Annu. Rev. Astron. Astrophys. 1981. 19: 77-113 Copyright © 1981 by Annual Reviews. All rights reserved

2.3. Metallicity Distribution

The narrow distribution of metallicities in long-lived stars, such as the G dwarfs (van den Bergh 1962, Schmidt 1963) or M dwarfs (Mould 1976), has been extensively discussed as a constraint on galactic enrichment models (e.g. Lynden-Bell 1975, Audouze & Tinsley 1976, Tinsley 1980, Pagel 1981); the G-dwarf data were discussed by Pagel & Patchett (1975), who found that the metallicities in a cylinder at right angles to the galactic plane fitted a Gaussian distribution with a mean [Fe/H] = - 0.3 relative to the Hyades (for which [Fe/H] = + 0.20 ± 0.10 relative to the Sun; Branch, Lambert & Tomkin 1980) and a dispersion in the raw data of about 0.25. After applying somewhat uncertain corrections for observational error and cosmic scatter (due to abundance gradients and perhaps other effects), Pagel & Patchett adopted for the intrinsic distribution due to the age-metallicity relation a truncated Gaussian with an effective dispersion between 0.075 and 0.18. Some implications for the past rate of star formation in the solar neighborhood have been discussed by Pagel (1981).

The above figures refer to disk stars, which form the vast majority of stars in the solar neighborhood. The distribution in the halo is relatively much wider (Hartwick 1976, Searle & Zinn 1978) and is closer in form to that predicted by the "simple" model of galactic chemical evolution. According to Bessell (1977) and Carney & Peterson (1980), stars can be found with [Fe/H] < - 3, but Bond (1980) finds the number of such stars to be very small, suggesting that a tiny degree of "prompt initial enrichment" may have preceded the formation of any low-mass stars even in the halo.