P. R. McCullough

Princeton University, Princeton, New Jersey, United States

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Publications (46)146.26 Total impact

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    ABSTRACT: Recent results from the Kepler mission indicate that super-Earths (planets with masses between 1-10 times that of the Earth) are the most common kind of planet around nearby Sun-like stars. These planets have no direct solar system analogue, and are currently one of the least well-understood classes of extrasolar planets. Many super-Earths have average densities that are consistent with a broad range of bulk compositions, including both water-dominated worlds and rocky planets covered by a thick hydrogen and helium atmosphere. Measurements of the transmission spectra of these planets offer the opportunity to resolve this degeneracy by directly constraining the scale heights and corresponding mean molecular weights of their atmospheres. We present Hubble Space Telescope near-infrared spectroscopy of two transits of the newly discovered transiting super-Earth HD 97658b. We use the Wide Field Camera 3's scanning mode to measure the wavelength-dependent transit depth in thirty individual bandpasses. Our averaged differential transmission spectrum has a median 1 sigma uncertainty of 19 ppm in individual bins, making this the most precise observation of an exoplanetary transmission spectrum obtained with WFC3 to date. Our data are inconsistent with a cloud-free solar metallicity atmosphere at the 17 sigma level. They are a good match for flat models corresponding to either a metal-rich atmosphere or a solar metallicity atmosphere with a cloud or haze layer located at pressures of a mbar or higher.
    03/2014;
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    ABSTRACT: We have used Hubble/WFC3 and the G141 grism to measure the secondary eclipse of the transiting very hot Jupiter CoRoT-2b in the 1.1-1.7$\mu$m spectral region. We find an eclipse depth averaged over this band equal to $395^{+69}_{-45}$ parts per million, equivalent to a blackbody temperature of $1788\pm18$K. We study and characterize several WFC3 instrumental effects, especially the "hook" phenomenon described by Deming et al. (2013). We use data from several transiting exoplanet systems to find a quantitative relation between the amplitude of the hook and the exposure level of a given pixel. Although the uncertainties in this relation are too large to allow us to develop an empirical correction for our data, our study provides a useful guide for optimizing exposure levels in future WFC3 observations. We derive the planet's spectrum using a differential method. The planet-to-star contrast increases to longer wavelength within the WFC3 bandpass, but without water absorption or emission to a $3\sigma$ limit of 85 ppm. The slope of the WFC3 spectrum is significantly less than the slope of the best-fit blackbody. We compare all existing eclipse data for this planet to a blackbody spectrum, and to spectra from both solar abundance and carbon-rich (C/O=1) models. A blackbody spectrum is an acceptable fit to the full dataset. Extra continuous opacity due to clouds or haze, and flattened temperature profiles, are strong candidates to produce quasi-blackbody spectra, and to account for the amplitude of the optical eclipses. Our results show ambiguous evidence for a temperature inversion in this planet.
    The Astrophysical Journal 01/2014; 783(2). · 6.73 Impact Factor
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    ABSTRACT: Exoplanetary transmission spectroscopy in the near-infrared using the Hubble Space Telescope (HST) NICMOS is currently ambiguous because different observational groups claim different results from the same data, depending on their analysis methodologies. Spatial scanning with HST/WFC3 provides an opportunity to resolve this ambiguity. We here report WFC3 spectroscopy of the giant planets HD 209458b and XO-1b in transit, using spatial scanning mode for maximum photon-collecting efficiency. We introduce an analysis technique that derives the exoplanetary transmission spectrum without the necessity of explicitly decorrelating instrumental effects, and achieves nearly photon-limited precision even at the high flux levels collected in spatial scan mode. Our errors are within 6% (XO-1) and 26% (HD 209458b) of the photon-limit at a resolving power of λ/δλ ~ 70, and are better than 0.01% per spectral channel. Both planets exhibit water absorption of approximately 200 ppm at the water peak near 1.38 μm. Our result for XO-1b contradicts the much larger absorption derived from NICMOS spectroscopy. The weak water absorption we measure for HD 209458b is reminiscent of the weakness of sodium absorption in the first transmission spectroscopy of an exoplanet atmosphere by Charbonneau et al. Model atmospheres having uniformly distributed extra opacity of 0.012 cm2 g–1 account approximately for both our water measurement and the sodium absorption. Our results for HD 209458b support the picture advocated by Pont et al. in which weak molecular absorptions are superposed on a transmission spectrum that is dominated by continuous opacity due to haze and/or dust. However, the extra opacity needed for HD 209458b is grayer than for HD 189733b, with a weaker Rayleigh component.
