Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo
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/ M 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/ M 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) x 109 M ⊙ 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 1996
- DOI:
- arXiv:
- arXiv:astro-ph/9507097
- Bibcode:
- 1996ApJ...456L..49G
- Keywords:
-
- COSMOLOGY: DARK MATTER;
- GALAXY: STELLAR CONTENT;
- GALAXY: HALO;
- STARS: LOW-MASS;
- BROWN DWARFS;
- STARS: POPULATION II;
- Astrophysics
- E-Print:
- 13 pages, 2 figures. Figure one only available via fax or snail-mail To be published in ApJL. fig. 2 now available in postscript. Some minor changes in dealing with disk forground. Some cosmetic changes. Updated references
