The long-period variables in ω Centauri^⋆

¹ Institute for Astrophysics (IfA), University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
e-mail: thomas.lebzelter@univie.ac.at
² Australian National University, 0200 Canberra, Australia
e-mail: wood@mso.anu.edu.au

Received: 5 September 2015
Accepted: 22 October 2015

Abstract

Context. Recent studies have detected multiple populations in globular clusters. The massive globular cluster ω Cen hosts at least three populations of different metallicity. The most metal-rich one is thought to show also an overabundance of He. These differences should become visible in the structure, evolution, and pulsation of its stars.

Aims. We aim to study the effects of the different starting compositions of the three populations in ω Cen on the most luminous red giants in this cluster.

Methods. The long-periodic variability of evolved stars in ω Cen opens a comparably easy access window to the structure and composition of these objects. We made a detailed search for long-period variables (LPVs) in ω Cen leading to the detection of many new variables and period determinations for a significant number of them. Periods and luminosities were then compared with the most recent pulsation models for these kinds of stars.

Results. Some of the LPVs belong to each of the main metallicity sub-groups of ω Cen. Almost all detected variables with periods are fundamental mode pulsators. For the metal-rich group, our comparison with pulsation models favours a high He abundance of Y = 0.4.

Conclusions. Our study can be considered an independent piece of evidence supporting a high helium abundance among the metal-rich stars in ω Cen.