Mon. Not. R. Astron. Soc., 372, 1407-1424 (2006/November-1)
A census of the Wolf-Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy.
CROWTHER P.A., HADFIELD L.J., CLARK J.S., NEGUERUELA I. and VACCA W.D.
Abstract (from CDS):
New Technology Telescope (NTT)/Son of Isaac (SOFI) imaging and spectroscopy of the Wolf-Rayet population in the massive cluster Westerlund 1 are presented. Narrow-band near-infrared (IR) imaging together with follow up spectroscopy reveals four new Wolf-Rayet stars, of which three were independently identified recently by Groh et al., bringing the confirmed Wolf-Rayet content to 24 (23 excluding source S) - representing 8 per cent of the known Galactic Wolf-Rayet population - comprising eight WC stars and 16 (15) WN stars. Revised coordinates and near-IR photometry are presented, whilst a quantitative near-IR spectral classification scheme for Wolf-Rayet stars is presented and applied to members of Westerlund 1. Late subtypes are dominant, with no subtypes earlier than WN5 or WC8 for the nitrogen and carbon sequences, respectively. A qualitative inspection of the WN stars suggests that most (∼75 per cent) are highly H deficient. The Wolf-Rayet binary fraction is high (≥62 per cent), on the basis of dust emission from WC stars, in addition to a significant WN binary fraction from hard X-ray detections according to Clark et al. We exploit the large WN population of Westerlund 1 to reassess its distance (∼5.0kpc) and extinction (AKS∼ 0.96mag), such that it is located at the edge of the Galactic bar, with an oxygen metallicity ∼60 per cent higher than Orion. The observed ratio of WR stars to red and yellow hypergiants, N(WR)/N(RSG + YHG) ∼3, favours an age of ∼4.5-5.0Myr, with individual Wolf-Rayet stars descended from progenitors of initial mass ∼40-55M☉. Qualitative estimates of current masses for non-dusty, H-free WR stars are presented, revealing 10-18M☉, such that ∼75 per cent of the initial stellar mass has been removed via stellar winds or close binary evolution. We present a revision to the cluster turn-off mass for other Milky Way clusters in which Wolf-Rayet stars are known, based upon the latest temperature calibration for OB stars. Finally, comparisons between the observed WR population and subtype distribution in Westerlund 1 and instantaneous burst evolutionary synthesis models are presented.
2006 The Authors. Journal compilation © 2006 RAS
stars: Wolf-Rayet - open clusters and associations: individual: Westerlund 1
erratum vol. 385, p. 544 (2008)
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