    The Astrophysical Journal 08/2013; 774(2):95. · 6.73 Impact Factor
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    ABSTRACT: Infrared Spectroscopy of the Exoplanets CoRoT-2b, XO-1b, HD 209458b Using the Wide-Field Camera-3 On the Hubble Space Telescope
    04/2013;
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    ABSTRACT: Exoplanetary transmission spectroscopy in the near-infrared using Hubble/NICMOS is currently ambiguous because different observational groups claim different results from the same data, depending on their analysis methodologies. Spatial scanning with Hubble/WFC3 provides an opportunity to resolve this ambiguity. We here report WFC3 spectroscopy of the giant planets HD209458b and XO-1b in transit, using spatial scanning mode for maximum photon-collecting efficiency. We introduce an analysis technique that derives the exoplanetary transmission spectrum without the necessity of explicitly decorrelating instrumental effects, and achieves nearly photon-limited precision even at the high flux levels collected in spatial scan mode. Our errors are within 6-percent (XO-1) and 26-percent (HD209458b) of the photon-limit at a spectral resolving power of 70, and are better than 0.01-percent per spectral channel. Both planets exhibit water absorption of approximately 200 ppm at the water peak near 1.38 microns. Our result for XO-1b contradicts the much larger absorption derived from NICMOS spectroscopy. The weak water absorption we measure for HD209458b is reminiscent of the weakness of sodium absorption in the first transmission spectroscopy of an exoplanet atmosphere by Charbonneau et al. (2002). Model atmospheres having uniformly-distributed extra opacity of 0.012 cm^2 per gram account approximately for both our water measurement and the sodium absorption in this planet. Our results for HD209458b support the picture advocated by Pont et al. (2013) in which weak molecular absorptions are superposed on a transmission spectrum that is dominated by continuous opacity due to haze and/or dust. However, the extra opacity needed for HD209458b is grayer than for HD189733b, with a weaker Rayleigh component.
    02/2013;
  • N. Crouzet, P. R. McCullough, C. Burke, D. Long
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    ABSTRACT: Spectroscopy during planetary transits is a powerful tool to probe exoplanet atmospheres. We present the near-infrared transit spectroscopy of XO-2b obtained with Hubble Space Telescope NICMOS. Uniquely for NICMOS transit spectroscopy, a companion star of similar properties to XO-2 is present in the field of view. We derive improved star and planet parameters through a photometric white-light analysis. We show a clear correlation of the spectrum noise with instrumental parameters, in particular the angle of the spectral trace on the detector. An MCMC method using a decorrelation from instrumental parameters is used to extract the planetary spectrum. Spectra derived independently from each of the three visits have an rms of 430, 510, and 1000 ppm, respectively. The same analysis is performed on the companion star after numerical injection of a transit with a depth constant at all wavelengths. The extracted spectra exhibit residuals of similar amplitude as for XO-2, which represent the level of remaining NICMOS systematics. This shows that extracting planetary spectra is at the limit of NICMOS's capability. We derive a spectrum for the planet XO-2b using the companion star as a reference. The derived spectrum can be represented by a theoretical model including atmospheric water vapor or by a flat spectrum model. We derive a 3σ upper limit of 1570 ppm on the presence of water vapor absorption in the atmosphere of XO-2b. In the Appendix, we perform a similar analysis for the gas giant planet XO-1b.
