14 research outputs found
A Link Between the Semi-Major Axis of Extrasolar Gas Giant Planets and Stellar Metallicity
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 their unexpected semi-major axes and eccentricities. We
propose a model, which correlates the metallicity of the host star with the
original semi-major axis of its most massive planet, prior to migration,
considering that the core accretion scenario governs giant gas planet
formation. The model predicts that the optimum regions for planetary formation
shift inward 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 also hints that the lack of planets
circling metal-poor stars may be partly caused by an enhanced destruction
probability during the migration process, since the planets lie initially
closer to the central stars.Comment: Nature of the replacement: According to recent simulations, the
temperature profile, T, is more adequately reproduced by beta = 1 rather than
beta = 2. We have introduced a distance scale factor that solves the very
fast drop of T for low metallicity and introduces naturally the inferior
distance limit of our ZAPO. Under this modification all the fitting process
was altere
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