Astronomy and Astrophysics, volume 285, 915-928 (1994/5-3)
CNO isotopes in red giants: theory versus observations.
EL EID M.F.
Abstract (from CDS):
Evolutionary models in the mass range 1.7 to 15 M☉ having solar-like initial composition are presented. The models are calculated with the latest opacities and with the Schwarzschild criterion for convective instability. In test cases, core overshooting has been included in a parameterized way. Up-dated nuclear reactions have been used to follow the nucleosynthesis during the evolution which has been carried out beyond core He-burning at masses > 2 M☉. The 2 M☉ model has been evolved through the core He-flash. The resulting surface CNO isotopic ratios after the first and second dredge-up phases are compared with observations of field giants as well as with data obtained for galactic open clusters. A comparison between recent theoretical predictions of these ratios is also presented. The goal of this work is twofold: (i) to test mass estimations of the observed giants on the basis of the predicted surface abundance ratios of 12C/13C, 16O/17O and 16O/18O, and the evolutionary tracks, (ii) to show that 16O/17O ratio obtained in giant models at M>2 M☉ is rather sensitive to the still uncertain 17O(p,α)14N reaction. The main results of the present work are as follows: (i) our one-dimensional dynamical calculations show that the 2 M☉ model survives the core He-flash, (ii) we are able to estimate the masses of 6 observed giants out of 11. In addition, the supergiant star α Sco is well described by a model of initially 15 M☉. (iii) the observed low 12C/13C ratios in many giants with M>2 M☉ cannot be explained in terms of a simple dredge-up scenario or by the variation of the initial value. These ratios are barely influenced by the effect of "moderate" overshooting, (iv) We find that the calculated Na enrichment resulting from the Ne-Na cycle does not match the values suggested by current observations.
stellar evolution - nucleosynthesis - red giant stars