The ¹³C Neutron Source and s-Processing in AGB Stars

Abstract

The main component of the s-process accounts for about 50% of elements heavier than Kr, through n-captures occurring in asymptotic giant branch (AGB) stars, where the 13 C(α,n) 16 O reaction is the main neutron source. Its activation below the convective envelope at third dredge-up (TDU) and its efficiency are still matters of debate, as: (i) the astrophysical factor is affected by a broad resonance near the reaction threshold and (ii) mixing mechanisms to locally produce 13 C were so far mimicked mainly parametrically. We discuss both problems and, in particular, we adopt one of the recent model proposed for producing 13 C and based on an exact multi-D analytical solution of MHD equations, where magnetic buoyancy induces partial mixing at the envelope border. The resulting distribution of 13 C is used, together with our upgraded prescription for the reaction rate, to reproduce solar abundances through AGB models. It can account for the chemical evolution of s-elements and for the s/(C/O) ratios in low-metallicity post-AGB stars.

A Trojan Horse Approach to the Production of 18F in Novae

“…The agreement between the THM and the direct cross sections at high energies and the subsequent scaling represents a necessary validation of the THM, in view of its application to measurements of reactions of astrophysical interest and constitutes a necessary step also for reactions induced by radioactive ion beams, such as the 18 F(p,α) 15 O. In the last years, an intensive experimental and theoretical activity has been carried out to upgrade the THM approach, with the aim of making normalisation to direct data unnecessary (La Cognata et al 2009, 2010Mukhamedzhanov 2011;La Cognata et al 2013;Trippella & La Cognata 2017). This is an important step in the application of the THM to reactions involving unstable nuclei, since in many cases no direct data exist for normalisation, chiefly in the case of reactions induced by neutrons on unstable nuclei.…”

Neutron-captures in Low Mass Stars and the Early Solar System Record of Short-lived Radioactivities

“…the neutron source itself [5,6]. Moreover, of high relevance are the cross sections for p-induced reactions on intermediate-mass isotopes, which must be considered as they hamper the possibility of those nuclei to capture neutrons or α particles.…”