Canada France Hawaii Telescope on Mauna Kea. We
find 9 of 57 objects included in our final analysis to be significantly
variable with >99% confidence, 5 of which are new discoveries. In our study,
strong variability (peak-to-peak amplitudes >2%) are confined to the L/T
transition (4/16 objects with L9-T3.5 spectral types and 0/41 objects for all
other spectral types). The probability that the observed occurrence rates for
strong variability inside and outside the L/T transition originate from the
same underlying true occurrence rate is excluded with >99.7% confidence. These
observations suggest that the settling of condensate clouds below the
photosphere in brown dwarf atmospheres does not occur in a spatially uniform
manner. Rather, the formation and sedimentation of dust grains at the L/T
transition is coupled to atmospheric dynamics, resulting in highly contrasting
regions of thick and thin clouds and/or clearings. Outside the L/T transition
we identify 5 weak variables (peak-to-peak amplitudes of 0.6%-1.6%). Excluding
L9-T3.5 spectral types, we infer that % of targets vary with
amplitudes of 0.5%1.6%, suggesting that surface heterogeneities are
ubiquitous among L and T dwarfs. Our survey establishes a significant link
between strong variability and L/T transition spectral types, providing
evidence in support of the hypothesis that cloud holes contribute to the abrupt
decline in condensate opacity and 1 micron brightening observed in this regime.
More generally, fractional cloud coverage is an important model parameter for
brown dwarfs and giant planets, especially those with L/T transition spectral
types and colors. [ABRIDGED]
We estimate the strength of the bandpass-integrated thermal emission from the extrasolar planet HD 209458b at 3.6, 4.5, 5.8, and 8.0 mum using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We observe a single secondary eclipse simultaneously in all four bandpasses and find relative eclipse depths of 0.00094+/-0.00009, 0.00213+/-0.00015, 0.00301+/-0.00043, and 0.00240+/-0.00026, respectively. These eclipse depths reveal that the shape of the inferred emission spectrum for the planet differs significantly from the predictions of standard atmosphere models; instead, the most plausible explanation would require the presence of an inversion layer high in the atmosphere leading to significant water emission in the 4.5 and 5.8 mum bandpasses. This is the first clear indication of such a temperature inversion in the atmosphere of a hot Jupiter, as previous observations of other planets appeared to be in reasonably good agreement with the predictions of models without such an inversion layer.
We describe the CASA (Common Astronomy Software Applications) package, its design and capabilities. CASA is a suite of applications for the reduction and analysis of radio astronomical data with a Python interface.
We present an updated analysis of Jupiter's equatorial meteorology from
Cassini observations. For two months preceding the spacecraft's closest
approach, the Imaging Science Subsystem (ISS) onboard regularly imaged the
atmosphere. We created time-lapse movies from this period in order to analyze
the dynamics of equatorial hot spots and their interactions with adjacent
latitudes. Hot spots are quasi-stable, rectangular dark areas on
visible-wavelength images, with defined eastern edges that sharply contrast
with surrounding clouds, but diffuse western edges serving as nebulous
boundaries with adjacent equatorial plumes. Hot spots exhibit significant
variations in size and shape over timescales of days and weeks. Some of these
changes correspond with passing vortex systems from adjacent latitudes
interacting with hot spots. Strong anticyclonic gyres present to the south and
southeast of the dark areas appear to circulate into hot spots. Impressive,
bright white plumes occupy spaces in between hot spots. Compact cirrus-like
'scooter' clouds flow rapidly through the plumes before disappearing within the
dark areas. These clouds travel at 150-200 m/s, much faster than the 100 m/s
hot spot and plume drift speed. This raises the possibility that the scooter
clouds may be more illustrative of the actual jet stream speed at these
latitudes. Most previously published zonal wind profiles represent the drift
speed of the hot spots at their latitude from pattern matching of the entire
longitudinal image strip. If a downward branch of an equatorially-trapped
Rossby waves controls the overall appearance of hot spots, however, the
westward phase velocity of the wave leads to underestimates of the true jet
stream speed.
