A. Burrows

Princeton University, Princeton, New Jersey, United States

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Publications (127)454.68 Total impact

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    ABSTRACT: Unlike hot Jupiters or other gas giants, super-Earths are expected to have a wide variety of compositions, ranging from terrestrial bodies like our own to more gaseous planets like Neptune. Observations of transiting systems, which allow us to directly measure planet masses and radii and constrain atmospheric properties, are key to understanding the compositional diversity of the planets in this mass range. Although Kepler has discovered hundreds of transiting super-Earth candidates over the past four years, the majority of these planets orbit stars that are too far away and too faint to allow for detailed atmospheric characterization and reliable mass estimates. Ground-based transit surveys focus on much brighter stars, but most lack the sensitivity to detect planets in this size range. One way to get around the difficulty of finding these smaller planets in transit is to start by choosing targets that are already known to contain super-Earth sized bodies detected using the radial velocity technique. Here we present results from a Spitzer program to observe six of the most favorable RV detected super-Earth systems, including HD 1461, HD 7924, HD 156668, HIP 57274, and GJ 876. We find no evidence for transits in any of their 4.5 micron flux light curves, and place limits on the allowed transit depths and corresponding planet radii that rule out even the most dense and iron-rich compositions for these objects. We also observed HD 97658, but the observation window was based on a possible ground-based transit detection (Henry et al. 2011) that was later ruled out; thus the window did not include the predicted time for the transit detection recently made by MOST (Dragomir et al. 2013).
    10/2013; 781(2).
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    ABSTRACT: The extrasolar planet HD 149026b, discovered in 2005, was among the first to be observed to transit its host star as seen from earth. Since its discovery, several observational campaigns have targeted HD 149026b in wavelengths from the visible to the infrared to obtain both transmission and emission broadband measurements. These measurements have revealed that HD 149026b has a radius similar to that of Saturn, but a density more akin to that of Neptune. This suggest that HD 149026b is enriched in heavy elements much like the ice giants of our solar system. Half-orbit phase curve observations of HD 149026b at 8 microns (Knutson et al., 2009) suggest that the day-to-night transport of heat is fairly efficient for this planet. However, further phase curve observations at other infrared wavelengths are needed to better constrain the planet’s energy budget, location of hot and cold regions, as well as possible chemical gradients in the planet’s atmosphere. Here we present an analysis of the full-orbit phase-curves of HD 149026b at 3.6 and 4.5 microns. We discuss the implications of the combined phase-curve information at 3.6, 4.5, and 8 microns and compare the observations to theoretical phase curves derived from three-dimensional atmospheric models that consider a range of possible heavy element enrichments in HD 149026b’s atmosphere.
    10/2013;
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    ABSTRACT: Several exoplanets have recently been imaged at wide separations of >10 AU from their parent stars. These span a limited range of ages (<50 Myr) and atmospheric properties, with temperatures of 800--1800 K and very red colors (J - H > 0.5 mag), implying thick cloud covers. Furthermore, substantial model uncertainties exist at these young ages due to the unknown initial conditions at formation, which can lead to an order of magnitude of uncertainty in the modeled planet mass. Here, we report the direct imaging discovery of a Jovian exoplanet around the Sun-like star GJ 504, detected as part of the SEEDS survey. The system is older than all other known directly-imaged planets; as a result, its estimated mass remains in the planetary regime independent of uncertainties related to choices of initial conditions in the exoplanet modeling. Using the most common exoplanet cooling model, and given the system age of 160 [+350, -60] Myr, GJ 504 b has an estimated mass of 4 [+4.5, -1.0] Jupiter masses, among the lowest of directly imaged planets. Its projected separation of 43.5 AU exceeds the typical outer boundary of ~30 AU predicted for the core accretion mechanism. GJ 504 b is also significantly cooler (510 [+30, -20] K) and has a bluer color (J-H = -0.23 mag) than previously imaged exoplanets, suggesting a largely cloud-free atmosphere accessible to spectroscopic characterization. Thus, it has the potential of providing novel insights into the origins of giant planets, as well as their atmospheric properties.
