Slow Neutron-Capture Process: Low-Mass Asymptotic Giant Branch Stars and Presolar Silicon Carbide Grains

Abstract

Presolar grains are microscopic dust grains that formed in the stellar winds or explosions of ancient stars that died before the formation of the solar system. The majority (~90% in number) of presolar silicon carbide (SiC) grains, including types mainstream (MS), Y, and Z, came from low-mass C-rich asymptotic giant branch (AGB) stars, which is supported by the ubiquitous presence of SiC dust observed in the circumstellar envelope of AGB stars and the signatures of slow neutron-capture process preserved in these grains. Here, we review the status of isotope studies of presolar AGB SiC grains with an emphasis on heavy element isotopes and highlight the importance of presolar grain studies for nuclear astrophysics. We discuss the sensitives of different types of nuclei to varying AGB stellar parameters and how their abundances in presolar AGB SiC grains can be used to provide independent, detailed constraints on stellar parameters, including 13C formation, stellar temperature, and nuclear reaction rates.

Magnetic and non-magnetic AGB mixing for s-processing

“…The complementary results obtained by [14] in comparing s-process model predictions from the above scenario with isotopic ratios of heavy nuclei measured in presolar SiC grains of AGB origin [16] added further credibility to the global picture.…”

“…• The extended 13 C pockets obtained in the above scenario make the nucleosynthesis of heavy nuclei from the 13 C(α,n) 16 O source more efficient than before.…”

“…This implies that, at the reignition of H-burning in the shell, proton captures do occur on 12 C producing 13 C, but minimal traces of 14 N can be synthesized (Figure 1, panel ii). Hence, neutrons made available by the 13 C(α,n) 16 O source remain largely unfiltered by intermediate-mass poisons, that would mainly result as daughters of 14 These peculiarities of the models are crucial to obtain good agreement with various observations (see discussion in [11,12,15]). They induce remarkble differences with respect to previous models.…”

“…In another lesson at this school [1] the general picture of neutron-capture nucleosynthesis was layed down and due space was dedicated to the slow (s) process of neutron addition, occurring in low mass stars (1 M/M 3). There, the involved evolutionary stages are those of the so-called Asymptotic Giant Branch, when the repeated activation of the neutron sources 13 C(α,n) 16 O and 22 Ne(α,n) 25 Mg make moderate neutron fluxes available, feeding nucleosynthesis along the valley of β stability [2,3].…”

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I outline a few features of recent models for the formation of the neutron source 13 C(α,n) 16 O in low mass stars (1 M/M 3, LMS ) ascending for the second time the Red Giant Branch, generally called Asymptotic Giant Branch, or AGB stars. I also briefly outline the nucleosynthesis results obtained trough them.

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