    The Astrophysical Journal 11/2012; 761(1):7. · 6.73 Impact Factor
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    ABSTRACT: Spectroscopy during planetary transits is a powerful tool to probe exoplanet atmospheres. We present the near-infrared transit spectroscopy of XO-2b obtained with HST NICMOS. Uniquely for NICMOS transit spectroscopy, a companion star of similar properties to XO-2 is present in the field of view. We derive improved star and planet parameters through a photometric white-light analysis. We show a clear correlation of the spectrum noise with instrumental parameters, in particular the angle of the spectral trace on the detector. An MCMC method using a decorrelation from instrumental parameters is used to extract the planetary spectrum. Spectra derived independently from each of the 3 visits have a RMS of 430, 510, and 1000 ppm respectively. The same analysis is performed on the companion star after numerical injection of a transit with a depth constant at all wavelengths. The extracted spectra exhibit residuals of similar amplitude as for XO-2, which represent the level of remaining NICMOS systematics. This shows that extracting planetary spectra is at the limit of NICMOS' capability. We derive a spectrum for the planet XO-2b using the companion star as a reference. The derived spectrum can be represented by a theoretical model including atmospheric water vapor or by a flat spectrum model. We derive a 3-sigma upper limit of 1570 ppm on the presence of water vapor absorption in the atmosphere of XO-2b. In an appendix, we perform a similar analysis for the gas giant planet XO-1b.
    10/2012;
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    ABSTRACT: Capitalizing on the observational advantage offered by its tiny M dwarf host, we present Hubble Space Telescope/Wide Field Camera 3 (WFC3) grism measurements of the transmission spectrum of the super-Earth exoplanet GJ1214b. These are the first published WFC3 observations of a transiting exoplanet atmosphere. After correcting for a ramp-like instrumental systematic, we achieve nearly photon-limited precision in these observations, finding the transmission spectrum of GJ1214b to be flat between 1.1 and 1.7 μm. Inconsistent with a cloud-free solar composition atmosphere at 8.2σ, the measured achromatic transit depth most likely implies a large mean molecular weight for GJ1214b's outer envelope. A dense atmosphere rules out bulk compositions for GJ1214b that explain its large radius by the presence of a very low density gas layer surrounding the planet. High-altitude clouds can alternatively explain the flat transmission spectrum, but they would need to be optically thick up to 10 mbar or consist of particles with a range of sizes approaching 1 μm in diameter.
    The Astrophysical Journal 02/2012; 747(1):35. · 6.73 Impact Factor
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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: We use HST+FGS to measure the parallax of the transiting planet host star XO-3. Our parallax measurement, with a precision of 0.2 mas, and resulting distance to XO-3 constrains the radius of the star, thereby providing the most accurate radius determination to date for the massive extrasolar planet (XO-3b) in orbit around XO-3. The result allows us to critically test current giant extrasolar planet structure models. The implied radius for XO-3b is smaller than previous determinations, bringing the size of XO-3b more in line with models which include only insolation by the parent star. As a result, their may be no need to invoke heating that may be produced inside XO-3b by tides raised on the planet as it moves through its 3.2 d eccentric (e 0.22) orbit.
    01/2012;
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    ABSTRACT: I will present new Hubble Space Telescope observations of the atmosphere of the enigmatic super-Earth GJ1214b. According to theoretical models, GJ1214b's low density requires it has either an H-rich outer envelope or a bulk composition with a large water-to-rock ratio. Because GJ1214b transits a very small M dwarf, these possibilities can be distinguished observationally using transmission spectroscopy, which probes the scale height (and thus H-content) of the planet's atmosphere. Using the Wide Field Camera 3 (WFC3)'s grism mode, we are measuring GJ1214b's transmission spectrum between 1.1 and 1.7 microns, where models predict absorption features due to water vapor to be strongest. Other observations of GJ1214b's atmosphere have been made, but are seemingly contradictory and difficult to interpret, in part because they do not probe this important wavelength range. Two of our three HST visits have already been completed; the data are of superb quality and will provide a robust test of the nature of GJ1214b's outer envelope. Our final HST visit is scheduled for 17 July 2011, so the results will presented for the first time at the Jackson Hole meeting. I am a member of the team that discovered GJ1214b and the principal investigator of the HST/WFC3 observations.
