A thesis submitted January 11, 2015 for the degree of Doctor of Philosophy and defended March 22, 2016.
The PhD School of Science
Faculty of Science,
Centre for Star and Planet Formation,
Niels Bohr Institute, University of Copenhagen
Uffe Gråe Jørgensen
Lars Astrup Buchhave
Modeling the Cloudy Atmospheres of Cool Stars, Brown Dwarfs and Hot Exoplanets
The search for exoplanets is one of the most exciting fields in astronomy and since the discovery of the first exoplanet orbiting a main sequence star in 1995, we have found thousands of new worlds beyond our own. The various techniques we use for the detection and characterization of exoplanets are based on very different principles, but they all have one important thing in common: The properties of an exoplanet cannot be determined without knowing the properties of its host star. It is therefore crucial that the stellar models linking the observations of a star to its properties are as precise as possible.
M-dwarfs are very attractive targets when searching for new exoplanets. Unfortunately, they are also very difficult to model since their temperatures are low enough for dust clouds to form in their atmospheres. The primary goal of this project is therefore to merge the model atmosphere code MARCS with the dust model code DRIFT, thus facilitating the computation of self-consistent cloudy atmosphere models that can be used to properly determine the stellar parameters of cool stars.
With this enhanced model atmosphere code we have created a small grid of cool, dusty atmosphere models ranging in effective temperatures from Teff = 2000 − 3000 K. We have found that dust formation appears in models with Teff < 2700 K and can have a significant effect on the structure and the spectrum of the atmosphere. 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 have also illustrated how these models - with additional development into the regime of exoplanet atmospheres - can be compared with spectrum observations to characterize the atmospheres of exoplanets.