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

Abstract

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(B−V) = 0.08 ± 0.02 (random) ± 0.02 (systematic) mag and a differential reddening of ΔE(B−V) < 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.

SPITZERSPECTROSCOPY OF MASS-LOSS AND DUST PRODUCTION BY EVOLVED STARS IN GLOBULAR CLUSTERS

“…Like us, they observed little dust in the spectrum, while earlier mid-infrared photometry (referenced by McDonald et al 2009) showed a clear excess at 10 μm. They concluded that this change in the measurements is likely real.…”

An Infrared Census of DUST in Nearby Galaxies with Spitzer (DUSTiNGS). IV. Discovery of High-redshift AGB Analogs*

“…Most of the M-type stars we identified in Section 2.4.2 are low-mass AGB stars and are expected to produce only modest amounts of dust (e.g., McDonald et al 2009McDonald et al , 2011Boyer et al 2015a). The significant IR excesses of the 26 M-type dusty stars identified here suggests that they are instead HBB AGB stars and are therefore more massive than their C-rich counterparts.…”

“…(Lagadec et al 2007;Matsuura et al 2007;Sloan et al 2009Sloan et al , 2012Whitelock et al 2009;Menzies et al 2010Menzies et al , 2011McDonald et al 2014) and in globular clusters with > -[ ] Fe H 1.6 (Boyer et al 2008(Boyer et al , 2009McDonald et al 2009McDonald et al , 2011. All of these examples are C stars or low-mass M-type stars.…”

The Acs Nearby Galaxy Survey Treasury. Ix. Constraining Asymptotic Giant Branch Evolution With Old Metal-Poor Galaxies

“…Thus, to obtain useful constraints on the AGB lifetime from star clusters, it is necessary to either sum the star counts in many clusters into age and metallicity bins (e.g., Girardi & Marigo 2007a), or embark on a more detailed study of the dust, chemical, and pulsational properties of individual cluster stars (see, e.g., Lebzelter & Wood 2007;Lebzelter et al 2008;Kamath et al 2010 for Magellanic Cloud clusters, and Lebzelter et al 2006;van Loon et al 2006;McDonald et al 2009McDonald et al , 2010Boyer et al 2009bBoyer et al , 2010 for MW globular clusters). Less direct constraints on AGB evolution come from integrated cluster properties, such as their colors and surface brightness fluctuations (see, e.g., Maraston 2005;Pessev et al 2008;Raimondo 2009;Conroy et al 2009;Conroy & Gunn 2010), which by their very nature cannot disentangle different evolutionary properties such as stellar luminosities, lifetimes, and chemical types, and hence often provide somewhat ambiguous constraints on the numbers of stars at different evolutionary stages.…”