The initial mass function for very low mass stars in the Hyades

We have computed theoretical luminosity functions at various evolutionary ages for stars and substellar objects (brown dwarfs), spanning the mass range 0.03 < M < 0.2 M_⊙. These functions constrain the distribution of very low mass objects in a star cluster of known age. In this Letter we compare our calculations with a 1988-1989 survey of faint members of the Hyades cluster by Leggett and Hawkins, a cluster whose age is 6 × 10⁸ yr. The comparison shows that the survey does not reach sufficiently low luminosities to reveal brown dwarfs. We obtain a strong constraint on the initial mass function (IMF) for very low mass stars in the Hyades and infer that its IMF does not increase with decreasing mass for the mass interval investigated here. Our results imply at most a moderate contribution from brown dwarfs to the cluster mass, and to the Galaxy's mass if the Hyades are representative of the Galaxy as a whole.