The role of NCO reaction chain on the He ignition in degenerate stellar structures

Some 25 years ago [5] it has been suggested that the 14 N (e − , γ) 14 C(α, γ) 18 O reaction (NCO reaction chain) plays a pivotal role in the onset of the Helium flash in the core of a low mass Red Giant Branch (RGB) star (see [3] and [4]). In fact in the He core the Fermi energy is large enough to approach the energy for the e-capture on 14 N well before the 3α reactions produce energy at a significant level so that a not negligible amount of 14 C is produced in the most internal regions. In the physical conditions typical of a star approaching the tip of the RGB α captures on 14 C dominate over 3α reactions. In 1980 [6] argued that the NCO reaction does not affect the He ignition in a star at the tip of RGB because the physical conditions in the inner region, particularly the density, do not allow the NCO reaction to be dominant over the 3α reaction. Successively, in 1986, Hashimoto, Nomoto, Arai & Kaminisi [1] recomputed the NCO cross sections and found that this reaction ". .. dominates over 3α reaction to heat up the central region " of a low mass star approaching the tip of the RGB [2]. Woosley & Weaver [8] accounted for this reactions chain to compute the pre-supernova models of cooled down CO WDs accreting He rich matter. They find that the inclusion of NCO reaction does not prevent the occurrence of an He detonation. Recently, Piersanti, Cassisi, Iben & Tornambé ([7]) have included the NCO chain in the evolution of a low mass CO WD accreting H and He rich matter at a rate suitable to obtain an He detonation. Their investigation shows that the differences determined by the inclusion of the NCO chain are negligible, the final outcome remaining an explosion. They point out that their result is due to the fact that they use an evolutionary model, obtained by evolving an intermediate mass star with moderate mass loss from the Main Sequence phase till the cooling sequence. In such a model the 14 N abundance at the base of the He shell, where the He flash takes place, is zero. In any case it is important to evaluate if the 14 N (e − , γ) 14 C(α, γ) 18 O reaction plays some role in stellar …