Curtis S. Cooper

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

‘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 μm, 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 of Hot Jupiters: Three-dimensional circulation models of HD 209458b and HD 189733b with Simplified Forcing

We present global, three-dimensional numerical simulations of the atmospheric circulation on HD 209458b and HD 189733b and calculate the infrared spectra and light curves predicted by these simulations, which we compare with available observations. Radiative heating/cooling is parameterized with a simplified Newtonian relaxation scheme. Our simulations develop day-night temperature contrasts that vary strongly with pressure. At low pressure (<10 mbar), air flows from the substellar point toward the antistellar point, both along the equator and over the poles. At deeper levels, the flow develops an eastward equatorial jet with speeds of 3-4 km s−1, with weaker westward flows at high latitudes. This basic flow pattern is robust to variations in model resolution, gravity, radiative time constant, and initial temperature structure. Nightside spectra show deep absorption bands of H2O, CO, and/or CH4, whereas on the dayside these absorption bands flatten out or even flip into emission. This results from the strong effect of dynamics on the vertical temperature-pressure structure; the temperature decreases strongly with altitude on the nightside but becomes almost isothermal on the dayside. In Spitzer bandpasses, our predicted planet-to-star flux ratios vary by a factor of ~2-10 with orbital phase, depending on the wavelength and chemistry. For HD 189733b, where a detailed 8 μm light curve has been obtained, we correctly produce the observed phase offset of the flux maximum, but we do not explain the flux minimum and we overpredict the total flux variation. This discrepancy likely results from the simplifications inherent in the Newtonian relaxation scheme and provides motivation for incorporating realistic radiative transfer in future studies.

DYNAMICS AND DISEQUILIBRIUM CARBON CHEMISTRY IN HOT JUPITER ATMOSPHERES, WITH APPLICATION TO HD 209458b

Chemical equilibrium considerations suggest that, assuming solar elemental abundances, carbon on HD 209458b is sequestered primarily as carbon monoxide (CO) and methane (CH4). The relative mole fractions of CO(g) and CH4(g) in chemical equilibrium are expected to vary greatly according to variations in local temperature and pressure. We show, however, that in the p = 1-1000 mbar range, chemical equilibrium does not hold. To explore disequilibrium effects, we couple the chemical kinetics of CO and CH4 to a three-dimensional numerical model of HD 209458bs atmospheric circulation. These simulations show that vigorous dynamics caused by uneven heating of this tidally locked planet homogenize the CO and CH4 concentrations at p < 1 bar, even in the presence of lateral temperature variations of ~500-1000 K. In the 1-1000 mbar pressure range we find that over 98% of the carbon is in CO. This is true even in cool regions where CH4 is much more stable thermodynamically. Our work shows, furthermore, that planets 300-500 K cooler than HD 209458b can also have abundant CO in their upper layers due to disequilibrium effects. We demonstrate several interesting observational consequences of these results.

Dynamic Meteorology at the Photosphere of HD 209458b

We calculate the meteorology of the close-in transiting extrasolar planet HD 209458b using a global, three-dimensional atmospheric circulation model. Dynamics are driven by perpetual irradiation of one hemisphere of this tidally locked planet. The simulation predicts global temperature contrasts of ~500 K at the photosphere and the development of a steady superrotating jet. The jet extends from the equator to midlatitudes and from the top model layer at 1 mbar down to 10 bar at the base of the heated region. Wind velocities near the equator exceed 4 km s-1 at 300 mbar. The hottest regions of the atmosphere are blown downstream from the substellar point by ~60° of longitude. We predict from these results a factor of ~2 ratio between the maximum and minimum observed radiation from the planet over a full orbital period, with peak infrared emission preceding the time of the secondary eclipse by ~14 hr.

