Metallicities on the Double Main Sequence of omega Centauri Imply Large Helium Enhancement

Abstract

Having shown in a recent paper that the main sequence of omega Centauri is split into two distinct branches, we now present spectroscopic results showing that the bluer sequence is_less_ metal-poor. We have carefully combined GIRAFFE@VLT spectra of 17 stars on each side of the split into a single spectrum for each branch, with adequate S/N to show clearly that the stars of the blue main sequence are less metal poor by 0.3 dex than those of the dominant red one. From an analysis of the individual spectra, we could not detect any abundance spread among the blue main sequence stars, while the red main sequence stars show a 0.2 dex spread in metallicity. We use stellar-structure models to show that only greatly enhanced helium can explain the color difference between the two main sequences, and we discuss ways in which this enhancement could have arisen.

arXiv comment24 pages, 7 figures, The Astrophysical Journal, accepted 30 Nov. 2004

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• A search for dormant binaries with degenerate components in omega Centauri and NGC 6397
• WFPC2 UV survey of Galactic Globular Clusters. The Horizontal Branch temperature distribution
• The Multiplicity of Main Sequence Turnoffs in Globular Clusters
• He-enriched STAREVOL models for globular cluster multiple populations. Self-consistent isochrones from ZAMS to the TP-AGB phase
• Lithium factories in the Galaxy: novae and AGB stars
• Study of the Helium Enrichment in Globular Clusters
• Impact of the low solar abundance on the ages of globular clusters
• The quite complex "Simple Stellar Populations" of Globular Clusters
• Galactic evolution of D, 3He and 4He