Journal Article

Giants in the globular cluster ω Centauri: dust production, mass-loss and distance

Iain McDonald, Jacco Th. Van Loon, Leen Decin, Martha L. Boyer, Andrea K. Dupree, Aneurin Evans, Robert D. Gehrz and Charles E. Woodward

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 394, issue 2, pages 831-856
Published in print April 2009 | ISSN: 0035-8711
Published online March 2009 | e-ISSN: 1365-2966 | DOI:
Giants in the globular cluster ω Centauri: dust production, mass-loss and distance

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We present spectral energy distribution modelling of 6875 stars in ω Centauri, obtaining stellar luminosities and temperatures by fitting literature photometry to state-of-the-art marcs stellar models. By comparison to four different sets of isochrones, we provide a new distance estimate to the cluster of 4850 ± 200 (random error) ± 120 (systematic error) pc, a reddening of E(BV) = 0.08 ± 0.02 (random) ± 0.02 (systematic) mag and a differential reddening of ΔE(BV) < 0.02 mag for an age of 12 Gyr. Several new post-early-AGB candidates are also found. Infrared excesses of stars were used to measure total mass-loss rates for individual stars down to ∼7 × 10−8 M yr−1. We find a total dust mass-loss rate from the cluster of 1.3 ±0.80.5× 10−9 M yr−1, with the total gas mass-loss rate being > 1.2 ±0.60.5× 10−6 M yr−1. Half of the cluster's dust production and 30 per cent of its gas production comes from the two most extreme stars – V6 and V42 – for which we present new Gemini/T-ReCS mid-infrared spectroscopy, possibly showing that V42 has carbon-rich dust. The cluster's dust temperatures are found to be typically ≳550 K. Mass-loss apparently does not vary significantly with metallicity within the cluster, but shows some correlation with barium enhancement, which appears to occur in cooler stars, and especially on the anomalous RGB. Limits to outflow velocities, dust-to-gas ratios for the dusty objects and the possibility of short-time-scale mass-loss variability are also discussed in the context of mass-loss from low-metallicity stars. The ubiquity of dust around stars near the RGB tip suggests significant dusty mass-loss on the RGB; we estimate that typically 0.20–0.25 M of mass-loss occurs on the RGB. From observational limits on intracluster material, we suggest the dust is being cleared on a time-scale of ≲105 yr.

Keywords: stars: AGB and post-AGB; circumstellar matter; stars: mass-loss; stars: winds, outflows; globular clusters: individual: ω Centauri; infrared: stars

Journal Article.  21060 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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