Massivestar populations in highmetallicity regions

At high metallicity, as a result of the metallicity dependence of the mass-loss rates, one expects larger fractions of WR stars (Maeder et al. 1980). This can be seen in the left-hand part of Figure 36.5, where the WR lifetimes of rotating models for four metallicities are plotted as a function of the initial mass. The metallicity dependence of the mass-loss rates is responsible for two features. (1) For a given initial mass and velocity the WR lifetimes are greater at higher metallicities. Typically at Z = 0.040 and for M > 60M0 the WR lifetime is of the order of 2 Myr, while at the metallicity of the SMC the WR lifetimes in this mass range are in the range 0.4-0.8 Myr. (2) The minimum mass necessary for a single star to evolve into the WR phase is lower at higher metallicity.

Comparisons with observed populations of WR stars are shown in Meynet & Maeder (2005). When the variation with the metallicity of the number ratio of WR to O-type stars is considered, good agreement is obtained provided that models with rotation are used. Models without rotation predict ratios of WR to O-type stars that are much too small, even at high metallicity. This illustrates the fact that, even at high metallicity, for which the effects of the mass-loss rates are dominant, one cannot neglect the effects of rotation.

Models well reproduce the variation with Z of the WC/WN ratio at low metallicity, but underestimate this ratio at high Z (Meynet & Maeder 2005, Figure 11). It might be that the mass-loss rates during the post-MS WNL phase are

Figure 36.5. Left panel: lifetimes of Wolf-Rayet stars of various initial masses for the four metallicities indicated. All the models begin their evolution with vinit = 300 km s-1 on the ZAMS. Right panel: the variation with metallicity of the number ratio of Type-Ib/Ic supernovae to Type-II supernovae. The crosses with the error bars correspond to the values deduced from observations by Prantzos & Boissier (2003). The dotted line is an analytical fit proposed by those authors. The continuous and dashed line show the predictions of rotating and non-rotating stellar models, respectively. Note that at high metallicity no Type-Ic supernovae are predicted by the models (see the text). Figures taken from Meynet & Maeder (2002).

Figure 36.5. Left panel: lifetimes of Wolf-Rayet stars of various initial masses for the four metallicities indicated. All the models begin their evolution with vinit = 300 km s-1 on the ZAMS. Right panel: the variation with metallicity of the number ratio of Type-Ib/Ic supernovae to Type-II supernovae. The crosses with the error bars correspond to the values deduced from observations by Prantzos & Boissier (2003). The dotted line is an analytical fit proposed by those authors. The continuous and dashed line show the predictions of rotating and non-rotating stellar models, respectively. Note that at high metallicity no Type-Ic supernovae are predicted by the models (see the text). Figures taken from Meynet & Maeder (2002).

underestimated. Also, the metallicity dependence of the WR stellar winds may help in resolving this disagreement (Eldridge and Vink 2006).

Current knowledge associates supernovae of Type Ib/Ic with the explosion of WR stars, the H-rich envelope of which has been completely removed by stellar winds and/or by mass transfer through Roche-lobe overflow in a close binary system. If we concentrate on the case of single-star models, theory predicts that the fraction of supernovae progenitors without H-rich envelopes with respect to H-rich supernovae should be higher at higher metallicity. The reason is the same as the one invoked to explain the increasing number ratio of WR to O-type stars with metallicity, namely the growth of the mass-loss rates with Z. Until recently very little observational evidence had been found confirming this predicted behavior. The situation began to change with the work by Prantzos & Boissier (2003). Those authors derived from published data the observed number ratios of Type-Ib/Ic supernovae to Type-II supernovae for various metallicities. The regions considered are regions of constant star-formation rate. Their results are plotted in the right-hand part of Figure 36.5.

On looking at this figure, it becomes clear that rotating models give a much better fit to the observed data than non-rotating models. This comparison can be viewed as a check of the lower initial-mass limit MWNE of the stars evolving into a WR phase without hydrogen. Let us note also at this point that, at high metallicity, no Type-Ic supernovae (supernovae with no trace of H and He) are expected to occur. This is a consequence of the high mass loss which allows the star to enter at an early stage into the WC phase when a lot of helium still has to be transformed into carbon and oxygen. The star thus keeps a high abundance of helium at its surface until the pre-supernova stage and explodes as a Type-Ib supernova. At low metallicities, since the mass-loss rates are weaker, the star may enter (if it does) at a later stage of the core He-burning phase, when most of the helium has already been transformed into carbon and oxygen (Smith & Maeder 1991). In that case the star may explode as a Type-Ic supernova. This might explain why the long soft gamma-ray bursts associated with Type-Ic supernovae occur only in metal-poor regions (Hirschi et al. 2005).

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