The age of the Galactic thin disk from Th/Eu nucleocosmochronology

III. Extended sample

¹ Observatório Nacional/MCT, Rua General José Cristino 77, 20921-400 Rio de Janeiro, Brazil e-mail: [epeloso;licio]@on.br
² Observatório do Valongo/UFRJ, Ladeira do Pedro Antônio 43, 20080-090 Rio de Janeiro, Brazil e-mail: [gustavo;lilia]@ov.ufrj.br

Received: 25 April 2005
Accepted: 9 June 2005

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 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 old, our result prompts groups developing Galactic formation models to include an hiatus of between the formation of halo and disk.