Nucleosynthesis of light and heavy elements across the Galaxy

The synthesis of elements mostly takes place in stars. Elements up to iron peak are a by-product of thermonuclear fusion reactions sustaining the stellar structure itself. This is the case of the Sun, whose energy generation is provided by the pp chain and (to a lesser extent) by the CNO cycle. However, a precise evaluation of their relative contributions is still lacking. On the other hand, elements heavier than iron are almost entirely produced via the slow (s) and rapid (r) neutron capture processes. Low-mass stars during their Asymptotic Giant Branch (AGB) phase are the site of the so-called main component of the s-process. In those objects a 13C n-source (the 13C pocket), formed due to partial mixing of hydrogen from the convective envelope, operates. The physical mechanism responsible for such mixing is still a matter of debate. Binary neutron star mergers are instead primary sites for the production of heavy elements through the r-process. This was recently confirmed by the detection of the kilonova AT2017gfo as the electromagnetic counterpart of the gravitational wave signal GW170817. Interestingly, the production of r-process elements can be accompanied by the synthesis of light elements, which could be detected during the early kilonova emission. With a multi-messenger approach, we investigate the nucleosynthesis occurring in these astrophysical scenarios by comparing results not only to stellar spectra, but also other observables: i.e. emission of neutrinos, isotopic measurements in primitive meteorites, and electromagnetic radiation emissions. Regarding the Sun, we use solar models to study the impact of a different 7Be electron-capture rate on the solar structure. We compare the results with the measured 7Be and 8B solar neutrino fluxes and find that the agreement with the Sudbury Neutrino Observatory measurements of the neutral current component of the 8B neutrino flux is improved. We then revise and generalize the relation connecting the measured solar photon luminosity and the total solar neutrino fluxes, namely the luminosity constraint. In particular, we study and determine the energy contributions due to non-equilibrium burning of 3He and 14N and due to solar expansion/contraction. We finally show the importance of such a relation for the search of CNO neutrinos by providing a specific relation that links CNO and pp neutrino fluxes. For what concerns the nucleosynthesis in AGB stars, we examine the origins of now-extinct radioactivities that were alive in the solar nebula, as testified by meteoritic measurements, in the light of most updated stellar models for intermediate-mass stars and massive stars. We find that, while the Galactic inheritance broadly explains most of the isotopes involved with lifetime τ ≳ 5 Myr, shorter-lived isotopes require nucleosynthesis events close in time to the solar formation. We investigate the possibility that an Asymptotic Giant Branch or a Massive star could have contaminated the solar nebula. We find that both scenarios meet serious problems to reproduce the presence of isotopic anomalies measured in early solar system solids of the solar system. In another work, we perform the first numerical simulations of the formation of a magnetically-induced 13C pocket in a stellar evolutionary code with fully coupled nucleosynthesis. We find that magnetic fields of the order 10^5 G can induce the formation and the buoyant rise of magnetic flux tubes in the He-intershell of AGB stars. The ensuing mixing can account for the downward penetration of poorly magnetized H-rich material, necessary for the formation of the 13C pocket. By adopting a single magnetic field configuration, new magnetic models provide a consistent explanation to the majority of the heavy-element isotope data detected in presolar SiC grains from AGB stars. Finally, we study the production of light elements in the ejecta of binary neutron star mergers. The outcome of numerical relativity merger simulations is combined with detailed r-process nucleosynthesis calculations performed with the Skynet nuclear reaction network. We find that hydrogen and helium are the most abundant light elements. However, despite their high abundance, the possibility of detecting hydrogen and helium features in kilonova spectra is very unlikely.