    The Astrophysical Journal 07/2013; 774(1). · 6.73 Impact Factor
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    ABSTRACT: We present a formulation for multigroup radiation hydrodynamics that is correct to order $O(v/c)$ using the comoving-frame approach and the flux-limited diffusion approximation. We describe a numerical algorithm for solving the system, implemented in the compressible astrophysics code, CASTRO. CASTRO uses an Eulerian grid with block-structured adaptive mesh refinement based on a nested hierarchy of logically-rectangular variable-sized grids with simultaneous refinement in both space and time. In our multigroup radiation solver, the system is split into three parts, one part that couples the radiation and fluid in a hyperbolic subsystem, another part that advects the radiation in frequency space, and a parabolic part that evolves radiation diffusion and source-sink terms. The hyperbolic subsystem and the frequency space advection are solved explicitly with high-order Godunov schemes, whereas the parabolic part is solved implicitly with a first-order backward Euler method. Our multigroup radiation solver works for both neutrino and photon radiation.
    The Astrophysical Journal Supplement Series 07/2012; 204(1). · 16.24 Impact Factor
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    ABSTRACT: We present results from a new set of 3D general-relativistic hydrodynamic simulations of rotating iron core collapse. We assume octant symmetry and focus on axisymmetric collapse, bounce, the early postbounce evolution, and the associated gravitational wave (GW) and neutrino signals. We employ a finite-temperature nuclear equation of state, parameterized electron capture in the collapse phase, and a multi-species neutrino leakage scheme after bounce. The latter captures the important effects of deleptonization, neutrino cooling and heating and enables approximate predictions for the neutrino luminosities in the early evolution after core bounce. We consider 12-solar-mass and 40-solar-mass presupernova models and systematically study the effects of (i) rotation, (ii) progenitor structure, and (iii) postbounce neutrino leakage on dynamics, GW, and, neutrino signals. We demonstrate, that the GW signal of rapidly rotating core collapse is practically independent of progenitor mass and precollapse structure. Moreover, we show that the effects of neutrino leakage on the GW signal are strong only in nonrotating or slowly rotating models in which GW emission is not dominated by inner core dynamics. In rapidly rotating cores, core bounce of the centrifugally-deformed inner core excites the fundamental quadrupole pulsation mode of the nascent protoneutron star. The ensuing global oscillations (f~700-800 Hz) lead to pronounced oscillations in the GW signal and correlated strong variations in the rising luminosities of antineutrino and heavy-lepton neutrinos. We find these features in cores that collapse to protoneutron stars with spin periods <~ 2.5 ms and rotational energies sufficient to drive hyper-energetic core-collapse supernova explosions. Hence, joint GW + neutrino observations of a core collapse event could deliver strong evidence for or against rapid core rotation. [abridged]
    Physical review D: Particles and fields 04/2012; 86(2).
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    ABSTRACT: Infrared transmission spectroscopy of the exoplanets HD189733b and XO-1 has been previously reported by Swain et al. and Tinetti et al. based on observations using the NICMOS instrument on the Hubble Space Telescope. The robustness of those results has been questioned, because derivation of the exoplanetary spectrum required decorrelating strong instrumental systematic effects in the NICMOS data. We here discuss results from HST/WFC3 grism 1.1-1.7 micron spectroscopy of these planets during transit. WFC3 instrumental signatures are smaller in both amplitude and complexity as compared to NICMOS. Moreover, we use a new spatial scan mode to trail the stars perpendicular to the dispersion direction during WFC3 exposures, and this increases the efficiency of the observations and reduces persistence effects in the detector. We derive the 1.4-micron water absorption spectrum of these planets during transit, discuss implications for these exoplanetary atmospheres, and compare our results to the NICMOS spectroscopy.
