Dust, pulsation, chromospheres and their rôle in driving mass loss from red giants in Galactic globular clusters

Astrophysics Group, School of Physical & Geographical Sciences, Keele University, Staffordshire ST5 5BG, UK e-mail: iain@astro.keele.ac.uk

Received: 20 August 2007
Accepted: 3 October 2007

Abstract

Context.Mass loss from red giants in old globular clusters affects the horizontal branch (HB) morphology and post-HB stellar evolution including the production of ultraviolet-bright stars, dredge up of nucleosynthesis products and replenishment of the intra-cluster medium. Study of mass loss in globular clusters also allows one to investigate the metallicity dependence of the mass loss from cool, low-mass stars down to very low metallicities.

Aims.We present an analysis of new VLT/UVES spectra of 47 red giants in the Galactic globular clusters 47 Tuc (NGC 104), NGC 362, ω Cen (NGC 5139), NGC 6388, M 54 (NGC 6715) and M 15 (NGC 7078). The spectra cover the wavelength region 6100-9900 Å at a resolving power of R = 110 000. Some of these stars are known to exhibit mid-infrared excess emission indicative of circumstellar dust. Our aim is to detect signatures of mass loss, identify the mechanism(s) responsible for such outflows, and measure the mass-loss rates.

Methods.We determine for each star its effective temperature, luminosity, radius and escape velocity. We analyse the Hα and near-infrared calcium triplet lines for evidence of outflows, pulsation and chromospheric activity, and present a simple model for estimating mass-loss rates from the Hα line profile. We compare our results with a variety of other, independent methods.

Results.We argue that a chromosphere persists in Galactic globular cluster giants and controls the mass-loss rate to late-K/early-M spectral types, where pulsation becomes strong enough to drive shock waves at luminosities above the RGB tip. This transition may be metallicity-dependent. We find mass-loss rates of ~10^-7 to 10^-5  yr^-1, largely independent of metallicity.