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Measuring helium abundance difference in giants of NGC 2808

By L. Pasquini, P. Mauas, H. U. Kaufl and C. Cacciari

Abstract

Multiple populations have been detected in several globular clusters (GC) that do not display a spread in metallicity. Unusual features of their CMD can be interpreted in terms of differences in the Helium content of the stars belonging to the sub-populations. Differences in He abundance have never been directly observed. We attempt to measure these differences in two giant stars of NGC 2808 with very similar parameters but different Na and O abundances, hence that presumably belong to different sub-populations, by directly comparing their He I 10830 {\AA} lines. The He 10830 {\AA} line forms in the upper chromosphere. Our detailed models derive the chromospheric structure using the Ca II and H$\alpha$, and simulate the corresponding He I 10830 line profiles. We show that, at a given value of He abundance, the He I 10830 equivalent width cannot significantly change without a corresponding much larger change in the Ca II lines. We have used the VLT-CRIRES to obtain high-resolution spectra in the 10830 {\AA} region, and the VLT-UVES to obtain spectra of the Ca II and H$\alpha$ lines of our target stars. The two target stars have very similar Ca II and H$\alpha$ lines, but different appearances in the He region. One line, blueshifted by 17 km s$^{-1}$ with respect to the He 10830 rest wavelength, is detected in the spectrum of the Na-rich star, whereas the Na-poor star spectrum is consistent with a non-detection. The difference in the spectra is consistent and most closely explained by an He abundance difference between the two stars of $\Delta Y \ge$ 0.17.We provide direct evidence of a significant He line strength difference in giant stars of NGC 2808 belonging to different sub-populations, which had been previously detected by other photometric and spectroscopic means.Comment: Accepted for publication in A&

Topics: Astrophysics - Solar and Stellar Astrophysics
Year: 2011
DOI identifier: 10.1051/0004-6361/201116592
OAI identifier: oai:arXiv.org:1105.0346
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