Observations of mean pulse shapes and pulse energies are reported for 38
pulsars at 2.7 and 4.9 GHz and for 7 of these at 10.7 GHz. The
observational system and procedure are described and average pulse
profiles are presented for 35 pulsars. These observations are compared
with similar observations at lower frequencies. It is shown that the 3
dB width of pulse profiles decreases with increasing frequency in most
cases, while the component separations either increase or decrease and
may be characterized by a critical frequency correlated with the cutoff
frequency in the spectra of the corresponding pulsars.
Multi-frequency synthesis is the technique of using measurements at
several observing frequencies when forming an image in radio
interferometric aperture synthesis. The main advantage of this over the
traditional single frequency approach is improved u-v coverage, allowing
more rapid synthesis of the aperture. This paper considers several
issues involved in a practical multi-frequency synthesis experiment. In
particular, variation of source brightness distribution with frequency
results in artifacts in the resultant images. A new, CLEAN-based,
algorithm is presented which removes these artifacts during the
deconvolution stage of image formation. As a demonstration of the
technique we present an image of the double-lobed radio source
PKSB1733-565.
Radio detections of ultracool dwarfs provide insight into their magnetic fields and the dynamos that maintain them, especially at the very bottom of the main sequence, where other activity indicators dramatically weaken. Until recently, radio emission was only detected in the M and L dwarf regimes, but this has changed with the Arecibo detection of rapid polarized flares from the T6.5 dwarf 2MASS J10475385+2124234. Here, we report the detection of quasi-quiescent radio emission from this source at 5.8 GHz using the Karl G. Jansky Very Large Array. The spectral luminosity is L
ν = (2.2 ± 0.7) × 1012 erg s–1 Hz–1, a factor of ~100 times fainter than the Arecibo flares. Our detection is the lowest luminosity yet achieved for an ultracool dwarf. Although the emission is fully consistent with being steady, unpolarized, and broad band, we find tantalizing hints for variability. We exclude the presence of short-duration flares as seen by Arecibo, although this is not unexpected given estimates of the duty cycle. Follow-up observations of this object will offer the potential to constrain its rotation period, electron density, and the strength and configuration of the magnetic field. Equally important, follow-up observations will address the question of whether the electron cyclotron maser instability, which is thought to produce the flares seen by Arecibo, also operates in the very different parameter regime of the emission we detect, or whether instead this ultracool dwarf exhibits both maser and gyrosynchrotron radiation, potentially originating from substantially different locations.
A variety of observations provide evidence for vigorous motion in the
atmospheres of brown dwarfs and directly imaged giant planets. Motivated by
these observations, we examine the dynamical regime of the circulation in the
atmospheres and interiors of these objects. Brown dwarfs rotate rapidly, and
for plausible wind speeds, the flow at large scales will be rotationally
dominated. We present 3D, global, numerical simulations of convection in the
interior, which demonstrate that, at large scales, the convection aligns in the
direction parallel to the rotation axis. Convection occurs more efficiently at
high latitudes than low latitudes, leading to systematic equator-to-pole
temperature differences that may reach ~1 K near the top of the convection
zone. The interaction of convection with the overlying, stably stratified
atmosphere will generate a wealth of atmospheric waves, and we argue that, as
in the stratospheres of planets in the solar system, the interaction of these
waves with the mean flow will cause a significant atmospheric circulation at
regional to global scales. At large scales, this should consist of stratified
turbulence (possibly organizing into coherent structures such as vortices and
jets) and an accompanying overturning circulation. We present an approximate
analytic theory of this circulation, which predicts characteristic horizontal
temperature variations of several to ~50 K, horizontal wind speeds of ~10-300
m/sec, and vertical velocities that advect air over a scale height in
~10^5-10^6 sec. This vertical mixing may help to explain the chemical
disequilibrium observed on some brown dwarfs. Moreover, the implied large-scale
organization of temperature perturbations and vertical velocities suggests
that, near the L/T transition, patchy clouds can form near the photosphere,
helping to explain recent observations of brown-dwarf variability in the
near-IR.