    01/2012;
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    ABSTRACT: Here we present HST eclipse spectroscopy spanning 1.1 to 1.7 μm of the CoRoT-2 system using the G141 grism on WFC3. These near-infrared data serve to complement the pre-existing and already-published optical CoRoT data and warm Spitzer infrared observations. CoRoT-2b, the sole planet known in the system, is a member of the Very Hot Jupiter (VHJ) class of exoplanets, and the secondary eclipse was measured with three separate HST visits. We find the albedo of CoRoT-2b upon comparison of optical data, which comprises both thermal and reflected spectral components, and the thermal spectrum as constrained by the Spitzer and WFC3 infrared data. Analysis of the data required characterization of the persistence on the WFC3 detector as it manifests for these observations; it is not insignificant. After compiling results of flux patterns for the majority of the seventeen exoplanets studied in this HST program, we find the extent of the persistence, a linear, additive effect that is strongly dependent upon stimulating flux and time of/since exposure, and subtract it to leave only the true flux received from the CoRoT-2 system in and out of secondary eclipse.
    01/2012;
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    ABSTRACT: We present new direct imaging results on the planet and debris disk surrounding 12 Myr-old beta Pictoris. Using an updated version of our reduction pipeline, we extract a new detection of beta Pic b from 2008 VLT/NaCo data at a sub-Jupiter projected separation ( 4 AU), about 1.5 lambda/D. We also obtain a high signal-to-noise rereduction of L' data taken in 2009 December. Intriguingly, the planet's orbit is aligned with the major axis of the outer disk (Omega 31 deg) but is probably misaligned with the warp/inclined disk at 80 AU, often cited as a signpost for the planet's existence. We also present new images of the beta Pic debris disk and discuss any evidence for a 2nd massive gas giant planet in the system.
    01/2012;
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    ABSTRACT: We present results from a suite of axisymmetric, core-collapse supernova simulations in which hydrodynamic recoil from an asymmetric explosion produces large proto-neutron star (PNS) velocities. We use the adaptive-mesh refinement code CASTRO to self-consistently follow core-collapse, the formation of the PNS and its subsequent acceleration. We obtain recoil velocities of up to 620 km/s at ~1 s after bounce. These velocities are consistent with the observed distribution of pulsar kicks and with PNS velocities obtained in other theoretical calculations. Our PNSs are still accelerating at several hundred km/s at the end of our calculations, suggesting that even the highest velocity pulsars may be explained by hydrodynamic recoil in generic, core-collapse supernovae.
    Monthly Notices of the Royal Astronomical Society 12/2011; 423(2). · 5.52 Impact Factor
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    J. Budaj, I. Hubeny, A. Burrows
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    ABSTRACT: The internal heat loss, or cooling, of a planet determines its structure and evolution. We study the effects of irradiation, metallicity of the atmosphere, heat redistribution, stratospheres, and the depth where the heat redistribution takes place on the atmospheric structure, the core entropy, and subsequently on the cooling of the interior of the planet. We address in a consistent fashion the coupling between the day and the night sides of a planet by means of model atmosphere calculations with heat redistribution. We assume that strong convection leads to the same entropy on the day and night sides and that gravity is the same on both hemispheres. We argue that the core cooling rates from the two hemispheres of a strongly irradiated planet may not be the same and that the difference depends on several important parameters. If the day-night heat redistribution is very efficient or if it takes place at the large optical depth, then the day-side and the night-side cooling may be comparable. However, if the day-night heat transport is not efficient or if it takes place at a shallow optical depth then there can be a large difference between the day- and the night-side cooling and the night side will cool more efficiently. If stellar irradiation becomes stronger, e.g. owing to stellar evolution or migration, cooling from both the day and the night sides is reduced. Enhanced metallicity of the atmosphere would act as an added "blanket" and reduces both the day- and the night-side cooling. However, a stratosphere on the planetary day side can enhance day-side cooling since its opacity acts as a "sunshade" that screens the stellar irradiation. These effects may also influence the well-known gravity darkening and bolometric albedo effects in interacting binaries, especially for strongly irradiated cold components.
