Observed Metallicity Dependence of Winds from WR stars

Paul A Crowther

Published 2005-10-03, updated 2005-12-12Version 2

A review of observational evidence in favour of a metallicity dependence of WN and WC stars is presented. New near-IR studies of Milky Way, LMC and SMC early-type WN stars are presented, with weake winds amongst WN stars containing hydrogen. A metallicity dependence is supported for WN stars with hydrogen, with dM/dt propto Z^alpha with alpha approx 0.8 +/- 0.2 The influence of CNO content upon WN subtypes is discussed. Earlier WN spectral types are expected (and observed) at lower metallicity due to the abundance sensitivity of NIII-IV classification diagnostics. Recent physical and chemical results of WC stars in the Milky Way and LMC are discussed, suggesting a metallicity dependence of alpha approx 0.6 +/- 0.1. Earlier WC spectral types are predicted (and observed) in lower metallicity galaxies, due to the dependence of the CIII classification diagnostic on wind density. WO stars reveal lower wind velocities at lower metallicity, whilst the situation for WN and WC stars is unclear. Finally, the influence of a WR metallicity dependence upon the ionizing flux distributions and optical line luminosities is addressed, with particular regard to I Zw 18. Weaker winds at low metallicity would imply harder ionizing flux distributions and lower line luminosities, arguing for an substantially increased number of WR stars with respect to standard calibrations, exacerbating difficulties with single star evolutionary models at very low metallicity.