The contribution of red dwarfs and white dwarfs to the halo dark matter

Abstract

Context. The nature of the several microlensing events observed by the MACHO team towards the LMC still remains controversial. Low-mass substellar objects and stars with masses larger than ∼1 Mhave been ruled out as major components of a MACHO Galactic halo, while stars of half solar masses are the most probable candidates. Aims. We assess jointly the relative contributions of both red dwarfs and white dwarfs to the mass budget of the Galactic halo. Methods. We use a Monte Carlo simulator that incorporates up-to-date evolutionary sequences of both red dwarfs and white dwarfs as well as detailed descriptions of both our Galaxy and the LMC. We explore a complete mass range between 0.08 and 1 Mof possible microlensing candidates and we compare the synthetic populations obtained with our simulator with the results obtained by the MACHO and EROS experiments. Results. The contribution of the red-dwarf population is insufficient to explain the number of events claimed by the MACHO team; this is even though the optical depth measured for the white-dwarf population alone, increases by a factor of two, after the addition of the red-dwarf population. Conclusions. We find that the contribution to the halo dark matter of the entire population under study, is smaller than 10%, at the 95% confidence level.