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Astron. Astrophys. 355, 176-180 (2000)

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1. Introduction

For long, the solar system has been the only location of the Galaxy with a known fluorine ([FORMULA]) abundance. At the same time, the production site(s) of this element has been a major nucleosynthetic puzzle, even if F is the least abundant (mass fraction of [FORMULA], following Grevesse & Sauval 1998) of the elements ranging from carbon to calcium.

These last years, the situation has changed quite dramatically, both observationally and theoretically. Fluorine overabundances (with respect to solar) in MS, S and C stars have been reported (Jorissen et al. 1992), and correlate in particular with s-process enrichments. These observations demonstrate that thermally pulsating Asymptotic Giant Branch (AGB) stars are fluorine producers, as predicted by Goriely et al. (1989), and confirmed by calculations conducted in the framework of detailed AGB models (Forestini et al. 1992, Mowlavi et al. 1996, 1998). It remains of course to determine the exact level of the contribution of these (mass losing) stars to the solar system and galactic F content.

In direct relation with this question, various calculations have been made in order to estimate the [FORMULA] yields from massive stars. The neutrino process operating during supernova explosions has been envisioned as a possible producer of primary [FORMULA] (e.g. Woosley & Weaver 1995). On the other hand, Meynet & Arnould (1993) have investigated on grounds of detailed stellar models the suggestion (Goriely et al. 1989) that the hydrostatically burning He-shell can synthesize [FORMULA] of secondary nature. They find that the level of production is relatively modest in [FORMULA]. In contrast, they show that stars which are massive enough to become Wolf-Rayet (WR) stars can eject through their winds substantial amounts of fluorine synthesized in the core at the beginning of the He-burning phase.

In the present work, we revisit the question of the galactic contribution of WR stars to [FORMULA] with the help of new stellar models that better account for many important observable properties of WR stars. In addition, we extend the range of masses and metallicities considered in our previous study. The broadening of the explored metallicity range may take some additional importance in relation with the recent claim by Timmes et al. (1997) that "positive detection of any fluorine at a sufficiently large redshift ([FORMULA]) would suggest strongly a positive detection of the neutrino process operating in massive stars ". The possibility of a significant thermonuclear production of [FORMULA] by WR stars of different metallicities might blur this picture, and might at least imply the necessity of establishing observationally the primary or secondary nature of the detected fluorine, if any.

The physical ingredients of the models are discussed in Sect. 2. Sect. 3 presents our predicted yields from individual WR stars, while Sect. 4 gives a rough estimate of the contribution of WR stars to the galactic [FORMULA] content. Some conclusions are drawn in Sect. 5.

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© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000