Self-consistent atmosphere modeling with cloud formation for low-mass stars and exoplanets

Context: Low-mass stars and extrasolar planets have ultra-cool atmospheres where a rich chemistry occurs and clouds form. The increasing amount of spectroscopic observations for extrasolar planets requires self-consistent model atmosphere simulations to consistently include the formation processes that determine cloud formation and their feedback onto the atmosphere. Aims: Complement the MARCS model atmosphere suit with simulations applicable to low-mass stars and exoplanets in preparation of E-ELT, JWST, PLATO and other upcoming facilities. Methods: The MARCS code calculates stellar atmosphere models, providing self-consistent solutions of the radiative transfer and the atmospheric structure and chemistry. We combine MARCS with DRIFT, a kinetic model that describes cloud formation in ultra-cool atmospheres (seed formation, growth/ evaporation, gravitational settling, convective mixing, element depletion). Results: We present a small grid of self-consistently calculated atmosphere models for $T_ \text{eff} = 2000 - 3000$ K with solar initial abundances and $\log(g) = 4.5$. Cloud formation in stellar and sub-stellar atmospheres appears for $T_\text{eff} < 2700$ K and has a significant effect on the structure and the spectrum of the atmosphere for $T_\text{eff} < 2400$ K. We have compared the synthetic spectra of our models with observed spectra and found that they fit the spectra of mid to late type M-dwarfs and early type L-dwarfs well. We also test DRIFT-MARCS for an example exoplanet and demonstrate that our simulations reproduce the Spitzer observations for WASP-19b rather well for $T_{\rm eff}=2600$ K, $\log(g)=3.2$ and solar abundances. Our model points at an exoplanet with a deep cloud-free atmosphere with a substantial day-night energy transport and no temperature inversion.