Bibcode
Rebolo, R.; Israelian, G.; García López, R. J.
Referencia bibliográfica
Galaxy Evolution: Connecting the Distant Universe with the Local Fossil Record. Proceedings of a Colloquium on this subject held at the Observatoire de Paris-Meudon from 21-25 September 1998. edited by Monique Spite, Observatoire de Paris, Meudon, France. Reprinted from Astrophysics and Space Science, 265:1-4 Kluwer Academic Publishers, Dordrecht, 1999, p.165
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1999
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Descripción
We have performed a detailed oxygen abundance analysis of 23 metal-poor
(-3.0 < [Fe/H] < -0.3) unevolved halo stars and one giant through
the OH bands in the near UV, using high-resolution echelle spectra.
Oxygen is found to be overabundant with respect to iron in these stars,
with the [O/Fe] ratio increasing from 0.6 to 1 between [Fe/H] = -1.5 and
-3.0. The behavior of the oxygen overabundance with respect to [Fe/H] is
similar to that seen in previous works based on the OI IR triplet data
(Abia & Rebolo 1989; Tomkin et al. 1992; Cavallo, Pilachowski, &
Rebolo 1997). Contrary to the previously accepted picture, our oxygen
abundances, derived from low-excitation OH lines, agree well with those
derived from high-excitation lines of the triplet. For nine stars in
common with Tomkin et al. we obtain a mean difference of 0.00 plus or
minus 0.11dex with respect to the abundances determined from the triplet
using the same stellar parameters and model photospheres. Our new
results show a smooth extension of the Edvardsson et al.'s (1993) [O/Fe]
versus metallicity curve to much lower abundances. The oxygen abundances
of unevolved stars when compared with values in the literature for
giants of similar metallicity imply that the latter may have suffered a
process of oxygen depletion. It appears that unevolved metal-poor stars
are better tracers of the early chemical evolution of the Galaxy. The
extrapolation of our results to very low metallicities indicates that
the ratio of oxygen to iron emerging from the first Type II SNe in the
early Galaxy was indeed close to unity. The higher [O/Fe] ratios we find
in dwarfs has an impact on the age determination of globular clusters,
and suggest that current age estimates have to be reduced by about 1-2
Gyr.