EDP Sciences
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Volume 434, Number 1, April IV 2005
Page(s) 275 - 300
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20047060

A&A 434, 275-300 (2005)
DOI: 10.1051/0004-6361:20047060

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

I. Determination of [Th/Eu] abundance ratios
E. F. del Peloso1, L. da Silva1 and G. F. Porto de Mello2

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

(Received 12 January 2004 / Accepted 15 December 2004 )

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. A stellar sample of 20 disk dwarfs/subgiants of F5 to G8 spectral types with $-0.8\le{\rm [Fe/H]}\le+0.3$ was selected. In stars with such spectral types and luminosity classes, spectral synthesis techniques must be employed if we wish to achieve acceptably accurate results. An homogeneous, self-consistent set of atmospheric parameters was determined. Effective temperatures were determined from photometric calibrations and H$\alpha$ profile fitting; surface gravities were obtained from $T_{{\rm eff}}$, stellar masses and luminosities; microturbulence velocities and metallicities were obtained from detailed, differential spectroscopic analysis, relative to the Sun, using equivalent widths of $\ion{Fe}{i}$ and $\ion{Fe}{ii}$ lines. Chemical abundances of the elements that contaminate the Th and Eu spectral regions (Ti, V, Cr, Mn, Co, Ni, Ce, Nd, and Sm) were determined through spectroscopic analysis. Abundance uncertainties were thoroughly scrutinised, their average value - ( $0.10\pm0.02$) dex - being found to be satisfactorily low. Eu and Th abundances were determined by spectral synthesis of one $\ion{Eu}{ii}$ line (4129.72 Å) and one $\ion{Th}{ii}$ line (4019.13 Å), taking into account the detailed hyperfine structures of contaminating Co lines, as well as the hyperfine structure and isotope shift of the Eu line. Comparison of our abundances with literature data shows that our results exhibit a similar behaviour, but a considerably lower scatter (36% lower for Eu, and 61% lower for Th). The [Th/Eu] abundance ratios thus obtained were used, in the second paper of this series, to determine the age of the Galactic disk.

Key words: Galaxy: disk -- Galaxy: evolution -- stars: late-type -- stars: fundamental parameters -- stars: abundances

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