اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدOn the radiative equilibrium of irradiated planetary atmospheres
140
0
0.0
(
0
)
تأليف
Tristan Guillot
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
The evolution of stars and planets is mostly controlled by the properties of their atmosphere. This is particularly true in the case of exoplanets close to their stars, for which one has to account both for an (often intense) irradiation flux, and from an intrinsic flux responsible for the progressive loss of the inner planetary heat. The goals of the present work are to help understanding the coupling between radiative transfer and advection in exoplanetary atmospheres and to provide constraints on the temperatures of the deep atmospheres. This is crucial in assessing whether modifying assumed opacity sources and/or heat transport may explain the inflated sizes of a significant number of giant exoplanets found so far. I use a simple analytical approach inspired by Eddingtons approximation for stellar atmospheres to derive a relation between temperature and optical depth valid for plane-parallel static grey atmospheres which are both transporting an intrinsic heat flux and receiving an outer radiation flux. The model is parameterized as a function of mean visible and thermal opacities, respectively. The model is shown to reproduce relatively well temperature profiles obtained from more sophisticated radiative transfer calculations of exoplanetary atmospheres. It naturally explains why a temperature inversion (stratosphere) appears when the opacity in the optical becomes significant compared to that in the infrared. I further show that the mean equivalent flux (proportional to T^4) is conserved in the presence of horizontal advection on constant optical depth levels. This implies with these hypotheses that the deep atmospheric temperature used as outer boundary for the evolution models should be calculated from models pertaining to the entire planetary atmosphere, not from ones that are relevant to the day side or to the substellar point. In these conditions, present-day models yield deep temperatures that are ~1000K too cold to explain the present size of planet HD 209458b. An tenfold increase in the infrared to visible opacity ratio would be required to slow the planetary cooling and contraction sufficiently to explain its size. However, the mean equivalent flux is not conserved anymore in the presence of opacity variations, or in the case of non-radiative vertical transport of energy: The presence of clouds on the night side or a downward transport of kinetic energy and its dissipation at deep levels would help making the deep atmosphere hotter and may explain the inflated sizes of giant exoplanets.
قيم البحث
اقرأ أيضاً
We describe the incorporation of polarized radiative transfer into the atmospheric radiative transfer modelling code VSTAR (Versatile Software for Transfer of Atmospheric Radiation). Using a vector discrete-ordinate radiative transfer code we are abl
e to generate maps of radiance and polarization across the disc of a planet, and integrate over these to get the full-disc polarization. In this way we are able to obtain disc-resolved, phase-resolved and spectrally-resolved intensity and polarization for any of the wide range of atmopsheres that can be modelled with VSTAR. We have tested the code by reproducing a standard benchmark problem, as well as by comparing with classic calculations of the polarization phase curves of Venus. We apply the code to modelling the polarization phase curves of the hot Jupiter system HD 189733b. We find that the highest polarization amplitudes are produced with optically thick Rayleigh scattering clouds and these would result in a polarization amplitude of 27 ppm for the planetary signal seen in the combined light of the star and planet. A more realistic cloud model consistent with the observed transmission spectrum results is an amplitude of ~20 ppm. Decreasing the optical depth of the cloud, or making the cloud particles more absorbing, both have the effect of increasing the polarization of the reflected light but reducing the amount of reflected light and hence the observed polarization amplitude.
Interactions between the winds of stars and the magnetospheres and atmospheres of planets involve many processes, including the acceleration of particles, heating of upper atmospheres, and a diverse range of atmospheric loss processes. Winds remove a
ngular momentum from their host stars causing rotational spin-down and a decay in magnetic activity, which protects atmospheres from erosion. While wind interactions are strongly influenced by the X-ray and ultraviolet activity of the star and the chemical composition of the atmosphere, the role of planetary magnetic fields is unclear. In this chapter, I review our knowledge of the properties and evolution of stellar activity and winds and discuss the influences of these processes on the long term evolution of planetary atmospheres. I do not consider the large number of important processes taking place at the surfaces of planets that cause exchanges between the atmosphere and the planets interior.
Transiting planets provide a unique opportunity to study the atmospheres of extra-solar planets. Radiative hydrodynamical models of the atmosphere provide a crucial link between the physical characteristics of the atmosphere and the observed properti
es. Here I present results from 3D simulations which solve the full Navier-Stokes equations coupled to a flux-limited diffusion treatment of radiation transfer for planets with 1, 3, and 7 day periods. Variations in opacity amongst models leads to a variation in the temperature differential across the planet, while atmospheric dynamics becomes much more variable at longer orbital periods. I also present 3D radiative simulations illustrating the importance of distinguishing between optical and infrared opacities.
We present new models for the X-ray photoevaporation of circumstellar discs which suggest that the resulting mass loss (occurring mainly over the radial range 10-40 AU) may be the dominant dispersal mechanism for gas around low mass pre-main sequence
stars, contrary to the conclusions of previous workers. Our models combine use of the MOCASSIN Monte Carlo radiative transfer code and a self-consistent solution of the hydrostatic structure of the irradiated disc. We estimate the resulting photoevaporation rates assuming sonic outflow at the surface where the gas temperature equals the local escape temperature and derive mass loss rates of ~10^{-9} M_sun/yr, typically a factor 2-10 times lower than the corresponding rates in our previous work (Ercolano et al., 2008) where we did not adjust the density structure of the irradiated disc. The somewhat lower rates, and the fact that mass loss is concentrated towards slightly smaller radii, result from the puffing up of the heated disc at a few AU which partially screens the disc at tens of AU. (.....) We highlight the fact that X-ray photoevaporation has two generic advantages for disc dispersal compared with photoevaporation by extreme ultraviolet (EUV) photons that are only modestly beyond the Lyman limit: the demonstrably large X-ray fluxes of young stars even after they have lost their discs and the fact that X-rays are effective at penetrating much larger columns of material close to the star (abridged).
Clouds are expected to form in a wide range of conditions in the atmosphere of exoplanets given the large range of possible condensible species. However this diversity might lead to very different small-scale dynamics depending on radiative transfer
in various thermal conditions: we aim at providing some insights into these dynamical regimes. We perform an analytical linear stability analysis of a compositional discontinuity with a heating source term that depends on composition. We also perform idealized two-dimensional (2D) simulations of an opacity discontinuity in a stratified medium with the code ARK. We use a two-stream grey model for radiative transfer and explore the brown-dwarf and earth-like regimes. We reveal the existence of a Radiative Rayleigh-Taylor Instability (RRTI hereafter, a particular case of diabatic Rayleigh-Taylor instability) when an opacity discontinuity is present in a stratified medium. This instability is similar in nature to diabatic convection and relies only on buoyancy with radiative transfer heating and cooling. When the temperature is decreasing with height in the atmosphere, a lower-opacity medium on top of a higher-opacity medium is dynamically unstable while a higher-opacity medium on top of a lower-opacity medium is stable. This stability/instability behavior is reversed if the temperature is increasing with height. The existence of the RRTI could have important implications for the stability of the cloud cover of a wide range of planetary atmospheres. In our solar system, it could help explain the formation of mammatus cloud in Earth atmospheres and the existence of Venus cloud deck. Likewise, it suggests that stable and large scale cloud covers could be ubiquitous in strongly irradiated exoplanets but might be more patchy in low-irradiated or isolated objects like brown dwarfs and directly imaged exoplanets.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
