Substellar mass function and maximum baryonic mass in the halo of the Galaxy

¹ Centre de Recherche Astrophysique de Lyon (UMR CNRS 142), Ecole Normale Supérieure - 69364 Lyon Cedex 07, France
² Département de Physique Théorique, CEA Saclay - 91191 Gif-sur-Yvette, France

Received: 23 October 1995
Accepted: 14 December 1995

Abstract

We use recently observed luminosity functions of halo M-dwarfs and an accurate mass-luminosity relationship to derive the mass function of Pop II low-mass stars in the galactic spheroid. All these luminosity functions yield rising mass functions near the hydrogen-burning limit (with ). Connecting the microlensing and star count constraints, we show that the mass function in the dark halo steepens significantly (). The minimum mass is found to be , which corresponds to an average mass , strongly suggesting that the dark objects responsible for the microlensing events are genuine brown dwarfs. We determine the expected spheroid and halo brown-dwarf density in the solar neighbourhood. The mass fraction under the form of substellar objects in the halo of the Galaxy is likely to represent only 5 to 15 % of the total dynamic mass.