    Astronomy and Astrophysics 11/2011; · 5.08 Impact Factor
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    ABSTRACT: Spitzer's extended warm mission gives us the opportunity to perform comparative studies of exoplanets' atmospheres. We describe the techniques and methods involved in our recently accepted Spitzer Exploration Science Proposal (Proposal ID 80016; PI: J. Krick; 619 hours) to obtain high-precision 4.5 micron phase curves for 22 transiting hot Jupiter systems, along with observations of secondary eclipses of 7 of these systems. The principal goal of this study, which will quadruple the number of phase curve observations to date, is to map longitudinal temperature distribution of the planetary atmospheres and to assess the following questions: (1) What is the contrast between exoplanetary day- and nightside temperatures, i. e., how efficiently is the incident energy redistributed? (2) Are the weather phenomena in the exoplanetary atmospheres stable over long periods of time? (3) How do the temperature distributions on the planetary surfaces correlate with astrophysical properties of the star-planet systems? To answer these questions with our proposed Spitzer observations, we will employ a novel observing technique involving snapshot observing to reduce telescope time requirements, and PCRS peak-up to IRAC to increase the pointing accuracy and thus minimize the photometric error due to intrapixel sensitivity variation.
    AAS/Division for Extreme Solar Systems Abstracts; 09/2011
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    ABSTRACT: We present transit and eclipse spectroscopy of Very Hot Jupiter atmospheres using the newly installed Wide Field Camera 3 (WFC3) spectrograph on the Hubble Space Telescope (HST). We include results from several hot Jupiter planets, but we focus particularly on spectra of WASP-4b in both transmission and emission, the first time this has been achieved using this instrument. These data are already in hand. Our preliminary analysis indicates they do not require substantial decorrelation against external measurements to correct for systematic errors, resulting in robust results that avoid the ambiguities in interpretation faced by earlier work with NICMOS. Recent work by Madhusudhan and Seager highlighted degeneracies in the interpretation of low-resolution Spitzer spectra of hot Jupiters. The G141 grism on WFC3 (1.1-1.7 microns) spans a 1.4 micron water feature, allowing us to constrain water vapor abundance and break this degeneracy. We measure the 1.5 micron continuum flux, which in conjunction with the Spitzer data allows us to constrain the thermal spectrum of the planet and obtain a more precise energy budget relevant to understanding WASP-4b’s abnormally large radius. This work is part of the Cycle-18 Large Program 12181, and we acknowledge support by NASA, NSF GRFP, and the HST Project.
    09/2011;
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    ABSTRACT: The short-period planet WASP-12b is among the hottest known transiting planets. Space- and ground-based secondary eclipse depths imply that this planet has a C/O ratio greater than 1 (Madhusudhan et al. 2011), in stark contrast to the chemistry in the Solar System and the assumed chemistry of other planets. These same eclipse data put the planet's day-side effective temperature at 3000 K. This indicates a low albedo and poor recirculation of heat to the night-side, as has been found for all of the hottest transiting giant planets (Cowan & Agol 2011b). But these trends were based solely on day-side observations (eclipse depths) rather than full phase variations, which directly probe night-side temperature. The short period (1.1 day) and inflated radius (1.8 RJ) of WASP-12b has led to speculation that it may be undergoing Roche-lobe overflow (Li et al. 2010, Lai et al. 2010), and UV observations by Fossati et al. (2010) seem to support this idea. We have recently obtained thermal phase curves of this planet with Warm Spitzer (PI:Machalek; PID 70060). Our data include two eclipses, a transit, and full phase coverage at each of 3.6 and 4.5 micron. Because of the planet's high temperature and large size, this is one of the highest S/N phase curves yet obtained with Spitzer. These data (currently being analyzed) will allow us to directly measure the planet's night-side temperature and the longitudinal offset of its day-side hot-spot. Since the 3.6 and 4.5 micron bands probe different depths in the atmosphere, we will strongly constrain climate models for the hottest gas giants. The high precision transit and eclipse photometry offered by Spitzer will also allow us to search for signs of accretion in this system.
