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Stellar Evolution with Rotation A&A 429, 581–598 (2005) Astronomy DOI: 10.1051/0004-6361:20047106 & c ESO 2004 Astrophysics Stellar evolution with rotation XI. Wolf-Rayet star populations at different metallicities G. Meynet and A. Maeder Geneva Observatory, 1290 Sauverny, Switzerland e-mail: [Georges.Meynet;Andre.Maeder]@obs.unige.ch Received 20 January 2004 / Accepted 28 June 2004 Abstract. Grids of models of massive stars (M ≥ 20 M) 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. Some possible remaining difficulties in these comparisons are mentioned. The evolution of the chemical abundances is largely influenced by rotation in all phases from the MS phase to the WN and WC phases. We also show that the interval of initial masses going through the LBV stage is changing with Z and Ω. The observed variation with metallicity of the fractions of type Ib/Ic supernovae with respect to type II supernovae as found by Prantzos & Boissier (2003) is very well reproduced by the rotating models, while non-rotating models predict much too low ratios. This indicates that the minimum initial masses of single stars going through a WR phase are consistently predicted. At Z = 0.040, stars with initial masses above 50 M reach a final mass at the time of supernova explosion between 5 and 7.5 M, while at Z = 0.004, like in the SMC, the final masses of stars are in the range of 17–29 M. On the whole, rotation appears to be an essential parameter even for the WR properties. Detailed tables describing the evolutionary tracks are available on the web. Key words. stars: evolution – stars: rotation – stars: Wolf-Rayet 1. Introduction amounts of 26Al (see e.g. Vuissoz et al. 2004), responsible for the diffuse emission at 1.8 MeV observed in the plane of our 19 Wolf-Rayet stars are considered to be bare stellar cores whose Galaxy (Prantzos & Diehl 1996), in F (Meynet & Arnould original H-rich envelopes have been removed either by strong 2000) whose origin still remains largely unknown (Cunha et al. stellar winds or by mass transfer through Roche Lobe Overflow 2003), and in s-process elements (see e.g. Arnould et al. 1997). 22 in close binary systems (Conti 1976; Chiosi & Maeder 1986; The WC star winds are also rich in Ne, which explains the 22 20 Abbott & Conti 1987). Their associations with young star high Ne/ Ne isotopic ratio observed in the galactic cosmic forming regions implies that their progenitors must be massive ray source material (see e.g. Meynet et al. 2001). WR stars are stars (see e.g. the recent review by Massey 2003, and refer- also the progenitors of type Ib/Ic supernovae (see the review by ences therein). WR stars have a deep impact on their surround- Hamuy 2003). Recently the spectrum of such a supernova was ings thanks to their high luminosity and their strong stellar observed in the optical transient of a γ-ray burst (see e.g. Hjorth winds. Their broad emission lines can be detected in the in- et al. 2003), confirming the suspected link between these stars tegrated spectrum of remote galaxies (Kunth & Sargent 1981; and the long γ-ray bursts (Woosley 1993). For all these rea- Schaerer et al. 1999) enabling us to study star formation and sons Wolf-Rayet stars appear as objects worthwhile to be well evolutioninverydifferent environments, from metal poor blue understood. compact dwarf galaxies to the vicinity of AGN (see e.g. Lípari In a previous paper (Meynet & Maeder 2003, Paper X), et al. 2003). They contribute to the enrichment of the inter- we discussed the consequences of rotation on the properties stellar medium by newly synthesized elements. In particular of WR stars at solar metallicity. One of the main conclu- their winds at high metallicity may be heavily loaded with sions is that the theoretical predictions for the number ratios of carbon (Maeder 1992). Their winds may also eject significant Wolf-Rayet to O-type stars, for the ratio of WN to WC stars and Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20047106 582 G. Meynet and A. Maeder: Stellar evolution with rotation. XI. for the fraction of WR stars in the transition WN/WC phase, SMC without assuming that a large fraction of WR stars owe are in good agreement with the observations when the effects their existence to mass transfer in close binary systems. of rotation are accounted for in stellar models. In contrast, the We shall also study the effects of rotation at higher metal- models with present-day mass loss rates and no rotation do not licity than solar, more precisely at twice the solar metallicity, succeed in reproducing the observed values. The main purpose a value often quoted for the galactic centre (although some au- of the present paper is to explore the case of metallicities lower thors quote for this region a value of the metallicity similar to and higher than solar and to see if the above conclusions still that of the solar neighbourhood, see Carr et al. 1999; Najarro hold. 2003). This region of the Milky Way is very rich in massive stars (according to Figer et al. 2002, the Arches cluster con- Let us recall that interesting questions arise from the obser- tains about 5% of all known WR stars in the Galaxy) and it is vations of WR stars both at low and high metallicity. At low thus interesting to derive the properties of the WR stars pre- metallicity, it has generally been thought that WR stars might dicted by the present rotating models. Moreover, populations preferentially be formed by mass transfer through Roche Lobe of Wolf-Rayet stars at a higher metallicity may also be of inter- Overflow in close binary systems. For instance, it was thought est for studying the stellar population in the vicinity of AGNs that the majority, if not all the WR stars in the SMC should be (see e.g. the review by Heckman 1999). born thanks to the mass transfer mechanism. The main reason is Section 2 briefly summarizes the physics of the models. that, at low metallicity, the mass loss rates are much lower than Evolution of the surface rotational velocities is discussed in at higher metallicity, thus making the ejection of the H-rich en- Sect. 3. The evolutionary tracks are presented in Sect. 4. The ef- velope by stellar winds more difficult. However, this idea was fects of rotation on WR star formation and WR lifetimes at dif- recently challenged by the works of Foellmi et al. (2003a,b). ferent metallicities are discussed in Sect. 5. Comparisons with They looked for periodic radial velocity variability in all the the observed WR populations are performed in Sect. 6. Finally WR stars in the Small Magellanic Cloud and in two thirds of the predicted surface abundances of the present WR stellar the WR stars in the Large Magellanic Cloud. They found that models are discussed in Sect. 7. the percentage of binaries among the WR stars is of the order of 40% for the SMC and 30% for the LMC, thus comparable or even below the percentage of binaries among the WR stars in 2. Physics of the models our Galaxy. This means that, in the SMC, at most 40% of the Most of the physical ingredients of the present models are the WR stars could originate from mass transfer through Roche same as in the solar metallicity models of Meynet & Maeder Lobe Overflow (RLOF) in a close binary system. The real frac- (2003, Paper X). They differ in only two points: tion is likely lower since RLOF has not necessarily occurred in all these systems. Therefore even in the SMC, a large fraction – The initial compositions are adapted for the different metal- of the WR stars likely originate via the single star scenario. licities considered here. For a given metallicity Z (in mass fraction), the initial helium mass fraction Y is given by How is it possible? Does this mean that the mass loss the relation Y = Y +∆Y/∆Z · Z,whereY is the pri- rates are larger than usually found or that another process is p p mordial helium abundance and ∆Y/∆Z the slope of the at work which favours the evolution of massive stars into the helium-to-metal enrichment law. We use the same values WR phase? In an earlier work (Maeder & Meynet 1994) we ex- as in Maeder & Meynet (2001) i.e.
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