Chemical Evolution of the Galaxy: the G-dwarf problem and radioactive chronology revisited taking account of the Thick Disk

Abstract

Thin and Thick disk metallicities overlap, but alpha(Fe) relations differ; there is also a hiatus in time before the first thin-disk stars, with a minimum metallicity [Fe/H] = -0.6. Thus there are two `G-dwarf' problems. We fit metallicity distribution functions (MDFs) for the two disks derived by Wyse & Gilmore (1995) with analytical models (for O and Mg). The Thick disk fits a simple inflow model with effective yield and final [Mg/H] about the same as for the Thin Disk, excluding continuous mass loss from the former as a source of prompt initial enrichment (PIE) for the latter. The Thin disk has a narrow MDF fitted (rather poorly) with a closed model having PIE from some other source, or more elegantly with a slow infall model similar to the `two-inflow' scenario of Matteucci et al., but with a PIE and a severe break in the age-metallicity relation. Implications for radio-active cosmochronology are investigated.

World Scientific LaTeX, 6 pages, 4 Postscript figures To appear in `Cosmic Evolution' Conference at IAp, Paris, honoring Jean Audouze and Jim Truran, 13-17 Nov 2000