Evolution and Infrared Spectra of Brown Dwarfs

The recent detection of a subluminous companion to the M dwarf star VB 8 has renewed interest in the characteristics of self-luminous objects spanning the mass range from Jupiter to hydrogen-burning stars. The authors have constructed atmospheric and interior models of objects up to 0.05 M_sun;, to understand the relationship of "brown dwarfs" such as the VB 8 companion both to low-mass stars and to the well-studied Jovian planets. Model atmospheres composed of a range of likely gaseous and particulate opacity sources have been calculated both to provide a boundary condition for interior temperature-pressure profiles and to determine the emergent spectra for such objects. The interior model incorporates our current best understanding of the behavior of hydrogen and helium at very high pressures and provides cooling time scales for objects of varying mass and composition. The results of these calculations are used to derive the luminosity and (assuming a rough age) the mass of VB 8B. The authors find VB 8B to be most probably a 0.05 M_sun; object with effective temperature in the range 1200 - 1500K and an atmosphere which very likely contains particulate absorbers. Finally, key changes in chemical oxidation state and condensation of major constituents during the evolution of brown dwarfs are presented, and the giant planets of the solar system are placed in context in the evolutionary scheme.