The Influence of Atmospheric Dynamics on the Infrared Spectra and Light Curves of Hot Jupiters

We explore the infrared spectrum of a three-dimensional dynamical model of planet HD 209458b as a function of orbital phase. The dynamical model predicts dayside atmospheric pressure-temperature profiles that are much more isothermal at pressures less than 1 bar than one-dimensional radiative-convective models have found. The resulting dayside thermal spectra are very similar to a blackbody, and only weak water absorption features are seen at short wavelengths. The dayside emission is consequently in better agreement with ground-based and space-based secondary eclipse data than any previous models, which predict strong flux peaks and deep absorption features. At other orbital phases, absorption due to carbon monoxide and methane is also predicted. We compute the spectra under two treatments of atmospheric chemistry: one uses the predictions of equilibrium chemistry, and the other uses nonequilibrium chemistry, which ties the timescales of methane and carbon monoxide chemistry to dynamical timescales. As a function of orbital phase, we predict planet-to-star flux ratios for standard infrared bands and all Spitzer Space Telescope bands. In Spitzer bands, we predict two- to fifteenfold variation in planetary flux as a function of orbital phase with equilibrium chemistry, and two- to fourfold variation with nonequilibrium chemistry. Variation is generally more pronounced in bands from 3 to 10 � m than at longer wavelengths. The orbital phase of maximum thermal emission in infrared bands is 15Y45 orbital degrees before the time of secondary eclipse. Changes influx as a function of orbital phase for HD 209458b should be observable with Spitzer, given the previously achieved observational error bars. Subject headingg binaries: eclipsing — planetary systems — radiative transfer — stars: individual (HD 209458)

Modeling the Formation of Clouds in Brown Dwarf Atmospheres

Because the opacity of clouds in substellar mass object (SMO) atmospheres depends on the composition and distribution of particle sizes within the cloud, a credible cloud model is essential for accurately modeling SMO spectra and colors. We present a one-dimensional model of cloud particle formation and subsequent growth based on a consideration of basic cloud microphysics. We apply this microphysical cloud model to a set of synthetic brown dwarf atmospheres spanning a broad range of surface gravities and effective temperatures (gsurf ¼ 1:78 � 10 3 3 � 10 5 cm s � 2 and Teff ¼ 600 1600 K) to obtain plausible particle sizes for several abundant species (Fe, Mg2SiO4, and Ca2Al2SiO7). At the base of the clouds, where the particles are largest, the particle sizes thus computed range from � 5 to over 300 lm in radius over the full range of atmospheric conditions considered. We show that average particle sizes decrease significantly with increasing brown dwarf surface gravity. We also find that brown dwarfs with higher effective temperatures have characteristically larger cloud particles than those with lower effective temperatures. We therefore conclude that it is unrealistic when modeling SMO spectra to apply a single particle size distribution to the entire class of objects. Subject headings: stars: atmospheres — stars: low-mass, brown dwarfs

On the indirect detection of sodium in the atmosphere of the planetary companion to HD 209458

Using a self-consistent atmosphere code, we construct a new model of the atmosphere of the transiting extrasolar giant planet HD 209458b to investigate the disparity between the observed strength of the sodium absorption feature at 589 nm and the predictions of previous models. For the atmospheric temperature-pressure profile we derive, silicate and iron clouds reside at a pressure of several millibars in the planets atmosphere. These clouds have significant vertical extent and optical depth because of our slant viewing geometry and lead to increased absorption in bands directly adjacent to the sodium line core. Using a non-LTE sodium ionization model that includes photoionization by stellar UV flux, collisional processes with H2, and radiative recombination, we show that the ionization depth in the planets atmosphere reaches ~ mbar at the day/night terminator. Ionization leads to a slight weakening of the sodium feature. We present our baseline model, including ionization and clouds, which falls near the observational error bars. The sensitivity of our conclusions to the derived atmospheric temperature-pressure profile is discussed.

