Annu. Rev. Astron. Astrophys. 1997. 35: 503-556 Copyright © 1997 by Annual Reviews. All rights reserved

4.2. The Halo

The Galactic halo does not appear to suffer from a severe G-dwarf problem (Laird et al 1988, Pagel 1989, Beers et al 1992). The halo metallicity ranges from -4 dex to just below the solar value, with a mean of ~ -1.6 (Laird et al 1988, Hartwick 1976), Hartwick (1976) noted that this low metallicity suggested that either the halo yield was much lower than in the disk or that gas was removed from halo star formation (e.g. Ostriker & Thuan 1975). The favored model is that the halo lost its gas before chemical evolution could go to completion. Carney et al (1990), Wyse & Gilmore (1992) suggested that the missing spheroid mass fell to the center of the Galaxy and contributed most of the bulge mass, based on angular momentum considerations.

Whether or not there is a minimum metallicity level, below which stars do not exist, has been debated for at least 20 years. Hartquist & Cameron (1977) predicted that there was an era of "pregalactic nucleosynthesis" by very massive zero metallicity objects; as a result, the Galactic halo would have formed with a non-zero metal content.

Bond (1981), Cayrel (1987) claimed that there is a paucity of stars below [Fe/H] ~ -3 relative to a Simple one-zone model of chemical evolution; this was attributed to a reduced efficiency of forming low-mass stars at low metallicity. Indeed several theoretical investigations (e.g. Kahn 1974, Wolfire & Cassinelli 1987, Yoshii & Saio 1986, Uehara et al 1996) have predicted that at low metallicity the IMF is skewed to high-mass stars. Contrary to Bond's suggestion, the huge increase in the number of known metal-poor halo stars (e.g. Beers et al 1985, 1992) led to agreement between the observed metallicity function and predictions from modified Simple models (Beers et al 1985, 1992, Laird et al 1988, Ryan & Norris 1991) down to the lowest measurable abundance, consistent with no metallicity dependence of the IMF.

Audouze & Silk (1995) claimed that there is a lower limit to the metallicity that can form stars, based on predictions concerning the amount of material that can dilute and cool SN ejecta; they estimated the lower limit to be approximately [Fe/H] ~ -4.

The most metal-poor star presently known is CD -38 245, at [Fe/H] = -4.01 (McWilliam et al 1995a, b), although it only narrowly beats CS 22949-037 for the record, at [Fe/H] = -3.99. This iron abundance for CD -38 245 is supported by Gratton & Sneden (1988), who found [Fe/H] = -3.97, but it is higher than the metallicity of Bessell & Norris (1984), who found [Fe/H] = -4.5. Norris et al (1993) also analyzed stars from the list of Beers et al (1992), one of which was CS 22885-096, with a measured [Fe/H] = -4.24. McWilliam et al (1995a) found [Fe/H] = -3.79 for this star and explained the difference as due to systematic analysis effects of 0.4 dex; if applied to the Bessell & Norris (1984) result, the same zero point would bring all three analyses into agreement at [Fe/H] = -4.0 for CD -38 245.

Thus, despite the heroic effort by George Preston of searching for metal-poor stars by visually inspecting over one million objective prism spectra (Beers et al 1985), the honor of the most metal-poor star known in the Galaxy still belongs to CD -38 245.