Fictitious Neutron Sinks to Trace Radiative S-Process Nucleosynthesis

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 (Ncapt) 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 Ncapt, 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.