Search

The complex reaction network connected to the 19F stellar abundance is very sensitive to the physical condition in stars. To overcome difficulties present in the direct measurements, mainly the very low cross section in the astrophysical energy range due to the Coulomb repulsion, an indirect measurement of the 19F(p, α) reaction, with the Trojan Horse Method (THM), was performed at INFN-LNS. While the method had been successfully used to study α0 channel, in the present analysis focus was given to the α1,2 channels, where better knowledge of the reaction rates at low energies is required, obtainable by employing the THM method.

Noticeable improvements were recently introduced in the modelling of n-capture nucleosynthesis in the advanced evolutionary stages of giant stars (Asymptotic Giant Branch, or AGB, stars). Two such improvements are closely linked together and concern the introduction of non-parameterized, physical models for extended mixing processes and the adoption of accurate reaction rates for H- and He-burning reactions, including the one for the main neutron source 13C(α,n)16O. These improvements profited of a longstanding collaboration between stellar physicists and C. Spitaleri’s team and of his seminal work both as a leader in the Nuclear Astrophysics scenario and as a talent-scout in the recruitment of young researchers in the field. We present an example of the innovative results that can be obtained thanks to the novelties introduced, by estimating the contributions from a nearby AGB star to the synthesis of short-lived (t1/2 ≤ 10 Myr) radioactive nuclei which were alive in early Solar System condensates. We find that the scenario indicating an AGB star as the source of such radioactivities, discussed for many years by researchers in this field, appears now to be no longer viable, when the mentioned improvements of AGB models and nuclear parameters are considered.

The nucleosynthesis of 7Li is one of the most crucial problems in nuclear as- trophysics, as its observations in several sites are hard to be explained. Concerning the Sun, the most common interpretations of the low Li abundance invoke either burning in early stages or non-convective mixing below the envelope. Here we apply a diffusive mechanism of mixing, together with a recent estimate of the rate for e−-captures on 7Be, to establish whether the solar Li destruction should be attributed to purely pre-Main Se- quence (MS) nuclear processes or if the coupling of mixing and nucleosynthesis on the MS can account for it. Our preliminary results indicate that, whether Li survives the pre- MS phase, the changes of the 7Be e−-capture rate do not affect its production/destruction. The low Li abundance should then depend only on diffusion processes from the bottom of the convective envelope to the lowerlying tachocline zone. We suggest that, if diffusive processes occurred over the age of the Sun, they required diffusive mass transfers of a few 10−13 M⊙/yr to explain the Li drop. This is a high estimate: future works will tell us if it is realistic or not. In this second case, pre-MS burning would remain the only alternative.

We present here the application of a model for a mass circulation mechanism induced by the stellar magnetic field to study peculiar aspects of AGB star nucleosynthesis. The mixing scheme is based on a previously suggested magnetic-buoyancy process [1, 2] and here shown to account adequately for the formation of the 13C neutron source for s-processes. In particular our analysis results are focused on addressing the constrains to AGB nucleosynthesis coming from the isotopic composition of presolar grains recovered in meteorites. It turns out that n-captures driven by the magnetically-induced mixing can account for the isotopic abundance ratios of s-elements recorded.

The 13C(α, n)16O reaction is considered to be the most important neutron source for the s-process main component in low-mass asymptotic giant branch stars. No direct experimental data exist at very low energies and measurements performed through direct techniques show inconsistent results, mostly in their absolute values. In this context, we reversed the usual normalization procedure combining two indirect approaches, the asymptotic normalization coefficient and the Trojan Horse Method, to unambiguously determine the absolute value of the 13C(α, n)16O astrophysical S(E)-factor in the most relevant energy-region for astrophysics. Adopting the new reaction rate for the n-source in the NEWTON s-process nucleosynthesis code, astrophysical calculations show only limited variations, less than 1%, for those nuclei whose production is considered to be totally due to slow neutron captures.