HALO STARS IN THE IMMEDIATE SOLAR NEIGHBORHOOD

We have searched the Fourth Catalogue of Nearby Stars for halo stars a nd identified 15 subdwarfs and a high velocity white dwarf in the sola r neighbourhood. This search was motivated mainly by the recent determ inations of MACHO masses of about 0.5M., which are typical for halo dw arfs. The local mass density of these stars is 1.5.10(-4) M. pc(-3), w hich is only 3% of the current estimate of the local mass density of t he MACHO population. We compare the local density of subdwarfs with co nstraints set by HST observations of distant red dwarfs. Using models of the stellar halo with density laws that fall off like r(-alpha), al pha=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 a bout 0.6. The non-detection of the analogues of MACHOs in the solar ne ighbourhood allows to set an upper limit to the luminosity of MACHOs o f M-B > 21 magnitudes.