Fictitious neutron sinks to trace radiative s-process nucleosynthesis

Institut d’Astronomie et d’Astrophysique (IAA), Université Libre de Bruxelles (ULB), CP226, Boulevard du Triomphe, 1050 Brussels, Belgium
e-mail: pawel.krynski@ulb.be

Received: 8 September 2022
Accepted: 29 November 2022

Abstract

Context. Asymptotic giant branch (AGB) stars are strong producers of s-process elements, which are synthesized by successive slow neutron captures on elements heavier than iron. The nucleosynthesis calculation involves solving large nuclear networks with hundreds of nuclei, which in a stellar evolution code can greatly extend the computational time. However, the s-process is often measured using a handful of elements located on the neutron magic shells and grouped into tracers called ls, hs, and vhs.

Aims. We propose a fictitious network that approximates the production of ls, hs, and vhs species at a minimal computational expense. The network is specifically designed for the radiative s-process in AGB stars. It is an alternative to methods using large networks that can be used as a fast exploratory tool to trace the production of s-elements.

Methods. The fictitious network was constructed by assembling species with Z ≥ 18 into seven fictitious particles whose abundances and reaction rates model the effective properties of the corresponding groups. The effective reaction rates were tabulated as a function of neutron density and number of neutrons captured per initial heavy seed (N_capt) using single-zone nucleosynthesis calculations. The accuracy of our network was tested by comparing the abundances obtained with the fictitious and large networks during the radiative burning of ¹³C during the interpulse period of a 2 M_⊙, [Fe/H] = −2 star.

Results. The fictitious network reliably reproduces the abundances of ls, hs, and vhs species during the radiative s-process. The accuracy of the method increases with the strength of the nucleosynthesis as measured by N_capt, but diminishes when the nuclear distribution is different from the initial distribution. This network is well suited to follow the s-process nucleosynthesis in low-mass AGB stars where neutrons are mainly produced below the envelope by the ¹³C(α, n) reaction.