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

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 excluding 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 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%) 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, comparisons between the observed WR population and subtype distribution in Westerlund 1 and instantaneous burst evolutionary synthesis models are presented.