A census of the Wolf–Rayet content in Westerlund 1 from near-infrared imaging and spectroscopy

By P. Crowther, L. Hadfield, J. Clark, I. Negueruela, W. Vacca. 2006

168 citations recorded by the source corpus.

New NTT/SOFI imaging and spectroscopy of the Wolf-Rayet population in Westerlund 1 are presented. Narrow-band near-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 excludin g source S) ‐ representing 8% of the known Galactic Wolf-Rayet population ‐ comprising 8 WC stars and 16 (15) WN stars. Revised coordinates and near-IR photometry are presented, whilst a quantitative nearIR spectral classification scheme for Wolf-Rayet stars is pr esented 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%) are highly H-deficient. The Wolf-Rayet binary fraction is high (>62%), 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 (�5.0 kpc) and extinction (AKS � 0.96 mag), such that it is located at the edge of the Galactic bar, with an oxygen metallicity �60% higher than Orion. The observed ratio of WR stars to red and yellow hypergiants, N(WR)/N(RSG+YHG)� 3, favours an age of�4.5‐5.0 Myr, 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% 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, c omparisons between the observed WR population and subtype distribution in Westerlund 1 and instantaneous burst evolutionary synthesis models are presented.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/ee1e138a695cb48b0544a1f2e3a9f834d787c99d

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In Astronomy and Astrophysics

1. N. Smith
2. A. Filippenko
3. S. Smartt
4. Ryan Chornock
5. Z. Cano
6. Weidong Li
7. P. Crowther
8. S. P. Portegies Zwart

Citations in the corpus

• Physical Properties of Wolf-Rayet Stars (2006, methodology): “A revised compilation is provided in Crowther et al. (2006b).”
• Observed Fractions of Core-Collapse Supernova Types and Initial Masses of their Single and Binary Progenitor Stars (2010, background): “…with star clusters suggest a value of MWR around roughly 35–40 M for most WR stars in the Milky Way (Schild & Maeder 1984; Humphreys, Nichols & Massey 1985; Massey, Johnson & DeGioia-Eastwood 1985; Massey, DeGioia-Eastwood & Waterhouse 2001; Massey 2003; Crowther et al. 2006; Crowther 2007).”
• Young Massive Star Clusters (2010, background): “In some relatively nearby cases, R136 (Portegies Zwart, Pooley & Lewin 2002), Wd1 (Clark et al. 2005, Crowther et al. 2006), and the Arches and Quintuplet clusters (Lang et al. 2005), the counterparts of the radio and xray sources have been identified.”
• The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 M⊙ stellar mass limit (2010, methodology): “Current mass-loss rates match solar-metallicity theoretical predictions (Vink et al. 2001) to within 0.2 dex. ages (Sung & Bessell 2004), while Crowther et al. (2006) estimated 1.3 ± 0.3 Myr for NGC 3603 from a comparison between massive O stars and theoretical isochrones (Lejeune & Schaerer 2001).”
• A new method for estimating the bolometric properties of Ibc supernovae (2013, methodology): “Furthermore, the mass ranges of the stellar progenitors for Ic-BL and GRB/XRF SNe is close to that seen for the ZAMS mass range of WR stars in the Milky Way, which is 35− 40 M (Humphreys et al. 1985; Massey et al. 1995; Massey et al. 2003; Crowther et al. 2006, Crowther 2007).”
• A Massive Cluster of Red Supergiants at the Base of the Scutum-Crux Arm (2007, background): “…enough to contain main-sequence O-stars up to the mass of 150 M (Figer et al. 2002), while the age and mass of Wd 1 is tuned in such a way that it contains 24 W-R stars—8% of all those known in the Galaxy (Clark & Negueruela 2002; Negueruela & Clark 2005; Groh et al. 2006; Crowther et al. 2006).”