M dwarfs, microlensing, and the mass budget of the Galaxy

We show that faint red stars do not contribute significantly to the mass budget of the Galaxy or to microlensing statistics. Our results are obtained by analyzing two long exposures of a high-latitude field taken with the Wide Field Camera (WFC) on the newly repaired Hubble Space Telescope (HST). Stars are easily distinguished from galaxies essentially to the limiting magnitudes of the images. We find five stars with 2.0 less than V - I less than 3.0 and I less than 25.3 and no stars with V - I greater than 3.0. Therefore, main-sequence stars with M(sub I) greater than 10 that are above the hydrogen-burning limit in 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 distribution of stars, Phi, as a function of luminosity L(sub V), d Phi/d ln L(sub V) proportional to L(sub V exp -gamma). For spheroid stars, gamma less than 0.32 over the range 6 less than M(sub V) less than 17, with 98% confidence. The disk luminosity function falls, gamma less than 0, for 15 approximately less than M(sub V) approximately less than 19. Faint red stars in the disk or thick disk, and stars with M(sub V) less than 16 in the spheroid contribute tau less than 10(exp -8) to the optical depth to microlensing toward the Large Magellanic Cloud.