Atmospheric structure of HD209458bTop of this page
Our model has been applied to the planet HD209458b. We determined a mean thermal structure as well as the corresponding atmospheric composition (Iro et al., 2005). In particular, we were able to confirm the importance of the absorption by alkali in the energy balance of the Pegasides and study the stellar flux absorption in their atmosphere, crucial issue for models of evolution. These models indeed have difficulties to explain the observed radius of HD209458b, unless an additional deep source of energy exists in the atmosphere of the planet (Guillot & Showman 2002). It has been proposed as source of this energy the penetration of several % of the incident stellar flux to the deep atmospheric layers. Only 1 % of the incident stellar flux reaches the 2 bar level. A negligible part reach the deep levels (see the figure below). This is then not this mecanism that can explain HD20948b's radius.
Net stellar flux as a function of pressure level for the Cold and Nominal profiles.
Radiative time constantsTop of this page
Our code allows us to estimate radiative time constants at any atmospheric level by perturbing the temperature from the equilibrium state and let it relax back to equilibrium. This is possible because we use a time-marching algorithm to solve for radiative equilibrium. Such calculations are particularly useful to modelers of atmospheric dynamics (Showman & Guillot 2002 ; Cho et al. 2003) who treat the radiative terms in their energy equation as a Newtonian cooling term. Our results were used by Cooper & Showman (2005) to construct a complete dynamical model of HD209458b.
Radiative time as a function of the pressure level. Symbols represent the 1-bar radiative time values calculated (resp. used) by Showman & Guillot 2002 ; (resp. Cho et al. 2003). Diamond corresponds to 1.1 days (Showman & Guillot 2002) ; square to 10 days (Cho et al. 2003).
Time-Dependent CalculationsTop of this page
I can calculate the diurnal variations of the temperature profile assuming a constant-with-height rotation of the planet, or its seasonal variation for a planet with a non-zero obliquity and/or an eccentric orbit. Note that, up to now, models published in the literature do not take into account time-dependent thermal structure. Showman & Guillot (2002), showed that the atmospheres of Pegasides should have strong winds (~ 1 km/s) and relatively strong day-night and equator-to-pole temperature contrasts (~ 500 K near optical depth unity). Assuming a constant angular velocity, which mimics a zonal atmospheric circulation, I calculated the longitudinal temperature variations of HD209458b and found a day-to-night contrast of 400-600 K at 0.1 bar, 30-200 K at 1 bar and less than 5 K at 10 bar. The large longitudinal temperature contrasts implies that species such as sodium will condense on the night side. Even with no settling, the morning limb (which is coldest) can be strongly depleted in the condensible species. From calculations of transit spectra representative of the morning and evening limbs, we found that the former shows a 3 times weaker sodium absorption than the latter. The Na dimming we calculated through the entire limb is then in very good agreement with the sodium absorption observed by Charbonneau et al. (2002) during planetary transits. These results are published in Iro et al. (2005).
Equatorial cut of the atmosphere between the 10-6 and the 10-bar levels for an equatorial wind velocity of 1 km/s. The level where condensation of sodium occurs (black line) goes deeper as the night wears on (anti-clockwise) and is deepest at the morning limb. Below 10 bar, the temperature field (not shown here) is uniform and depends only on the bottom boundary condition.
Extrasolar giant planets evolutionTop of this page
My work on exoplanets can provide boundary conditions for models of extrasolar planet evolution. A collaboration with T. Guillot, N. Santos, F. Pont and C. Melo has been established in 2005 in order to relate the metallicity of the central star to the radius of the orbiting planet. We can relate the radius of the transiting extrasolar planets with the initial amount of heavy elements in the protoplanetary disk. This can explain the observed anomalously large radius of some extrasolar giant planets such as HD209458 and HD189733b (Guillot et al., 2005).
More to come soon...