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Salaris M.,Liverpool John Moores University | Cassisi S.,INAF Osservatorio Astronomico di Collurania | Pietrinferni A.,INAF Osservatorio Astronomico di Collurania | Kowalski P.M.,Helmholtz Center Potsdam | And 2 more authors.
Astrophysical Journal | Year: 2010

We present a new set of cooling models and isochrones for both H- and He-atmosphere white dwarfs (WDs), incorporating accurate boundary conditions from detailed model atmosphere calculations, and carbon-oxygen chemical abundance profiles based on updated stellar evolution calculations from the BaSTI stellar evolution archive - a theoretical data center for the Virtual Observatory. We discuss and quantify the uncertainties in the cooling times predicted by the models, arising from the treatment of mixing during the central H- and He-burning phases, the number of thermal pulses experienced by the progenitors, progenitor metallicity, and the 12C(α, γ)16O reaction rate. The largest sources of uncertainty turn out to be related to the treatment of convection during the last stages of the progenitor central He-burning phase and the 12C(α, γ)16O reaction rate. We compare our new models to previous calculations performed with the same stellar evolution code, and discuss their application to the estimate of the age of the solar neighborhood and the interpretation of the observed number ratios between H- and He-atmosphere WDs. The new WD sequences and an extensive set of WD isochrones that cover a large range of ages and progenitor metallicities are made publicly available at the official BaSTI Web site. © 2010. The American Astronomical Society. Source


Bedin L.R.,US Space Telescope Science Institute | Salaris M.,Liverpool John Moores University | King I.R.,University of Washington | Piotto G.,University of Padua | And 2 more authors.
Astrophysical Journal Letters | Year: 2010

We use 10 orbits of Advanced Camera for Surveys observations to reach the end of the white dwarf (WD) cooling sequence in the solar-metallicity open cluster NGC 2158. Our photometry and completeness tests show that the end falls at magnitude mF606W = 27.5±0.15, which implies an age between ∼1.8 and ∼2.0 Gyr, consistent with the age of 1.9±0.2 Gyr obtained from fits to the main-sequence turn-off. The faintest WDs show a clear turn toward bluer colors, as predicted by theoretical isochrones. © 2010. The American Astronomical Society. All rights reserved.. Source


Calamida A.,National institute for astrophysics | Bono G.,University of Rome Tor Vergata | Corsi C.E.,National institute for astrophysics | Iannicola G.,National institute for astrophysics | And 10 more authors.
Astrophysical Journal Letters | Year: 2011

We present a new theoretical calibration of the Strömgren metallicity index hk using α-enhanced evolutionary models transformed into the observational plane by using atmosphere models with the same chemical mixture. We apply the new metallicity-index-color (MIC) relations to a sample of 85 field red giants (RGs) and find that the difference between photometric estimates and spectroscopic measurements is on average smaller than 0.1dex with a dispersion of σ= 0.19 dex. The outcome is the same if we apply the MIC relations to a sample of eight RGs in the bulge globular cluster NGC 6522, but the standard deviation ranges from 0.26 (hk, v-y) to 0.49 (hk, u-y). The difference is mainly caused by a difference in photometric accuracy. The new MIC relations based on the Ca-y color provide metallicities systematically more metal-rich than the spectroscopic ones. We found that the Ca band is affected by Ca abundance and possibly by chromospheric activity. © 2011. The American Astronomical Society. All rights reserved.. Source


Cristallo S.,University of Granada | Piersanti L.,INAF Osservatorio Astronomico di Collurania | Straniero O.,INAF Osservatorio Astronomico di Collurania | Gallino R.,INAF Osservatorio Astronomico di Collurania | And 6 more authors.
Astrophysical Journal, Supplement Series | Year: 2011

By using updated stellar low-mass stars models, we systematically investigate the nucleosynthesis processes occurring in asymptotic giant branch (AGB) stars. In this paper, we present a database dedicated to the nucleosynthesis of AGB stars: FRANEC Repository of Updated Isotopic Tables & Yields (FRUITY). An interactive Web-based interface allows users to freely download the full (from H to Bi) isotopic composition, as it changes after each third dredge-up (TDU) episode and the stellar yields the models produce. A first set of AGB models, having masses in the range 1.5 ≤M/M ≤ 3.0 and metallicities 1 × 10-3 ≤ Z ≤ 2 × 10-2, is discussed. For each model, a detailed description of the physical and the chemical evolution is provided. In particular, we illustrate the details of the s-process and we evaluate the theoretical uncertainties due to the parameterization adopted to model convection and mass loss. The resulting nucleosynthesis scenario is checked by comparing the theoretical [hs/ls] and [Pb/hs] ratios to those obtained from the available abundance analysis of s-enhanced stars. On the average, the variation with the metallicity of these spectroscopic indexes is well reproduced by theoretical models, although the predicted spread at a given metallicity is substantially smaller than the observed one. Possible explanations for such a difference are briefly discussed. An independent check of the TDU efficiency is provided by the C-stars luminosity function. Consequently, theoretical C-stars luminosity functions for the Galactic disk and the Magellanic Clouds have been derived. We generally find good agreement with observations. © 2011. The American Astronomical Society. All rights reserved. Source

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