    09/2011;
  • Nicolas Crouzet, P. R. McCullough, C. Burke, D. Long
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    ABSTRACT: We present the photometric and spectroscopic analysis of the transiting planet XO-2 b, from HST NICMOS. Three transits of the planet have been observed. The field of view contains not only the XO-2 star but also a second star of very similar properties, which we use as a comparison star. We first perform a white-light photometric analysis of the data. The high signal to noise ratio yields improved parameters for the planet and host star. We then build the spectrum of this system in the range 1.2-1.8 microns with a spectral resolution of R 35, taking into account systematic effects. To investigate the validity of the method and the reliability of NICMOS data, we also build a spectrum for the exoplanet XO-1 b. We find an overall shape close to Tinetti et al. (ApJL 2010), although differences appear for some particular features. In the context of a debate about planetary spectra obtained with NICMOS, this study suggests that NICMOS data, although clearly affected by systematics, contain enough information to derive planetary spectra at least at low resolution.
    09/2011;
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    ABSTRACT: Observations conducted with the Fine Guidance Sensor on the Hubble Space Telescope (HST) providing high cadence and precision time-series photometry were obtained over 10 consecutive days in 2008 December on the host star of the transiting exoplanet HD 17156b. During this time, 1.0 × 1012 photons (corrected for detector dead time) were collected in which a noise level of 163 parts per million per 30 s sum resulted, thus providing excellent sensitivity to the detection of the analog of the solar 5-minute p-mode oscillations. For HD 17156, robust detection of p modes supports the determination of the stellar mean density of ρ* = 0.5301 ± 0.0044 g cm–3 from a detailed fit to the observed frequencies of modes of degree l = 0, 1, and 2. This is the first star for which the direct determination of ρ* has been possible using both asteroseismology and detailed analysis of a transiting planet light curve. Using the density constraint from asteroseismology, and stellar evolution modeling results in M * = 1.285 ± 0.026 M ☉, R * = 1.507 ± 0.012 R ☉, and a stellar age of 3.2 ± 0.3 Gyr.
    The Astrophysical Journal 12/2010; 726(1):2. · 6.73 Impact Factor
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    ABSTRACT: We present observations of three distinct transits of HD 17156b obtained with the Fine Guidance Sensors on board the Hubble Space Telescope. We analyzed both the transit photometry and previously published radial velocities to find the planet-star radius ratio Rp /R = 0.07454 ± 0.00035, inclination i = 86.49+0.24 –0.20 deg, and scaled semimajor axis a/R = 23.19+0.32 –0.27. This last value translates directly to a mean stellar density determination ρ = 0.522+0.021 –0.018 g cm–3. Analysis of asteroseismology observations by the companion paper of Gilliland et al. provides a consistent but significantly refined measurement of ρ = 0.5308 ± 0.0040. We compare stellar isochrones to this density estimate and find M = 1.275 ± 0.018 M ☉ and a stellar age of 3.37+0.20 –0.47 Gyr. Using this estimate of M and incorporating the density constraint from asteroseismology, we model both the photometry and published radial velocities to estimate the planet radius Rp = 1.0870 ± 0.0066 RJ and the stellar radius R = 1.5007 ± 0.0076 R ☉. The planet radius is larger than that found in previous studies and consistent with theoretical models of a solar-composition gas giant of the same mass and equilibrium temperature. For the three transits, we determine the times of mid-transit to a precision of 6.2 s, 7.6 s, and 6.9 s, and the transit times for HD 17156 do not show any significant departures from a constant period. The joint analysis of transit photometry and asteroseismology presages similar studies that will be enabled by the NASA Kepler Mission.
