MACHOs in the solar neighbourhood

Abstract

We rediscuss the question of what fraction of the population of MACHOs in the halo of the Milky Way might be in the form of halo dwarfs, because recent determinations indicate masses of the MACHOs, which are typical for such stars. For this purpose we have analyzed the Third Catalogue of Nearby Stars and identified the subdwarfs and a high velocity white dwarf in the solar neighbourhood. The local mass density of these stars is 1.5*10^{-4} M_sun/pc^3, which is only 3% of the current estimate of the local mass density of the MACHO population. We compare the local density of subdwarfs with constraints set by HST observations of distant red dwarfs. Using models of the stellar halo with density laws that fall off like r^{-alpha}, alpha = 3.5 to 4, we find that the HST constraints can only be matched, if we assume that the stellar halo is flattened with an axial ratio of about 0.6. The non-detection of the analogs of MACHOs in the solar neighbourhood allows to set an upper limit to the luminosity of MACHOs of M_B > 21 magnitudes.