The age of the Galactic thin disk from Th/Eu nucleocosmochronology II. Chronological analysis

The purpose of this work is to resume investigation of Galactic thin disk dating using nucleocosmochronology with Th/Eu stellar abundance ratios, a theme absent from the literature since 1990. [Th/Eu] abundance ratios for a sample of 20 disk dwarfs/subgiants of F5 to G8 spectral type with -0.8 <= [Fe/H] <= +0.3, determined in the first paper of this series, were adopted for this analysis. We developed a Galactic chemical evolution model that includes the effect of refuses, which are composed of stellar remnants (white dwarfs, neutron stars and black holes) and low-mass stellar formation residues (terrestrial planets, comets, etc.), contributing to a better fit to observational constraints. Two Galactic disk ages were estimated, by comparing literature data on Th/Eu production and solar abundance ratios to the model ((8.7 +5.8-4.1) Gyr), and by comparing [Th/Eu] vs. [Fe/H] curves from the model to our stellar abundance ratio data ((8.2 +/- 1.9) Gyr), yielding the final, average value (8.3 +/- 1.8) Gyr. This is the first Galactic disk age determined via Th/Eu nucleocosmochronology, and corroborates the most recent white dwarf ages determined via cooling sequence calculations, which indicate a low age (<~ 10 Gyr) for the disk.Comment: 9 pages, 7 Postscript figures, accepted for publication in Astronomy & Astrophysics, final versio

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

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.Comment: 7 pages, 5 Postscript figures, accepted for publication in Astronomy & Astrophysic