NEUTRON-STARS AND WHITE-DWARFS IN GALACTIC HALOS

We consider the possibility that galactic halos are composed of stellar remnants such as neutron stars and white dwarfs. On the basis of a simple model for the evolution of galactic halos, we follow the history of halo matter, luminosity, and metal and helium abundances. We assume conventional yields for helium and the heavier elements. By comparing with the observational constraints, which may be considered as fairly conser-vative, we find that, for an exponentially decreasing star-formation rate (SFR) with e-folding time ??, only values between 6 ?? 108 ≲ ?? ≲ 2 ?? 109 yr are allowed together with a very limited range of masses for the initial mass function (IMF). Star formation is allowed for 2 M??? ≲ m ≲ 8 M??? if ?? = 2 ?? 109 yr, and for 4 M??? ≲ m ≲ 6 M??? if ?? = 109 yr. For ?? = 6 ?? 108 yr, the lower and upper mass limits merge to ???5 M???. We conclude that, even though the possibility of neutron stars as halo matter may be ruled out, that of white dwarfs may still be a viable hypothesis, though with very stringent constraints on allowed parameters, that merits further consideration.open544