Where are the halo stars?

In the usual and most widespread textbook picture of the Milky Way Galaxy, disk stars like the Sun are referred to as Population I, the spheroidal or halo component in turn as Population H. The latter is thought of as the pressure-supported, metal-poor relic of the early Galaxy, with renewed interest in recent years in the search for dark matter via microlensing. Modelling the putative massive compact halo objects however, faces the problem that the stellar halo is generally considered to consist of only a few billion solar masses. Here we present observational evidence that even this low budget may be a factor ten too high. If so, this immediately implies that the classical population II of halo stars is fairly irrelevant, not only in the dark matter context, but, in particular, in models of the formation and evolution of the Milky Way Galaxy. (C) 2002 Elsevier Science B.V. All rights reserved.