Stellar evolution with rotation XI: Wolf-Rayet star populations at different metallicities

Abstract

Grids of models of massive stars ($M \ge$ 20 $M_\odot$) with rotation are computed for metallicities $Z$ ranging from that of the Small Magellanic Cloud (SMC) to that of the Galactic Centre. The hydrostatic effects of rotation, the rotational mixing and the enhancements of the mass loss rates by rotation are included. The evolution of the surface rotational velocities of the most massive O--stars mainly depends on the mass loss rates and thus on the initial $Z$ value. The minimum initial mass for a star for entering the Wolf--Rayet (WR) phase is lowered by rotation. For all metallicities, rotating stars enter the WR phase at an earlier stage of evolution and the WR lifetimes are increased, mainly as a result of the increased duration of the eWNL phase. Models of WR stars predict in general rather low rotation velocities ($< 50$ km s$^{-1}$) with a few possible exceptions, particularly at metallicities lower than solar where WR star models have in general faster rotation and more chance to reach the break--up limit.The properties of the WR populations as predicted by the rotating models are in general in much better agreement with the observations in nearby galaxies. The observed variation with metallicity of the fractions of type Ib/Ic supernovae with respect to type II supernovae as found by Prantzos & Boissier (\cite{Pr03}) is very well reproduced by the rotating models, while non--rotating models predict much too low ratios.Comment: 20 pages, 16 figure, Astronomy and Astrophysics, in pres