Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo

We re-examine a deep {\it Hubble Space Telescope} pencil-beam search for red dwarfs, stars just massive enough to burn Hydrogen. The authors of this search (Bahcall, Flynn, Gould & Kirhakos 1994) found that red dwarfs make up less than 6% of the galactic halo. First, we extrapolate this result to include brown dwarfs, stars not quite massive enough to burn hydrogen; we assume a $1\mathrm{/}\mathcal{M}$ mass function. Then the total mass of red dwarfs and brown dwarfs is $\le$18% of the halo. This result is consistent with microlensing results assuming a popular halo model. However, using new stellar models and parallax observations of low mass, low metallicity stars, we obtain much tighter bounds on low mass stars. We find the halo red dwarf density to be $<1\mathrm{\%}$ of the halo, while our best estimate of this value is 0.14-0.37%. Thus our estimate of the halo mass density of red dwarfs drops to 16-40 times less than the reported result of Bahcall et al (1994). For a $1\mathrm{/}\mathcal{M}$ mass function, this suggests a total density of red dwarfs and brown dwarfs of $\sim$0.25-0.67% of the halo, \ie , $(0.9-2.5)\times {10}^9\text{msun}$ out to 50 kpc. Such a low result would conflict with microlensing estimates by the \macho\ group (Alcock \etal 1995a,b).