News on the s process from young open clusters

TL;DR

This paper proposes that low-mass AGB stars with enhanced ^{13}C neutron sources can explain the observed increase in neutron-rich element abundances in young open clusters, aligning with solar s-process distribution without ad-hoc assumptions.

Contribution

It introduces a stronger neutron source in low-mass AGB stars as a key factor in reproducing solar-like s-process abundances in young stellar populations.

Findings

Enhanced ^{13}C neutron source increases s-process efficiency.

Stellar yields now match solar distribution at higher metallicity.

Stable conditions over Gyr timescales allow these yields to influence subsequent star formation.

Abstract

Recent spectroscopic measurements in open clusters younger than the Sun, with [Fe/H]>=0, showed that the abundances of neutron-rich elements have continued to increase in the Galaxy after the formation of the Sun, roughly maintaining a solar-like distribution. Such a trend requires neutron fluences larger than those so far assumed, as these last would have too few neutrons per iron seed. We suggest that the observed enhancements can be produced by nucleosynthesis in AGB stars of low mass (M < 1.5M\odot), if they release neutrons from the ^{13}C({\alpha},n)^{16}O reaction in reservoirs larger by a factor of 4 than assumed in more massive AGBs (M > 1.5M\odot). Adopting such a stronger neutron source as a contributor to the abundances at the time of formation of the Sun, we show that this affects also the solar s-process distribution, so that its main component is well reproduced, without the need of assuming ad-hoc primary sources for the synthesis of s elements up to A \sim 130, contrary to suggestions from other works. The changes in the expected abundances that we find are primarily due to the following reasons. i) Enhancing the neutron source increases the efficiency of the s process, so that the ensuing stellar yields now mimic the solar distribution at a metallicity higher than before ([Fe/H]>=-0.1). ii) The age-metallicity relation is rather flat for several Gyr in that metallicity regime, so that those conditions remain stable and the enhanced nuclear yields, which are necessary to maintain a solar-like abundance pattern, can dominate the composition of the interstellar medium from which subsequent stars are formed.