BROWN DWARFS AS DARK GALACTIC HALOS

We examine the possibility that the dark matter in galactic halos can consist of a population of brown dwarf stars. The mass range for brown dwarfs is limited from above by the hydrogen burning limit (0.08 M⊙). Since the current theory of star formation suggests that deuterium burning is essential for the formation of stars, the allowed mass range is limited from below by the deuterium burning limit (0.015 M⊙). For any initial mass function of brown dwarfs within this mass range, we find that brown dwarf halos are consistent with presently available observations. While such brown dwarf solutions to the halo dark matter problem may obviate the need for nonbaryonic halos, they cannot be the explanation for the recently observed near-infrared background. We show that future satellite missions will either detect brown dwarf halos or place tight constraints on the allowed range of the initial mass function. Finally, we note that star formation in the early galaxy may not proceed as in present-day molecular clouds and hence may not require deuterium burning; in this case, the lower limit on the mass of brown dwarfs could be smaller and the corresponding radiative signature would be harder to detect.