부산대학교 도서관

To purchase or authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1365-2966.2006.10952.x Byline: Paul A. Crowther (1), L. J. Hadfield (1), J. S. Clark (2), I. Negueruela (3), W. D. Vacca (4) Keywords: stars: Wolf-Rayet; open clusters and associations: individual: Westerlund 1 Abstract: ABSTRACT 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 ([proportional to]75 per cent) are highly H deficient. The Wolf-Rayet binary fraction is high ([greater than or equal to]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 ([proportional to]5.0 kpc) and extinction (AK.sub.S[proportional to] 0.96 mag) , such that it is located at the edge of the Galactic bar, with an oxygen metallicity [proportional to]60 per cent higher than Orion. The observed ratio of WR stars to red and yellow hypergiants, N(WR)/N(RSG + YHG) [proportional to]3 , favours an age of [proportional to]4.5-5.0 Myr, with individual Wolf-Rayet stars descended from progenitors of initial mass [proportional to]40-55 M.sub.⊙ . Qualitative estimates of current masses for non-dusty, H-free WR stars are presented, revealing 10-18 M.sub.⊙ , such that [proportional to]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. Author Affiliation: (1)Department of Physics & Astronomy, University of Sheffield, Hicks Building, Hounsfield Rd, Sheffield S3 7RH (2)Department of Physics & Astronomy, The Open University, Milton Keynes MK7 6AA (3)Dpto de Fisica, Ingenieria de Sistemas y Teoria de la Senal, Universidad de Alicante, Apdo. 99, E03080 Alicante, Spain (4)SOFIA-URSA, NASA Ames Research Center, MS N211-3, Moffett Field, CA 94035, USA Article History: Accepted 2006 August 16. Received 2006 August 2; in original form 2006 June 21 Article note: ([dagger]) E-mail: Paul.crowther@sheffield.ac.uk