Resolving the Surfaces of Extrasolar Planets with Secondary Eclipse Light Curves

We present a method that employs the secondary eclipse light curves of transiting extrasolar planets to probe the spatial variation of their thermal emission. This technique permits an observer to resolve the surface of the planet withouttheneedtospatiallyisolateitslightfromthatofthecentralstar.Weevaluatethefeasibilityofthistechnique for the HD 209458 system by simulating observations made with the Spitzer Infrared Array Camera (IRAC). We consider two representations of the planetary thermal emission: a simple model parameterized by a sinusoidal dependence on longitude and latitude, and the results of a three-dimensional dynamical simulation of the planetary atmosphere previously published by Cooper & Showman. Wefind that observations of the secondary eclipse light curve are most sensitive to a longitudinal asymmetry in the dayside planetary emission. To quantify this signal, we define a new parameter,the‘‘uniformtimeoffset,’’whichmeasuresthetimelagbetweentheobservedsecondaryeclipseandthatpredictedbyaplanetwithspatiallyuniformemission.WecomparethepredictedamplitudeofthisparameterforHD20948 with the precision with which it could be measured with IRAC. We find that IRAC observations at 3.6� m of a single secondary eclipse should permit sufficient precision to confirm or reject the Cooper & Showman model of the surface fluxdistributionforthisplanet.Wequantifythesignal-to-noiseratioforthisoffsetintheremainingIRACbandsandfind that a modest improvement in photometric precision should permit a similarly robust detection. Subjectheadinggbinaries:eclipsing — infrared:stars — planetarysystems — stars:individual(HD 209458) — techniques: high angular resolution — techniques: photometric

DETECTING THE WIND-DRIVEN SHAPES OF EXTRASOLAR GIANT PLANETS FROM TRANSIT PHOTOMETRY

Several processes can cause the shape of an extrasolar giant planets shadow, as viewed in transit, to depart from circular. In addition to rotational effects, cloud formation, non-homogenous haze production and movement, and dynamical effects (winds) could also be important. When such a planet transits its host star as seen from the Earth, the asphericity will introduce a deviation in the transit light curve relative to the transit of a perfectly spherical (or perfectly oblate) planet. We develop a theoretical framework to interpret planetary shapes. We then generate predictions for transiting planet shapes based on a published theoretical dynamical model of HD189733b. Using these shape models we show that planet shapes are unlikely to introduce detectable light-curve deviations (those >1 × 10–5 of the host star), but that the shapes may lead to astrophysical sources of systematic error when measuring planetary oblateness, transit time, and impact parameter.

Erratum: “Modeling the Formation of Clouds in Brown Dwarf Atmospheres” (ApJ, 586, 1320 [2003])

Because the opacity of clouds in substellar mass object (SMO) atmospheres depends on the composition and distribution of particle sizes within the cloud, a credible cloud model is essential for accurately modeling SMO spectra and colors. We present a one--dimensional model of cloud particle formation and subsequent growth based on a consideration of basic cloud microphysics. We apply this microphysical cloud model to a set of synthetic brown dwarf atmospheres spanning a broad range of surface gravities and effective temperatures (g_surf = 1.78 * 10^3 -- 3 * 10^5 cm/s^2 and T_eff = 600 -- 1600 K) to obtain plausible particle sizes for several abundant species (Fe, Mg2SiO4, and Ca2Al2SiO7). At the base of the clouds, where the particles are largest, the particle sizes thus computed range from ~5 microns to over 300 microns in radius over the full range of atmospheric conditions considered. We show that average particle sizes decrease significantly with increasing brown dwarf surface gravity. We also find that brown dwarfs with higher effective temperatures have characteristically larger cloud particles than those with lower effective temperatures. We therefore conclude that it is unrealistic when modeling SMO spectra to apply a single particle size distribution to the entire class of objects.Because the opacity of clouds in substellar mass object (SMO) atmospheres depends on the composition and distribution of particle sizes within the cloud, a credible cloud model is essential for accurately modeling SMO spectra and colors. We present a one-dimensional model of cloud particle formation and subsequent growth based on a consideration of basic cloud microphysics. We apply this microphysical cloud model to a set of synthetic brown dwarf atmospheres spanning a broad range of surface gravities and effective temperatures (gsurf = 1.78×10 – 3×10 cm s and Teff = 600 – 1600 K) to obtain plausible particle sizes for several abundant species (Fe, Mg2SiO4, and Ca2Al2SiO7). The particle sizes we have thus computed range from ∼5 μm to over 300 μm in radius over the full range of atmospheric conditions considered. We show that modal particle sizes decrease significantly with increasing brown dwarf surface gravity. We also find that brown dwarfs with higher effective temperatures have characteristically larger cloud particles than those with lower effective temperatures. We thus conclude that it is unrealistic when modeling SMO spectra to apply a single particle size distribution to the entire class of objects. Subject headings: atmospheres: clouds, condensation, grains: fundamental parameters — stars: low mass, brown dwarfs, substellar mass objects, L dwarfs, T dwarfs, spectroscopy, atmospheres, spectral