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

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Abstract
We reexamine a deep Hubble Space Telescope pencil-beam search for red dwarfs, stars just massive enough to burn hydrogen. The authors of this search (Bahcall et al.) 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/ mass function. Then the total mass of red dwarfs and brown dwarfs is ≤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 less than 1% 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 result reported by Bahcall et al. in 1994. For a 1/ mass function, this suggests a total density of red dwarfs and brown dwarfs of ~0.25%-0.67% of the halo, i.e., (0.9-2.5) × 109 ☉ out to 50 kpc. Such a low result would conflict with microlensing estimates by the MACHO group. We suggest that either the halo mass function must rise very steeply below the hydrogen-burning limit or the microlensing results should be reinterpreted with a different halo model or mass function.

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