On the possibility of 18O and 19F synthesis in helium-rich zones of hydrostatically evolving stars

On grounds of a recent experimental analysis of the 18O (α,γ) 22Ne rate, it is shown that 18O could be an important end product of quiescent core He burning in M <= 5 Mo stars. However such a conclusion depends to some extend upon the uncercainties still affecting that rate at T <= 2 x 108 K. A lower limit on the interesting mass range may be provided by the fact that 18O probably cannot survive strong enough core He flashes which precede quiescent core He burning in M <= 2.2 Mo stars. We also emphasize that 19F might be produced during quiescent core He burning in a more extended star mass range than 18O. Furthermore, we speculate that 18O might accumulate in certain regions of quiescently burning He shells or in shell He flashes associated to low enough core masses. Finally, the transport of 18O (and 19F) to the stellar surface is discussed qualitatively. On grounds of classical stellar models, such a possibility appears to be quite remote. However, certain theoretical considerations and observational data leave room for some hope. (orig.)