Adam P. Showman

Bio: Adam P. Showman is an academic researcher from University of Arizona. The author has contributed to research in topics: Planet & Hot Jupiter. The author has an hindex of 92, co-authored 331 publications receiving 26340 citations. Previous affiliations of Adam P. Showman include Peking University & Ames Research Center.

A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion

Abstract: Thousands of transiting exoplanets have been discovered, but spectral analysis of their atmospheres has so far been dominated by a small number of exoplanets and data spanning relatively narrow wavelength ranges (such as 1.1-1.7 micrometres). Recent studies show that some hot-Jupiter exoplanets have much weaker water absorption features in their near-infrared spectra than predicted. The low amplitude of water signatures could be explained by very low water abundances, which may be a sign that water was depleted in the protoplanetary disk at the planet's formation location, but it is unclear whether this level of depletion can actually occur. Alternatively, these weak signals could be the result of obscuration by clouds or hazes, as found in some optical spectra. Here we report results from a comparative study of ten hot Jupiters covering the wavelength range 0.3-5 micrometres, which allows us to resolve both the optical scattering and infrared molecular absorption spectroscopically. Our results reveal a diverse group of hot Jupiters that exhibit a continuum from clear to cloudy atmospheres. We find that the difference between the planetary radius measured at optical and infrared wavelengths is an effective metric for distinguishing different atmosphere types. The difference correlates with the spectral strength of water, so that strong water absorption lines are seen in clear-atmosphere planets and the weakest features are associated with clouds and hazes. This result strongly suggests that primordial water depletion during formation is unlikely and that clouds and hazes are the cause of weaker spectral signatures.

A map of the day–night contrast of the extrasolar planet HD 189733b

Abstract: 'Hot Jupiter' extrasolar planets are expected to be tidally locked because they are close (<0.05 astronomical units, where 1 au is the average Sun–Earth distance) to their parent stars, resulting in permanent daysides and nightsides. By observing systems where the planet and star periodically eclipse each other, several groups have been able to estimate the temperatures of the daysides of these planets. A key question is whether the atmosphere is able to transport the energy incident upon the dayside to the nightside, which will determine the temperature at different points on the planet's surface. Here we report observations of HD 189733, the closest of these eclipsing planetary systems, over half an orbital period, from which we can construct a 'map' of the distribution of temperatures. We detected the increase in brightness as the dayside of the planet rotated into view. We estimate a minimum brightness temperature of 973 ± 33 K and a maximum brightness temperature of 1,212 ± 11 K at a wavelength of 8 mum, indicating that energy from the irradiated dayside is efficiently redistributed throughout the atmosphere, in contrast to a recent claim for another hot Jupiter. Our data indicate that the peak hemisphere-integrated brightness occurs 16 ± 6° before opposition, corresponding to a hotspot shifted east of the substellar point. The secondary eclipse (when the planet moves behind the star) occurs 120 ± 24 s later than predicted, which may indicate a slightly eccentric orbit.

Atmospheric circulation and tides of ``51 Pegasus b-like'' planets

Abstract: We examine the properties of the atmospheres of extrasolar giant planets at orbital distances smaller than 0.1 AU from their stars. We show that these \51 Peg b-like" planets are rapidly synchronized by tidal in- teractions, but that small departures from synchronous rotation can occur because of fluid-dynamical torques within these planets. Previous radiative-transfer and evolution models of such planets assume a homogeneous atmosphere. Nevertheless, we show using simple arguments that, at the photosphere, the day-night temperature dierence and characteristic wind speeds may reach500 K, and 2k m s 1 , respectively. Substantial departures from chemical equilibrium are expected. The cloud coverage depends sensitively on the dynamics; clouds could exist predominantly either on the dayside or nightside, depending on the circulation regime. Radiative-transfer models that assume homogeneous conditions are therefore inadequate in describing the atmospheric properties of 51 Peg b-like planets. We present preliminary three-dimensional, nonlinear simulations of the atmospheric cir- culation of HD 209458b that indicate plausible patterns for the circulation and generally agree with our simpler estimates. Furthermore, we show that kinetic energy production in the atmosphere can lead to the deposition of substantial energy in the interior, with crucial consequences for the evolution of these planets. Future measure- ments of reflected and thermally-emitted radiation from these planets will help test our ideas.

