ANALYSIS OF A HUBBLE-SPACE-TELESCOPE SEARCH FOR RED DWARFS - LIMITS ON BARYONIC MATTER IN THE GALACTIC HALO

We reexamine a deep Hubble Space Telescope pencil-beam search for red dwarfs, stars just massive enough to burn hydrogen. The authors of thi s search (Bahcall et al.) found that red dwarfs make up less than 6% o f the Galactic halo. First, we extrapolate this result to include brow n dwarfs, stars not quite massive enough to burn hydrogen; we assume a 1/M mass function. Then the total mass of red dwarfs and brown dwarfs is less than or equal to 18% of the halo. This result is consistent w ith microlensing results, assuming a popular halo model.However, using new stellar models and parallax observations of low-mass, low-metalli city stars, we obtain much tighter bounds on low-mass stars. We find t he halo red dwarf density to be less than 1% of the halo, while our be st estimate of this value is 0.14%-0.37%. Thus, our estimate of the ha lo mass density of red dwarfs drops to 16-40 times less than the resul t reported by Bahcall et al. in 1994. For a 1/M mass function, this su ggests a total density of red dwarfs and brown dwarfs of similar to 0. 25%-0.67% of the halo, i.e., (0.9-2.5) x 10(9) M. out to 50 kpc. Such a low result would conflict with microlensing estimates by the MACHO g roup. We suggest that either the halo mass function must rise very ste eply below the hydrogen-burning limit or the microlensing results shou ld be reinterpreted with a different halo model or mass function.