Chemical Evolution of the Galaxy: the G-dwarf Problem and Radioactive Chronology Revisited Taking Account of the Thick Disk

Thin and Thick disk metallicities overlap, but α(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.