M-DWARFS, MICROLENSING, AND THE MASS BUDGET OF THE GALAXY

We show that faint red stars do not contribute significantly to the ma ss budget of the Galaxy or to microlensing statistics. Our results are obtained by analyzing two long exposures of a high-latitude field tak en with the Wide Field Camera on the newly repaired Hubble Space Teles cope. Stars are easily distinguished from galaxies essentially to the limiting magnitudes of the images. We find five stars with 2.0 < V - I < 3.0 and I < 25.3 and no stars with V - I > 3.0. Therefore, main-seq uence stars with M(I) > 10 that are above the hydrogen-burning limit i n the dark halo or the spheroid contribute less than 6% of the unseen matter. Faint red disk stars, M-dwarfs, contribute at most 15% to the mass of the disk. We parameterize the faint end of the cumulative dist ribution of stars, PHI, as a function of luminosity L(v), dPHI/dln L(v ) is-proportional-to L(v)-gamma. For spheroid stars, gamma < 0.32 over the range 6 < M(v) < 17, with 98% confidence. The disk luminosity fun ction falls, gamma < 0, for 15 approximately-less-than M(V) approximat ely-less-than 19. Faint red stars in the disk or thick disk, and stars with M(v) < 16 in the spheroid contribute tau < 10(-8) to the optical depth to microlensing toward the Large Magellanic Cloud.