Atmospheric circulation of hot Jupiters: Coupled radiative-dynamical general circulation model simulations of HD 189733b and HD 209458b

Abstract: We present global, three-dimensional numerical simulations of HD 189733b and HD 209458b that couple the atmospheric dynamics to a realistic representation of nongray cloud-free radiative transfer. The model, which we call the Substellar and Planetary Atmospheric Radiation and Circulation model, adopts the MITgcm for the dynamics and uses the radiative model of McKay, Marley, Fortney, and collaborators for the radiation. Like earlier work with simplified forcing, our simulations develop a broad eastward equatorial jet, mean westward flow at higher latitudes, and substantial flow over the poles at low pressure. For HD 189733b, our simulations without TiO and VO opacity can explain the broad features of the observed 8 and 24 ?m light curves, including the modest day-night flux variation and the fact that the planet/star flux ratio peaks before the secondary eclipse. Our simulations also provide reasonable matches to the Spitzer secondary-eclipse depths at 4.5, 5.8, 8, 16, and 24 ?m and the ground-based upper limit at 2.2 ?m. However, we substantially underpredict the 3.6 ?m secondary-eclipse depth, suggesting that our simulations are too cold in the 0.1-1 bar region. Predicted temporal variability in secondary-eclipse depths is ~1% at Spitzer bandpasses, consistent with recent observational upper limits at 8 ?m. We also show that nonsynchronous rotation can significantly alter the jet structure. For HD 209458b, we include TiO and VO opacity; these simulations develop a hot (>2000 K) dayside stratosphere whose horizontal dimensions are small at depth but widen with altitude. Despite this stratosphere, we do not reproduce current Spitzer photometry of this planet. Light curves in Spitzer bandpasses show modest phase variation and satisfy the observational upper limit on day-night phase variation at 8 ?m.

Atmospheric Circulation and Tides of "51Peg b-like" Planets

Abstract: We examine the properties of the atmospheres of extrasolar giant planets at orbital distances smaller than 0.1 AU from their stars. We show that these ``51Peg b-like'' planets are rapidly synchronized by tidal interactions, but that small departures from synchronous rotation can occur because of fluid-dynamical torques within these planets. Previous radiative-transfer and evolution models of such planets assume a homogeneous atmosphere. Nevertheless, we show using simple arguments that, at the photosphere, the day-night temperature difference and characteristic wind speeds may reach ~500 K and ~2 km/s, respectively. Substantial departures from chemical equilibrium are expected. The cloud coverage depends sensitively on the dynamics; clouds could exist predominantly either on the dayside or nightside, depending on the circulation regime. Radiative-transfer models that assume homogeneous conditions are therefore inadequate in describing the atmospheric properties of 51Peg b-like planets. We present preliminary three-dimensional, nonlinear simulations of the atmospheric circulation of HD209458b that indicate plausible patterns for the circulation and generally agree with our simpler estimates. Furthermore, we show that kinetic energy production in the atmosphere can lead to the deposition of substantial energy in the interior, with crucial consequences for the evolution of these planets. Future measurements of reflected and thermally-emitted radiation from these planets will help test our ideas.

Evolutionary models for cool brown dwarfs and extrasolar giant planets. The case of HD 209458

Abstract: We present evolutionary models for cool brown dwarfs and extra-solar giant planets. The models reproduce the main trends of observed methane dwarfs in near-IR color-magnitude diagrams. We also present evolutionary models for irradiated planets, coupling for the first time irradiated atmosphere profiles and inner structures. We focus on HD 209458-like systems and show that irradiation effects can substantially affect the radius of sub-jovian mass giant planets. Irradiation effects, however, cannot alone explain the large observed radius of HD 209458b. Adopting assumptions which optimise irradiation effects and taking into account the extension of the outer atmospheric layers, we still find $\\sim$ 20% discrepancy between observed and theoretical radii. An extra source of energy seems to be required to explain the observed value of the first transit planet.

Geophysical fluid dynamics.

