The age of the Galactic thin disk from Th/Eu nucleocosmochronology III. Extended sample

Abstract

The first determination of the age of the Galactic thin disk from Th/Eu nucleocosmochronology was accomplished by us in Papers I and II. The present work aimed at reducing the age uncertainty by expanding the stellar sample with the inclusion of seven new objects - an increase by 37%. A set of [Th/Eu] abundance ratios was determined from spectral synthesis and merged with the results from Paper I. Abundances for the new, extended sample were analyzed with the aid of a Galactic disk chemical evolution (GDCE) model developed by us is Paper II. The result was averaged with an estimate obtained in Paper II from a conjunction of literature data and our GDCE model, providing our final, adopted disk age T_G = (8.8 +/- 1.7) Gyr with a reduction of 0.1 Gyr (6%) in the uncertainty. This value is compatible with the most up-to-date white dwarf age determinations (<~ 10 Gyr). Considering that the halo is currently presumed to be (13.5 +/- 0.7) Gyr old, our result prompts groups developing Galactic formation models to include an hiatus of (4.7 +/- 1.8) Gyr between the formation of halo and disk.

7 pages, 5 Postscript figures, accepted for publication in Astronomy & Astrophysics