ON THE SODIUM EXCESS IN THE ATMOSPHERES OF YELLOW SUPERGIANTS

In order to explain quantitatively the reported Na excess at the surface of some yellow giant stars we explore the possibility of an enhanced rate of the Ne-20(p, gamma) reaction. Detailed stellar nucleosynthesis calculations show that an enhancement factor f approximately 10 in H-burning temperatures (T approximately 20-40 x 10(6) K) is required to explain the observations. A careful analysis of our current understanding of that reaction shows that such an enhancement is unlikely; in lack of reasonable alternatives we feel, however, that a reinvestigation of the Ne-Na cycle in the low-temperature regime is worthwhile.