The Importance of the ¹³C(α,n)¹⁶O 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 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.

A new approach to monitor $$^{13}\hbox {C}$$-targets degradation in situ for $$^{13}\hbox {C}(\alpha ,\hbox {n})^{16}\hbox {O}$$ cross-section measurements at LUNA

“…This was the case of the astrophysically important 13 C(α,n) 16 O reaction [10] recently studied in direct kinematics at the Laboratory for Underground Nuclear Astrophysics (LUNA) [11,12] of the Laboratori Nazionali del Gran Sasso (LNGS), INFN, Italy. Because of the small cross-sections involved at the energies investigated (E α = 305 − 400 keV), intense α-particle beams were needed, leading to severe target degradation and frequent target replacements.…”

The Study of Key Reactions Shaping the Post-Main Sequence Evolution of Massive Stars in Underground Facilities

“…A reservoir of 13 C, produced via the 12 C(p, c) 13 N(β + ]) 13 C reaction chain, forms the so-called 13 C pocket at the interface between the He-rich intershell and the H-rich envelope. As of today, the exact formation mechanism of such a pocket is still debated, as stated by Cristallo et al (2018). During this phase, which lasts some 10 4 years, the 13 C(α,n) 16 O reaction takes place and provides neutrons for the main s-process.…”

“…During this phase, which lasts some 10 4 years, the 13 C(α,n) 16 O reaction takes place and provides neutrons for the main s-process. In the article by Cristallo et al (2018), the authors claim that, in the most metal-rich stellar models with an almost solar composition, a small amount of 13 C might survive and be engulfed into the convective zone generated by the incoming thermal pulse. This scenario would affect several branching points along the s-process path, and excesses of 60 Fe, 86 Kr, 87 Rb, or 96 Zr are expected compared to the radiative (low neutron density) 13 C burst.…”

Resonant reactions of astrophysical interest studied by means of the Trojan Horse Method. Two case studies

“…Recently, two works [43,44] have determined new interesting spectroscopic information about the 6.356 MeV. The consequences on the 13 C(α, n) 16 O S-factor are presently under investigation [45,46].…”