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

The hydrogen burning minimum mass (HBMM) is determined for solar composition by performing some numerical computations. Pre-main sequence evolution is followed for masses down to 0.04 solar mass, from initial models having central temperatures of 200,000 K, through deuterium burning, until either stable hydrogen burning is reached or the object has cooled down to roughly 10 to the -5th solar luminosity. The opacities are interpolated among the recent opacity tables by Alexander, Johnson, and Rypma (1983), which include a number of important molecules below 4000 K and grain formation below 1500 K. The HBMM turns out to be slightly smaller than predicted with Co and Stewart (1970) opacities, but mainly displays a luminosity ten times smaller. A physical interpretation of this result is presented, and the number of brown dwarfs expected to be detectable near the sun is discussed.