The Importance of the 13 C(α,n) 16 O Reaction in Asymptotic Giant Branch Stars

Abstract

Low-mass asymptotic giant branch stars are among the most important polluters of the interstellar medium. In their interiors, the main component ( A  ≳ 90) of the slow neutron capture process (the s -process) is synthesized, the most important neutron source being the 13 C( α ,n) 16 O reaction. In this paper, we review its current experimental status, discussing possible future synergies between some experiments currently focused on the determination of its rate. Moreover, in order to determine the level of precision needed to fully characterize this reaction, we present a theoretical sensitivity study, carried out with the FUNS evolutionary stellar code and the NEWTON post-process code. We modify the rate up to a factor of 2 with respect to a reference case. We find that variations of the 13 C( α ,n) 16 O rate do not appreciably affect s -process distributions for masses above 3 M ⊙ at any metallicity. Apart from a few isotopes, in fact, the differences are always below 5%. The situation is completely different if some 13 C burns in a convective environment: this occurs in FUNS models with M  < 3 M ⊙ at solar-like metallicities. In this case, a change of the 13 C( α ,n) 16 O reaction rate leads to nonnegligible variations of the element surface distribution (10% on average), with larger peaks for some elements (such as rubidium) and neutron-rich isotopes (such as 86 Kr and 96 Zr). Larger variations are found in low-mass, low-metallicity models if protons are mixed and burned at very high temperatures. In this case, the surface abundances of the heavier elements may vary by more than a factor of 50.