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.

The fact that most extrasolar planets found to date are orbiting metal‐rich stars lends credence to the core accretion mechanism of gas giant planet formation over its competitor, the disc instability mechanism. However, the core accretion mechanism is not refined to the point of explaining orbital parameters such as the unexpected semimajor axes and eccentricities. We propose a model that correlates the metallicity of the host star with the original semimajor axis of its most massive planet, prior to migration, assuming that the core accretion scenario governs giant gas planet formation. The model predicts that the optimum regions for planetary formation shift inwards as stellar metallicity decreases, providing an explanation for the observed absence of long‐period planets in metal‐poor stars. We compare our predictions with the available data on extrasolar planets for stars with masses similar to the mass of the Sun. A fitting procedure produces an estimate of what we define as the zero‐age planetary orbit (ZAPO) curve as a function of the metallicity of the star. The model hints that the lack of planets circling metal‐poor stars may be partly caused by an enhanced destruction probability during the migration process, because the planets lie initially closer to their central star.

Abstract. 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 refuse, 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.Key words. Galaxy: disk -Galaxy: evolution -stars: late-type -stars: fundamental parameters -stars: abundances IntroductionCurrent estimates of the age of the Galactic thin disk 1 are obtained by dating the oldest open clusters or white dwarfs. Ages of open clusters are determined by fitting isochrones, and ages of white dwarfs are determined using cooling sequences. Isochrone and cooling sequence calculations require deep knowledge of stellar evolution, are very complex, and depend on a large number of physical parameters known at different levels of uncertainty. Many important aspects of stellar evolution, like the influence of rotation, are not well known, and may have a strong influence on the outcome of isochrone calculations. Furthermore, white dwarf cooling sequences also depend on calculations of degeneratematter physics. Nucleocosmochronology is a dating method that makes use of only a few results of main sequence stellar evolution models, therefore allowing a quasi-independent verification of the afore mentioned techniques.1 All references to the Galactic disk must be regarded, in this work, as references to the thin disk, unless otherwise specified.Nucleocosmochronology estimates timescales for astrophysical objects and events by using abundances of radioactive nuclides. These are compared with the abundances of their daughter nuclides, or of other nuclides that are created by the same or a similar nucleosynthetic process. Depending on the half-life of the chosen nuclide, different timescales can be probed. The Th/Eu chronometer, first proposed by Pagel (1989), is adequate to assess the age of the Galactic disk, since 232 Th is a radioactive nuclide with a 14.05 Gyr half-life (i.e., of the order of magnitude of the age being assessed). Eu is a satisfactory element for comparison, b...

A model is presented for the chemical evolution of the solar neighbourhood which takes into account three families of galactic objects, according to their condensation states: stars, refuses and gas. Stars are defined as every condensed objects with masses greater than or equal to the minimum mass which ignites hydrogen and which will give rise to an evolutionary track on the HR diagram to the left of Hayashi's limit; refuses include the remnants, which are compact objects resulting from stellar deaths, and the residues, which have masses not large enough to ignite hydrogen; gas is defined as the mass which can be condensed to form stars and/or residues. We have developed equations for the mass evolution of each family, and have studied the gas metallicity distribution within the framework of the instantaneous recycling approximation, adopting different initial conditions. In order to constrain the model parameters we have also used preliminary evaluations of comet cloud masses to investigate the role of the residues as sinks of heavy elements in the Galaxy.