Heterogeneous clouds or temperature perturbations in rotating brown dwarfs
produce variability in the observed flux. We report time-resolved simultaneous
observations of the variable T6.5 brown dwarf 2MASSJ22282889-431026 over the
wavelength ranges 1.1-1.7 microns and broadband 4.5 microns. Spectroscopic
observations were taken with Wide Field Camera 3 on board the Hubble Space
Telescope and photometry with the Spitzer Space Telescope. The object shows
sinusoidal infrared variability with a period of 1.4 hr at most wavelengths
with peak-to-peak amplitudes between 1.45% and 5.3% of the mean flux. While the
light curve shapes are similar at all wavelengths, their phases differ from
wavelength to wavelength with a maximum difference of more than half of a
rotational period. We compare the spectra with atmospheric models of different
cloud prescriptions, from which we determine the pressure levels probed at
different wavelengths. We find that the phase lag increases with decreasing
pressure level, or higher altitude. We discuss a number of plausible scenarios
that could cause this trend of light curve phase with probed pressure level.
These observations are the first to probe heterogeneity in an ultracool
atmosphere in both horizontal and vertical directions, and thus are an ideal
test case for realistic three dimensional simulations of the atmospheric
structure with clouds in brown dwarfs and extrasolar planets.
Five years of Cassini Imaging Science Subsystem images, from 2004 to 2009, are analyzed in this work to retrieve global zonal wind profiles of Saturn's northern and southern hemispheres in the methane absorbing bands at 890 and 727 nm and in their respective adjacent continuum wavelengths of 939 and 752 nm. A complete view of Saturn's global circulation, including the equator, at two pressure levels, in the tropopause (60 mbar to 250 mbar with the MT filters) and in the upper troposphere (from similar to 350 mbar to similar to 500 mbar with the CB filter set), is presented. Both zonal wind profiles (available at the Supplementary Material Section), show the same structure but with significant differences in the peak of the eastward jets and the equatorial region, including a region of positive vertical shear symmetrically located around the equator between the 10 degrees < vertical bar phi(c)vertical bar < 25 degrees where zonal velocities close to the tropopause are higher than at 500 mbar. A comparison of previously published zonal wind sets obtained by Voyager 1 and 2(1980-1981), Hubble Space Telescope, and ground-based telescopes (1990-2004) with the present Cassini profiles (2004-2009) covering a full Saturn year shows that the shape of the zonal wind profile and intensity of the jets has remained almost unchanged except at the equator, despite the seasonal insolation cycle and the variability of Saturn's emitted power. The major wind changes occurred at equatorial latitudes, perhaps following the Great White Spot eruption in 1990. It is not evident from our study if the seasonal insolation cycle and its associated ring shadowing influence the equatorial circulation at cloud level. (c) 2011 Elsevier Inc. All rights reserved.
We describe and compare several post-correlation radio frequency interference (RFI) classification methods. As data sizes of observations grow with new and improved telescopes, the need for completely automated, robust methods for RFI mitigation is pressing. We investigated several classification methods and find that, for the data sets we used, the most accurate among them is the SumThreshold method. This is a new method formed from a combination of existing techniques, including a new way of thresholding. This iterative method estimates the astronomical signal by carrying out a surface fit in the time-frequency plane. With a theoretical accuracy of 95 per cent recognition and an approximately 0.1 per cent false probability rate in simple simulated cases, the method is in practice as good as the human eye in finding RFI. In addition, it is fast, robust, does not need a data model before it can be executed and works in almost all configurations with its default parameters. The method has been compared using simulated data with several other mitigation techniques, including one based upon the singular value decomposition of the time-frequency matrix, and has shown better results than the rest.
The statistical properties of least-squares frequency analysis of unequally spaced data are examined. It is shown that, in the least-squares spectrum of gaussian noise, the reduction in the sum of squares at a particular frequency is aX
2
2
variable. The reductions at different frequencies are not independent, as there is a correlation between the height of the spectrum at any two frequencies,f
1 andf
2, which is equal to the mean height of the spectrum due to a sinusoidal signal of frequencyf
1, at the frequencyf
2. These correlations reduce the distortion in the spectrum of a signal affected by noise. Some numerical illustrations of the properties of least-squares frequency spectra are also given.