High mass planets and low mass stars

Abstract

There are three main ingredients in brown dwarf theoretical models: (i) The equation of state; (ii) The entropy equation, which relates the internal and atmospheric thermodynamic states; and (iii) The atmospheric boundary condition (the infrared opacity). It is argued that the first two ingredients are very well understood. The opacity is less well understood and the major unresolved problem. Simple scaling laws are described and discussed for the relationships between luminosity (L), mass, opacity and age (t), assuming no thermonuclear energy sources. In the limit of extreme degeneracy, L ∝ t^(-1.25). However, detectable brown dwarfs (including VB8B) are still significantly contracting (i.e. actual radius ~10% larger than the zero temperature limit). As a consequence, dlnL/dlnt ~ -1.0 to -1.1 at this epoch for VB8B. Large opacity increases the effect of non-degeneracy. Complicating factors in brown dwarf evolution (super- or sub-adiabaticity, Debye cooling, freezing, differentiation, variable opacity) are discussed but only the latter two seem like to be important.

Files

Additional details