    The Astrophysical Journal 12/2010; 726(1):3. · 6.73 Impact Factor
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    ABSTRACT: Observations conducted with the Fine Guidance Sensor on Hubble Space Telescope (HST) providing high cadence and precision time-series photometry were obtained over 10 consecutive days in December 2008 on the host star of the transiting exoplanet HD 17156b. During this time 10^12 photons (corrected for detector deadtime) were collected in which a noise level of 163 parts per million per 30 second sum resulted, thus providing excellent sensitivity to detection of the analog of the solar 5-minute p-mode oscillations. For HD 17156 robust detection of p-modes supports determination of the stellar mean density of 0.5301 +/- 0.0044 g/cm^3 from a detailed fit to the observed frequencies of modes of degree l = 0, 1, and 2. This is the first star for which direct determination of the mean stellar density has been possible using both asteroseismology and detailed analysis of a transiting planet light curve. Using the density constraint from asteroseismology, and stellar evolution modeling results in M_star = 1.285 +/- 0.026 solar, R_star = 1.507 +/- 0.012 solar, and a stellar age of 3.2 +/- 0.3 Gyr. Comment: Accepted by ApJ; 16 pages, 18 figures
    11/2010;
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    ABSTRACT: We present observations of three distinct transits of HD 17156b obtained with the Fine Guidance Sensors (FGS) on board the Hubble Space Telescope} (HST). We analyzed both the transit photometry and previously published radial velocities to find the planet-star radius ratio R_p/R_s = 0.07454 +/- 0.00035, inclination i=86.49 +0.24/-0.20 deg, and scaled semi-major axis a/R = 23.19 +0.32/-0.27. This last value translates directly to a mean stellar density determination of 0.522 +0.021/-0.018 g cm^-3. Analysis of asteroseismology observations by the companion paper of Gilliland et al. (2009) provides a consistent but significantly refined measurement of the stellar mean density. We compare stellar isochrones to this density estimate and find M_s = 1.275 +/- 0.018 M_sun and a stellar age of $3.37 +0.20/-0.47 Gyr. Using this estimate of M_s and incorporating the density constraint from asteroseismology, we model both the photometry and published radial velocities to estimate the planet radius R_p= 1.0870 +/- 0.0066 Jupiter radii and the stellar radius R_s = 1.5007 +/- 0.0076 R_sun. The planet radius is larger than that found in previous studies and consistent with theoretical models of a solar-composition gas giant of the same mass and equilibrium temperature. For the three transits, we determine the times of mid-transit to a precision of 6.2 s, 7.6 s, and 6.9 s, and the transit times for HD 17156 do not show any significant departures from a constant period. The joint analysis of transit photometry and asteroseismology presages similar studies that will be enabled by the NASA Kepler Mission. Comment: Accepted for publication to ApJ
    11/2010;
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    ABSTRACT: We refine the physical parameters of the transiting hot Jupiter planet XO-1b and its stellar host XO-1 using Hubble Space Telescope (HST) NICMOS observations. XO-1b has a radius Rp = 1.21 ± 0.03 RJ , and XO-1 has a radius R = 0.94 ± 0.02 R ☉, where the uncertainty in the mass of XO-1 dominates the uncertainty of Rp and R . There are no significant differences in the XO-1 system properties between these broadband NIR observations and previous determinations based upon ground-based optical observations. We measure two transit timings from these observations with 9 s and 15 s precision. As a residual to a linear ephemeris model, there is a 2.0σ timing difference between the two HST visits that are separated by three transit events (11.8 days). These two transit timings and additional timings from the literature are sufficient to rule out the presence of an Earth mass planet orbiting in 2:1 mean motion resonance coplanar with XO-1b. We identify and correct for poorly understood "gain-like" variations present in NICMOS time series data. This correction reduces the effective noise in time series photometry by a factor of 2 for the case of XO-1.
