High-resolution spectroscopy of the relatively hydrogen-poor metal-rich giants in the globular cluster ω centauri

Abstract

High-resolution optical spectra are analyzed for two of the four metal-rich, mildly hydrogen-poor or helium-enhanced giants discovered by Hema & Pandey, along with their comparison normal (hydrogen-rich) giants. The strengths of the MgH bands in the spectra of the program stars are analyzed for their derived stellar parameters. The observed spectra of the sample (hydrogen-poor) stars (LEID 39048 and LEID 34225) show weaker MgH bands, unlike in the spectra of the normal comparison giants (LEID 61067 and LEID 32169). The magnesium abundance derived from MgH bands is less by 0.3 dex or more for LEID 39048 and LEID 34225 than that derived from Mg i lines. This difference cannot be reconciled by making the changes to the stellar parameters within the uncertainties. This difference in the magnesium abundances derived from Mg i lines and from the MgH band is unacceptable. This difference is attributed to the hydrogen deficiency or helium enhancement in their atmospheres. These metal-rich, hydrogen-poor or helium-rich giants provide an important link to the evolution of the metal-rich subpopulation of ω Cen. These stars provide the first direct spectroscopic evidence for the presence of the He enhancement in the metal-rich giants of ω Cen.