Abstract: The potential for sea ice-albedo feedback to give rise to nonlinear climate change in the Arctic Ocean – defined as a nonlinear relationship between polar and global temperature change or, equivalently, a time-varying polar amplification – is explored in IPCC AR4 climate models. Five models supplying SRES A1B ensembles for the 21 st century are examined and very linear relationships are found between polar and global temperatures (indicating linear Arctic Ocean climate change), and between polar temperature and albedo (the potential source of nonlinearity). Two of the climate models have Arctic Ocean simulations that become annually sea ice-free under the stronger CO 2 increase to quadrupling forcing. Both of these runs show increases in polar amplification at polar temperatures above-5 o C and one exhibits heat budget changes that are consistent with the small ice cap instability of simple energy balance models. Both models show linear warming up to a polar temperature of-5 o C, well above the disappearance of their September ice covers at about-9 o C. Below-5 o C, surface albedo decreases smoothly as reductions move, progressively, to earlier parts of the sunlit period. Atmospheric heat transport exerts a strong cooling effect during the transition to annually ice-free conditions. Specialized experiments with atmosphere and coupled models show that the main damping mechanism for sea ice region surface temperature is reduced upward heat flux through the adjacent ice-free oceans resulting in reduced atmospheric heat transport into the region.

VizieR Online Data Catalog: A Stellar Spectral Flux Library: 1150 - 25000 A (Pickles 1998)

Abstract: A stellar spectral flux library of wide spectral coverage and an example of its application are presented. The new library consists of 131 flux-calibrated spectra, encompassing all normal spectral types and luminosity classes at solar abundance, and metal-weak and metal-rich F-K dwarf and G-K giant components. Each library spectrum was formed by combining data from several sources overlapping in wavelength coverage. The SIMBAD database, measured colors, and line strengths were used to check that each input component has closely similar stellar type. The library has complete spectral coverage from 1150 to 10620 Afor all components and to 25000 Afor about half of them, mainly later types of solar abundance. Missing spectral coverage in the infrared currently consists of a smooth energy distribution formed from standard colors for the relevant types. The library is designed to permit inclusion of additional digital spectra, particularly of non-solar abundance stars in the infrared, as they become available. The library spectra are each given as Fl versus l, from 1150 to 25000 Ain steps of 5 A ˚. A program to combine the library spectra in the ratios appropriate to a selected isochrone is described and an example of a spectral component signature of a composite population of solar age and metallicity is illustrated. The library spectra and associated tables are available as text files by remote electronic access.

Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation: Application to Transits

Abstract: Toaidinthephysicalinterpretationofplanetaryradii constrainedthroughobservationsoftransitingplanets,oreventuallydirectdetections,wecomputemodelradiiofpurehydrogen-helium,water,rock,andironplanets,alongwithvarious mixtures. Masses ranging from 0.01 Earth masses to 10 Jupiter masses at orbital distances of 0.02–10 AU are considered. For hydrogen-helium rich planets, our models are the first to couple planetary evolution to stellar irradiation over a wide range of orbital separations (0.02–10 AU) through a nongray radiative-convective equilibrium atmosphere model. Stellar irradiation retards the contraction of giant planets, but its effect is not a simple function of theirradiationlevel:aplanetat1AUcontractsasslowlyasaplanetat0.1AU.WeconfirmtheassertionofGuillotthat very old giant planets under modest stellar irradiation (like that received by Jupiter and Saturn) develop isothermal atmospheric radiative zones once the planet’s intrinsic flux drops to a small fraction of the incident flux. For hydrogenhelium planets, we consider cores up to 90% of the total planet mass, comparable to those of Uranus and Neptune. If ‘‘hot Neptunes’’ have maintained their original masses and are not remnants of more massive planets, radii of � 0.30– 0.45RJ areexpected.Waterplanetsare � 40%–50%largerthanrockyplanets,independentofmass.Finally,weprovide tables of planetary radii at various ages and compositions, and for ice-rock-iron planets we fit our results to analytic functions, which will allow for quick composition estimates, given masses and radii, or mass estimates, given only planetary radii. These results will assist in the interpretation of observations for both the current transiting planet surveys as well as upcoming space missions, including COROT and Kepler.