Massivestar nucleosynthesis in highmetallicity regions

Let us begin this section by recalling a few orders of magnitude. When single stars form with a Salpeter initial mass function, about 14% of the mass locked into stars consists of massive stars, i.e. stars with masses greater than 8M0, 25% is locked into stars of masses between M0 and 8M0, and 61% is in stars with masses between 0.1 Mq and M0. When all the stars with masses above M0 have died, about 13% of the mass initially locked into stars is ejected by massive stars (1% remains locked into black holes or neutron stars). The intermediate-mass stars (masses from M0 to 8M0) eject about 18.5% of the mass initially locked into stars (6.5% remains locked in white dwarfs).

Figure 36.6 shows for several models at various metallicities the fraction of the mass in stars which is eventually ejected in the form of new elements. The part ejected by stellar winds is distinguished from that ejected at the time of the supernova explosion. One sees that, on the whole, the fraction of the mass initially locked into stars and transformed into new elements by the massive stars does not depend too much either on the model or on the metallicity. All the results are between 3.5% and 4.5%.

One notes, however, that more variations appear when one looks at the proportions of these new elements ejected by the winds and the supernova explosion. Indeed, at higher metallicity a greater part of the new elements synthesized by stars is ejected by stellar winds, as can be see in Figure 36.6. Typically the non-rotating stellar models of Schaller et al. (1992) predict that a stellar population at Z = 0.001 ejects in the form of new elements during the supernova explosion a little less than 4% of the total mass used to form the stars. Models at Solar metallicity and higher eject about half of their new elements at the time of the supernova explosion and the rest through stellar winds. This may have a great influence on the final yields, as has been shown by Maeder (1992). Indeed, mass loss removes matter at earlier

Schaller et al. (92) MM(03) Rotating Rotating High M;

Figure 36.6. Masses of new elements ejected by stars more massive than 8M0 per unit mass in stars given by five stellar models. A Salpeter initial mass function was used. The labels MM(03) and MM(05) are for Meynet & Maeder (2003) and Meynet & Maeder (2005), respectively.

Schaller et al. (92) MM(03) Rotating Rotating High M;

Figure 36.6. Masses of new elements ejected by stars more massive than 8M0 per unit mass in stars given by five stellar models. A Salpeter initial mass function was used. The labels MM(03) and MM(05) are for Meynet & Maeder (2003) and Meynet & Maeder (2005), respectively.

evolutionary stages. The ejected matter has therefore been partially processed by nuclear burning and has a chemical composition different from the one it would have if the matter had remained locked in the star.

This effect is responsible for many specific enrichments at high metallicity: for instance, because of this effect, massive-star models are expected to be stronger sources of 4He, 12C, 22Ne, and 26Al and to be less important sources of 16O at high metallicity than at low metallicity. The cases of helium, carbon, and oxygen are shown in Figure 36.7. We can see the importance of mass loss for the carbon yields. Indeed, the larger yields are obtained for the models computed with enhanced mass-loss rates. It is interesting here to note that high mass loss is not a sufficient condition for obtaining high carbon yields. High carbon yields are obtained only when the star enters into the WC phase at an early stage of the core He-burning phase. This can be seen by comparing the 60M0 stellar model at Z = 0.04 computed by Meynet & Maeder (2005) with the 60M0 stellar model at Z = 0.02 with enhanced mass-loss rate computed by Maeder (1992). These two models end their lifetimes with similar final masses, the model of Meynet & Maeder (2005) with 6.7M0 and the model of Maeder (1992) with 7.8M0), thus the mass of new carbon ejected at the time of the supernova explosion is quite similar for these two models and is around 0.45M0. When one compares the predictions for the mass of new carbon ejected by the winds, we obtain 7.3M0 and 0.16M0 for the model of Maeder (1992) and that of Meynet & Maeder (2005), respectively. This difference arises from the fact that the model with enhanced mass loss enters the WC phase at a much earlier time of the core He-burning phase, typically when the mass fraction of helium at the

Figure 36.7. Masses of new helium, carbon, and oxygen ejected by stars more massive than 8M0 per unit mass initially in stars given by five stellar models. A Salpeter initial mass function was used. The labels MM(03) and MM(05) are for Meynet & Maeder (2003) and Meynet & Maeder (2005), respectively.

Figure 36.7. Masses of new helium, carbon, and oxygen ejected by stars more massive than 8M0 per unit mass initially in stars given by five stellar models. A Salpeter initial mass function was used. The labels MM(03) and MM(05) are for Meynet & Maeder (2003) and Meynet & Maeder (2005), respectively.

center, Yc, is 0.43 and the actual mass of the star is 25.8M0, whereas the model of Meynet & Maeder (2005) enters the WC stage when Yc is 0.24 and the actual mass is 8.6M0.

The example above shows that entry at an early stage into the WC phase is more favored by strong mass loss than by rotation. This comes from the fact that rotation favors an early entry into the WN phase, while the star has still an important H-rich envelope. It takes time for the whole H-rich envelope to be removed and, when this has been done, the star is already well advanced into the core He-burning phase. Of course, this conclusion is quite dependent on the magnitudes of the mass-loss rates. For instance, higher mass-loss rates during (a part of) the WNL phase would favor an early entrance into the WC phase. This would give a better agreement with the number ratio of WC to WN observed and would lead to higher carbon yields.

Let us conclude this paper by estimating an empirical yield in carbon from the WR stars in the Solar neighborhood. From the catalog published by van der Hucht (2001) one obtains that the number of WC stars within 3 kpc of the Sun is 44. The mass-loss rate during the WC phase is estimated to be 10-4 8M0 per year, and the mass fraction of carbon observed in the WC stellar wind is around 0.35 (Crowther 2006, Table 2). If we consider a star formation rate of (2-4)M0 per square parsec and per Gyr, we obtain that the mass of (new) carbon ejected by WC wind per

Rotating

Rotating

unit mass used to form stars is between 0.25% and 0.5%. These empirical yields are well in line with the range of values given by the models, namely 0.2%-0.6%; see Figure 36.7, the greatest value corresponding to the case of Maeder (1992). Let us note that incorporating the yields of Maeder (1992) into chemical-evolution models has an important impact (e.g. Prantzos et al. 1996). If the upper value of the empirical yields is the correct one then this indicates that WC stars are very important sources of carbon in metal-rich regions.

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