Models of Stars, Brown Dwarfs and Exoplanets

Within the next few years, GAIA and several instruments aiming at imag- ing extrasolar planets will see first light. In parallel, low mass planets are being searched around red dwarfs which offer more favourable conditions, both for radial velocity de- tection and transit studies, than solar-type stars. Authors of the model atmosphere code which has allowed the detection of water vapour in the atmosphere of Hot Jupiters re- view recent advancement in modelling the stellar to substellar transition. The revised solar oxygen abundances and cloud model allow for the first time to reproduce the pho- tometric and spectroscopic properties of this transition. Also presented are highlight results of a model atmosphere grid for stars, brown dwarfs and extrasolar planets.Comment: Refereed paper submitted to the british journal Philosophical Transactions A as an invited review to the Theo Murphy Meeting entitled "Water in the gas phase" held by the Kavli Royal Society in Chichely, GB, June 13-14th 201

Alkali Line Profiles in Degenerate Dwarfs

Ultracool stellar atmospheres show absorption by alkali resonance lines severely broadened by collisions with neutral perturbers. In the coolest and densest atmospheres, such as those of T dwarfs, Na I and K I broadened by molecular hydrogen and helium can come to dominate the entire optical spectrum. Their profiles have been successfully modelled with accurate interaction potentials in the adiabatic theory, computing line profiles from the first few orders of a density expansion of the autocorrelation function. The line shapes in the emergent spectrum also depend on the distribution of absorbers as a function of depth, which can be modelled with improved accuracy by new models of dust condensation and settling. The far red K I wings of the latest T dwarfs still show missing opacity in these models, a phenomenon similar to what has been found for the Na I line profiles observed in extremely cool, metal-rich white dwarfs. We show that the line profile in both cases is strongly determined by multiple-perturber interactions at short distances and can no longer be reproduced by a density expansion, but requires calculation of the full profile in a unified theory. Including such line profiles in stellar atmosphere codes will further improve models for the coolest and densest dwarfs as well as for the deeper atmosphere layers of substellar objects in general.Comment: VI Serbian Conference on Spectral Line Shapes in Astrophysics; to be published by the American Institute of Physics, eds. Milan S. Dimitrijevic and Luka C. Popovic; 6 pages, 6 figure

The role of convection, overshoot, and gravity waves for the transport of dust in M dwarf and brown dwarf atmospheres

Observationally, spectra of brown dwarfs indicate the presence of dust in their atmospheres while theoretically it is not clear what prevents the dust from settling and disappearing from the regions of spectrum formation. Consequently, standard models have to rely on ad hoc assumptions about the mechanism that keeps dust grains aloft in the atmosphere. We apply hydrodynamical simulations to develop an improved physical understanding of the mixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres, in particular of the influence of the underlying convection zone. We performed 2D radiation hydrodynamics simulations including a description of dust grain formation and transport with the CO5BOLD code. The simulations cover the very top of the convection zone and the photosphere including the dust layers for effective temperatures between 900K and 2800K, all with logg=5 assuming solar chemical composition. Convective overshoot occurs in the form of exponentially declining velocities with small scale heights, so that it affects only the region immediately above the almost adiabatic convective layers. From there on, mixing is provided by gravity waves that are strong enough to maintain thin dust clouds in the hotter models. With decreasing effective temperature, the amplitudes of the waves become smaller but the clouds become thicker and develop internal convective flows that are more efficient in mixing material than gravity waves. The presence of clouds leads to a highly structured appearance of the stellar surface on short temporal and small spatial scales. We identify convectively excited gravity waves as an essential mixing process in M dwarf and brown dwarf atmospheres. Under conditions of strong cloud formation, dust convection is the dominant self-sustaining mixing component

Parallel Implementation of the PHOENIX Generalized Stellar Atmosphere Program

We describe the parallel implementation of our generalized stellar atmosphere and NLTE radiative transfer computer program PHOENIX. We discuss the parallel algorithms we have developed for radiative transfer, spectral line opacity, and NLTE opacity and rate calculations. Our implementation uses a MIMD design based on a relatively small number of MPI library calls. We report the results of test calculations on a number of different parallel computers and discuss the results of scalability tests.Comment: To appear in ApJ, 1997, vol 483. LaTeX, 34 pages, 3 Figures, uses AASTeX macros and styles natbib.sty, and psfig.st