Journal
ASTROPHYSICAL JOURNAL
Volume 621, Issue 2, Pages 777-784Publisher
IOP PUBLISHING LTD
DOI: 10.1086/427796
Keywords
Galaxy : abundances; globular clusters : individual (NGC 5139); Hertzsprung-Russell diagram
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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 VLT's GIRAFFE spectra of 17 stars on each side of the split into a single spectrum for each branch, with adequate signal-to-noise ratio, 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, whereas 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.
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