    The Astrophysical Journal 08/2010; 719(2):1796. · 6.73 Impact Factor
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    ABSTRACT: Searches for planetary transits find many astrophysical false positives as a by-product. There are four main types analyzed in the literature: a grazing-incidence eclipsing binary (EB) star, an EB star with a small radius companion star, a blend of one or more stars with an unrelated EB star, and a physical triple star system. We present a list of 69 astrophysical false positives that had been identified as candidates of transiting planets of the on-going XO survey. This list may be useful in order to avoid redundant observation and characterization of these particular candidates that have been independently identified by other wide-field searches for transiting planets. The list may be useful for those modeling the yield of the XO survey and surveys similar to it. Subsequent observations of some of the listed stars may improve mass-radius relations, especially for low-mass stars. From the candidates exhibiting eclipses, we report three new spectroscopic double-line binaries and give mass function estimations for 15 single-line spectroscopic binaries.
    The Astrophysical Journal Supplement Series 06/2010; 189(1):134. · 16.24 Impact Factor
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    ABSTRACT: We report here the first infrared spectrum of the hot-Jupiter XO-1b. The observations were obtained with NICMOS instrument onboard the Hubble Space Telescope during a primary eclipse of the XO-1 system. Near photon-noise-limited spectroscopy between 1.2 and 1.8 micron allows us to determine the main composition of this hot-Jupiter's planetary atmosphere with good precision. This is the third hot-Jupiter's atmosphere for which spectroscopic data are available in the near IR. The spectrum shows the presence of water vapor (H2O), methane (CH4) and carbon dioxide (CO2), and suggests the possible presence of carbon monoxide (CO). We show that the published IRAC secondary transit emission photometric data are compatible with the atmospheric composition at the terminator determined from the NICMOS spectrum, with a range of possible mixing-ratios and thermal profiles; additional emission spectroscopy data are needed to reduce the degeneracy of the possible solutions. Finally, we note the similarity between the 1.2-1.8 micron transmission spectra of XO-1b and HD 209458b, suggesting that in addition to having similar stellar/orbital and planetary parameters the two systems may also have a similar exoplanetary atmospheric composition. Comment: ApJ accepted, 4 figures
    The Astrophysical Journal Letters 02/2010; · 6.35 Impact Factor
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    ABSTRACT: We determined the flux ratios of the heavy and eccentric planet XO-3b to its parent star in the four Infrared Array Camera bands of the Spitzer Space Telescope: 0.101% ± 0.004% at 3.6 μm; 0.143% ± 0.006% at 4.5 μm; 0.134% ± 0.049% at 5.8 μm; and 0.150% ± 0.036% at 8.0 μm. The flux ratios are within [–2.2, 0.3, –0.8, and –1.7]σ of the model of XO-3b with a thermally inverted stratosphere in the 3.6 μm, 4.5 μm, 5.8 μm, and 8.0 μm channels, respectively. XO-3b has a high illumination from its parent star (Fp ~ (1.9-4.2) × 109 erg cm–2 s–1) and is thus expected to have a thermal inversion, which we indeed observe. When combined with existing data for other planets, the correlation between the presence of an atmospheric temperature inversion and the substellar flux is insufficient to explain why some high insolation planets like TrES-3 do not have stratospheric inversions and some low insolation planets like XO-1b do have inversions. Secondary factors such as sulfur chemistry, atmospheric metallicity, amounts of macroscopic mixing in the stratosphere, or even dynamical weather effects likely play a role. Using the secondary eclipse timing centroids, we determined the orbital eccentricity of XO-3b as e = 0.277 ± 0.009. The model radius-age trajectories for XO-3b imply that at least some amount of tidal heating is required to inflate the radius of XO-3b, and the tidal heating parameter of the planet is constrained to Qp 106.
    The Astrophysical Journal 02/2010; 711(1):111. · 6.73 Impact Factor

Publication Stats

268 Citations
205 Downloads
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146.26 Total Impact Points

Institutions

  • 2014
    • Princeton University
      • Department of Astrophysical Sciences
      Princeton, New Jersey, United States
  • 2013
    • University of Maryland, College Park
      • Department of Astronomy
      Maryland, United States
  • 2006–2012
    • Space Telescope Science Institute
      Baltimore, Maryland, United States