THE MASS FUNCTION OF LOW-MASS HALO STARS - LIMITS ON BARYONIC HALO DARK-MATTER

We derive mass functions (MF) for halo red dwarfs (the faintest hydrog en burning stars), and then we extrapolate to place limits on the tota l mass of halo brown dwarfs (stars not quite massive enough to burn hy drogen). The mass functions are obtained from the luminosity function of a sample of 114 local halo stars in the US Naval Observatory parall ax survey (Dahn et al.). We use stellar models of Alexander et al., an d we make varying assumptions about metallicity, as well as possible u nresolved binaries in the sample. We find that the MF for halo red dwa rfs cannot rise more quickly than 1 m(-2) as one approaches the hydrog en burning limit. Using recent results from star formation theory, we extrapolate the MF into the brown dwarf regime. We see that likely ext rapolations imply that the total mass of brown dwarfs in the halo is l ess than similar to 3% of the local mass density of the halo (similar to 0.3% for the more realistic models we consider). Our limits apply t o brown dwarfs in the halo that come from the same stellar population as the red dwarfs.