EDP Sciences
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Volume 476, Number 3, December IV 2007
Page(s) 1261 - 1282
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20078514

A&A 476, 1261-1282 (2007)
DOI: 10.1051/0004-6361:20078514

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

I. McDonald and J. Th. van Loon

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)

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, $\omega$ 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 H$\alpha$ 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 H$\alpha$ 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 $M_{\odot}$ yr-1, largely independent of metallicity.

Key words: stars: AGB and post-AGB -- stars: circumstellar matter -- stars: mass-loss -- stars: chromospheres -- infrared: stars -- Galaxy: globular clusters: general

© ESO 2007

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