COOL ZERO-METALLICITY STELLAR ATMOSPHERES

Very low mass stars and brown dwarfs have low effective temperatures a nd their atmospheric opacities are provided by heavy elements which fo rm a host of strongly absorbing molecules. We present a study of the a tmospheres and emergent spectra of these objects in the limiting case of zero metallicity. A grid of atmosphere models is presented, based o n updated opacities for H/He mixtures, in particular for the collision -induced absorption by H-2 which plays an important role in these atmo spheres. The emergent spectra bear little resemblance to the correspon ding blackbody emission and become bluer as the effective temperature is lowered. The atmosphere models are coupled with interior calculatio ns to obtain accurate physical and photometric characteristics for the low-mass end of the zero-metallicity main sequence. This provides a u seful boundary in the Hertzsprung Russel diagram to interpret observat ions of low-metallicity stars.