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

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 ¹³C(α,n)¹⁶O 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 ¹³C(α,n)¹⁶O 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 ¹³C 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 ¹³C(α,n)¹⁶O 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 ⁸⁶Kr and ⁹⁶Zr). 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.