    AAS/Division for Extreme Solar Systems Abstracts; 09/2011
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    ABSTRACT: The atmospheres of close-in extrasolar planets experience strong, asymmetrically distributed radiative forcing that can potentially lead to dramatic variations in both temperature and composition between the day- and night-side hemispheres. However, secondary eclipse observations only tell us about the properties of the dayside atmosphere, while transmission spectroscopy probes the region around the day-night terminator. By measuring changes in the infrared emission spectra of these planets as a function of orbital phase, we can resolve thermal and compositional gradients in these atmospheres, allowing us to obtain a complete picture of their local properties. The most extensively studied planet to date, HD 189733b, appears to have a relatively modest day-night temperature gradient as seen in the 8 and 24 micron Spitzer bands, suggesting that compositional gradients in this atmosphere are likely to be minimal. We present new, full-orbit phase curves at 3.6 and 4.5 um obtained with warm Spitzer, which we use to construct improved multi-color maps and to constrain variations in the pressure-temperature profile and atmospheric composition as a function of longitude. We also present preliminary results for complementary full-orbit observations of HAT-P-7b in the same bands, and discuss an emerging pattern in which the most highly irradiated (>2000 K) planets appear to undergo a shift towards large day-night temperature gradients, perhaps due to Lorentz braking or other MHD processes.
    American Astronomical Society, ESS meeting #2, #12.01. 09/2011; 2:1201.
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    ABSTRACT: The Spitzer warm mission has already greatly expanded the field of exoplanet characterization with over 3000 hours of time dedicated to exoplanet observations. Observations of eclipsing systems with Spitzer are at the heart of these advances, as they allow us to move beyond simple mass and period estimates to determine planetary radius, dayside emission, and emission variations as a function of orbital phase. The eclipsing system HAT-P-2 is of special interest because the massive Jovian sized planet in this system is on a highly eccentric orbit (e=0.5171). Because HAT-P-2b's orbit is eccentric, the planet is subject to time variable heating and probable non-synchronous rotation. Circulation patterns that we expect to develop in HAT-P-2b's atmosphere will likely vary with both planetary local time and orbital phase. Here we present an analysis of two full-orbit light curves for the HAT-P-2 system obtained at 3.6 and 4.5 microns during the first two years of the Spitzer warm mission and discuss the observational constraints imposed on the atmospheric circulation of HAT-P-2b. Additionally, three-dimensional atmospheric models that incorporate realistic radiative transfer will be presented to further elucidate possible global scale circulations patterns present in the atmosphere of HAT-P-2b. Support for this work was provided by NASA.
    09/2011;
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    ABSTRACT: We describe the development of a flux-limited gray radiation solver for the compressible astrophysics code, CASTRO. CASTRO uses an Eulerian grid with block-structured adaptive mesh refinement based on a nested hierarchy of logically-rectangular variable-sized grids with simultaneous refinement in both space and time. The gray radiation solver is based on a mixed-frame formulation of radiation hydrodynamics. In our approach, the system is split into two parts, one part that couples the radiation and fluid in a hyperbolic subsystem, and another parabolic part that evolves radiation diffusion and source-sink terms. The hyperbolic subsystem is solved explicitly with a high-order Godunov scheme, whereas the parabolic part is solved implicitly with a first-order backward Euler method.
