Bringing the light collecting power, resolution, and extended wavelength coverage of
UVES/VLT to bear on the abundance analysis of CS 22949-37 has provided important new
results as well as refined the results of previous analyses. For the first time in
such a metal-poor star, we have measured the forbidden O I line at 630.0 nm: we found
.
We found a mild carbon enhancement
and a very
low
ratio, close to the equilibrium value
The elemental abundances of the extremely metal-poor stars
CS 22949-037 are very unusual. The strong enhancement of oxygen (not typical
for this very low metallicity) could be explained by pair-instability
supernovae, but the strong odd-even effect predicted in these models,
which is not observed, rules out these very massive objects.
The enhancement of O and C is explained by models of zero-metallicity (or
very metal-poor) core-collapse supernovae. The observed enhancement of
N has to be explained by the CN processing of C, in the supernova or
in the star itself (or its companion, if binary). The fair agreement between the observed
elemental abundances in CS 22949-037 and those predicted in the Z35Z
model of Woosley and Heger (private communication) suggests that the most likely interpretation
is that this star exhibits the ejecta of a single core-collapse supernova.
However, more complex scenarios, in which the combined ejecta of several progenitors
are responsible, are not excluded by the present data. Clearly,
interpretation of the exceptional pattern of the neutron-capture elements
in CS 22949-037 merits further study. The identification of other
extremely metal-poor stars that exhibit similar patterns would be most illuminating.
Acknowledgements
We thank A. Heger, S. Woosley, I. Baraffe, A. Chieffi and L. Limongi for useful discussions pertaining to the interpretation of the elemental abundance patterns in CS 22949-037, in particular A. Heger and S. Woosley for kindly providing the modified supernova yields and A. Chieffi and L. Limongi for sending us a copy of their paper in advance of publication their paper. We thank N. Jacquinet-Husson for helping to extract accurate wavelengths of the telluric lines from the GEISA database. We also thank the referee, J. Cowan, for useful comments that helped to improve the paper. J.A. and B.N. received partial support for this work from the Carlsberg foundation and the Danish Natural Science Research Council. T.B. acknowledges partial support of this work from the U.S. National Science Foundation, in the form of grants AST 00-98549 and AST 00-98508.
Copyright ESO 2002