Evolution of very low mass stars and brown dwarfs. I - The minimum main-sequence mass and luminosity

Abstract

The hydrogen burning minimum mass (HBMM) is determined for solarcomposition by performing some numerical computations. Pre-main sequenceevolution is followed for masses down to 0.04 solar mass, from initialmodels having central temperatures of 200,000 K, through deuteriumburning, until either stable hydrogen burning is reached or the objecthas cooled down to roughly 10 to the -5th solar luminosity. Theopacities are interpolated among the recent opacity tables by Alexander,Johnson, and Rypma (1983), which include a number of important moleculesbelow 4000 K and grain formation below 1500 K. The HBMM turns out to beslightly smaller than predicted with Co and Stewart (1970) opacities,but mainly displays a luminosity ten times smaller. A physicalinterpretation of this result is presented, and the number of browndwarfs expected to be detectable near the sun is discussed.