The subgiant branch of omega Cen seen through high-resolution spectroscopy. II. The most metal-rich population

Abstract

We analyze spectra of 18 stars belonging to the faintest subgiant branch in omega Centauri (the SGB-a), obtained with GIRAFFE@VLT at a resolution o R~17000 and a S/N ratio between 25 and 50. We measure abundances of Al, Ba, Ca, Fe, Ni, Si, and Ti and we find that these stars have <[Fe/H]>=-0.73 +/- 0.14 dex, similarly to the corresponding red giant branch population (the RGB-a). We also measure <[alpha/Fe]>=+0.40 +/- 0.16 dex, and <[Ba/Fe]>=+0.87 +/- 0.23 dex, in general agreement with past studies. It is very interesting to note that we found a uniform Al abundance, <[Al/Fe]>=+0.32 +/- 0.14 dex, for all the 18 SGB-a stars analysed here, thus supporting past evidence that the usual (anti-)correlations are not present in this population, and suggesting a non globular cluster-like origin of this particular population. In the dwarf galaxy hypothesis for the formation of omega Cen, this population might be the best candidate for the field population of its putative parent galaxy, although some of its properties appear contradictory. It has also been suggested that the most metal-rich population in omega Cen is significantly enriched in helium. If this is true, the traditional abundance analysis techniques, based on model atmospheres with normal helium content, might lead to errors. We have computed helium enhanced atmospheres for three stars in our sample and verified that the abundance errors due to the use of non-enhanced atmospheres are negligible. Additional, indirect support to the enhanced helium content of the SGB-a population comes from our Li upper limits.

11 pages, 5 figures, accepted for publication in A&A

References in corpus (9)

• Spectroscopic and Photometric evidence of two stellar populations in the Galactic Globular Cluster NGC6121 (M4)
• DAOSPEC: an automatic code for measuring equivalent widths in high-resolution stellar spectra
• The exotic chemical composition of the Sagittarius dwarf Spheroidal galaxy
• Origin of multiple stellar populations in globular clusters and their helium enrichment
• Looking outside the Galaxy: the discovery of chemical anomalies in 3 old Large Magellanic Cloud clusters
• NLTE determination of the aluminium abundance in a homogeneous sample of extremely metal-poor stars
• VLT/UVES Spectroscopy of Individual Stars in Three Globular Clusters in the Fornax Dwarf Spheroidal Galaxy
• The Rotation of Sub-Populations in omega Centauri
• A Spectroscopic Survey for Omega Centauri Members at and beyond the Cluster Tidal Radius

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• Chemical Compositions of Field and Globular Cluster RR~Lyrae Stars: II. omega Centauri
• From the CMD of Omega Centauri and (super-)AGB stellar models to a Galactic plane passage gas purging chemical evolution scenario
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