    The Astrophysical Journal Supplement Series 05/2011; 196(2). · 16.24 Impact Factor
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    ABSTRACT: Spitzer's extended warm mission gives us the opportunity to contribute to its legacy by performing comparative science on atmospheres of extrasolar planets. The goal of this proposal is to obtain high quality 4.5 micron phase curves for 22 transiting hot Jupiter systems, which represent a complete sample of systems that can be studied with Spitzer in a reasonable amount of time. The resulting dataset will not only quadruple the number of phase curve observations to date, but also populate gaps in parameter space explored by current phase curve studies. The combination of our phase curves with well-known literature ephemerides and observations of secondary eclipses will produce maps of the longitudinal brightness/temperature distributions in the planetary atmospheres. These maps can be used to calculate energy redistribution efficiencies between the hot dayside and cooler nightside -- exoplanetary weather. Our observations focus on the following three questions: (1) What is the contrast between exoplanetary day- and nightside temperatures, i.e., how efficiently is the incident energy redistributed? (2) Are the weather phenomena in the exoplanetary atmospheres stable over long periods of time? (3) How do the temperature distributions on the planetary surfaces correlate with astrophysical properties of the star-planet systems? To answer these questions with our proposed Spitzer observations, we will employ a novel observing technique using snapshot observing to reduce telescope time requirements, and PCRS peak-up to IRAC to increase the pointing accuracy and thus minimize the photometric error due to intrapixel sensitivity variation. The analysis of this comprehensive data set will elevate the study of phase curves to the level of comparative atmospheric studies outside the solar system.
    Spitzer Proposal. 05/2011;
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    ABSTRACT: The Spitzer warm mission has already greatly expanded the field of exoplanet characterization with over 3000 hours of time dedicated to exoplanet observations. Observations of eclipsing systems with Spitzer are at the heart of these advances, as they allow us to move beyond simple mass and period estimates to determine planetary radius, dayside emission, and emission variations as a function of orbital phase. The eclipsing system HAT-P-2 is of special interest because the massive Jovian sized planet in this system is on a highly eccentric orbit (e=0.5171). Because HAT-P-2b's orbit is eccentric, the planet is subject to time variable heating and probable non-synchronous rotation. Circulation patterns that we expect to develop in HAT-P-2b's atmosphere will likely vary with both planetary local time and orbital phase. Here we present an analysis of a full orbit light curve from the HAT-P-2 system obtained during the most recent cycle of the Spitzer warm mission and discuss the constraints it imposes on the atmospheric circulation of HAT-P-2b. Support for this work was provided by NASA.
    05/2011;
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    ABSTRACT: We perform 3+1 general relativistic simulations of rotating core collapse in the context of the collapsar model for long gamma-ray bursts. We employ a realistic progenitor, rotation based on results of stellar evolution calculations, and a simplified equation of state. Our simulations track self-consistently collapse, bounce, the postbounce phase, black hole formation, and the subsequent early hyperaccretion phase. We extract gravitational waves from the spacetime curvature and identify a unique gravitational wave signature associated with the early phase of collapsar formation.
    Physical Review Letters 04/2011; 106(16):161103. · 7.73 Impact Factor
  • David S. Spiegel, A. Burrows
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    ABSTRACT: We review recent results on the spectra of giant planets, including calculations specific to objects in the Kepler field (HAT-P-7b and TrES-2), and calculations of emergent radiation and transit spectra associated with general circulation models. We also present recent results on the potential habitability of terrestrial planets on eccentric orbits.
    01/2011;

Publication Stats

4k Citations
454.68 Total Impact Points

Institutions

  • 2008–2013
    • Princeton University
      • Department of Astrophysical Sciences
      Princeton, New Jersey, United States
  • 2011
    • California Institute of Technology
      Pasadena, California, United States
  • 2010
    • TRI/Princeton
      Princeton, New Jersey, United States
  • 1995–2009
    • The University of Arizona
      • • Department of Astronomy
      • • Department of Planetary Sciences
      Tucson, Arizona, United States
  • 1997
    • University of Reading
      • Department of Meteorology
      Reading, England, United Kingdom
  • 1996
    • Vanderbilt University
      • Department of Physics and Astronomy
      Nashville, Michigan, United States
    • New Mexico State University
      • Department of Astronomy
      Las Cruces, NM, United States