Improved astrophysical rate for the ¹⁸O(p,α)¹⁵N reaction by underground measurements

Abstract

The ¹⁸O(p,α)¹⁵N reaction affects the synthesis of ¹⁵N, ¹⁸O and ¹⁹F isotopes, whose abundances can be used to probe the nucleosynthesis and mixing processes occurring deep inside asymptotic giant branch (AGB) stars. We performed a low-background direct measurement of the ¹⁸O(p,α)¹⁵N reaction cross-section at the Laboratory for Underground Nuclear Astrophysics (LUNA) from center of mass energy $E_{\text{c.m.}}=340$ keV down to $E_{\text{c.m.}}=55$ keV, the lowest energy measured to date corresponding to a cross-section of less than 1 picobarn/sr. The strength of a key resonance at center of mass energy $E_r=90$ keV was found to be a factor of 10 higher than previously reported. A multi-channel R-matrix analysis of our and other data available in the literature was performed. Over a wide temperature range, $T=0.01\text{–}1.00$ GK, our new astrophysical rate is both more accurate and precise than recent evaluations. Stronger constraints can now be placed on the physical processes controlling nucleosynthesis in AGB stars with interesting consequences on the abundance of ¹⁸O in these stars and in stardust grains, specifically on the production sites of oxygen-rich Group II grains.