The Galactic Disk Mass Function: Reconciliation of the ITALHubble Space Telescope/ITAL and Nearby Determinations

We derive and parametrize the Galactic mass function (MF) below 1 characteristic of both single objects and binary systems. We resolve the long standing discrepancy between the MFs derived from the HST and from the nearby luminosity functions, respectively. We show that this discrepancy stemmed from two cumulative effects, namely (i) incorrect color-magnitude determined distances, due a substantial fraction of M dwarfs in the HST sample belonging to the metal-depleted, thick-disk population, as corrected recently by Zheng et al. and (ii) unresolved binaries. We show that both the nearby and HST MF for unresolved systems are consistent with a fraction 50% of M-dwarf binaries, with the mass of both the primaries and the companions originating from the same underlying single MF. This implies that 30% of M dwarfs should have an M dwarf companion and 20% should have a brown dwarf companion, in agreement with recent determinations. The present calculations show that the so-called "brown-dwarf desert" should be reinterpreted as a lack of high mass-ratio (m₂/m₁ 0. 1) systems, and does not preclude a substantial fraction of brown dwarfs as companions of M dwarfs or for other brown dwarfs.