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

Carlo Bruno ,M Junker,G Gervino,F Cavanna,Z Elekes,A Formicola,V Mossa,A Guglielmetti,Gy Gyürky,P Corvisiero,K Stöckel,Zs Fülöp,A Di Leva,A Boeltzig,T Chillery,C Gustavino,R Menegazzo,A Caciolli,P Prati,A Best,F Ferraro,C Broggini,G F Ciani ,D Trezzi,G Imbriani,Paola Marigo ,D Piatti,D Bemmerer,F Strieder,F R Pantaleo ,R Depalo,T Szücs,Maria Lugaro ,O Straniero,T Davinson,M P Takács ,Pierre Descouvemont ,M Aliotta

doi:10.1016/j.physletb.2019.01.017

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 Ec.m.=340 keV down to Ec.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 Er=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=0.01–1.00 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.

Highlights

The 18O(p,α)15N reaction influences the abundances of 15N, 18O and 19F isotopes [1, 2], critical to constrain a wide variety of stellar models
The O isotopic ratios observed in asymptotic giant branch (AGB) stars of different masses [3, 4, 5] can be used to probe the nucleosynthesis and mixing processes in these stars
The 18O/16O abundance ratio is critical in classifying stardust oxide and silicate grains that originally condensed in AGB stars, supernovae, and novae and Preprint submitted to Physics Letters B

Summary

Introduction

The 18O(p,α)15N reaction influences the abundances of 15N, 18O and 19F isotopes [1, 2], critical to constrain a wide variety of stellar models. A low-background measurement of the 18O(p,α)15N reaction at energies of astrophysical interest can result in more accurate predictions for the O isotopic composition and place stronger constraints on the stellar sites from where these grains originate [8, 9]. At temperatures of astrophysical interest (T = 0.01 − 1.00 GK), its rate is dominated by the interference of three Jπ = 1/2+ resonances at center of mass energies Er = 143, 610 and 800 keV, respectively For the latter two resonances, results on their energy and partial widths are largely inconsistent [14], and tensions have been reported between the cross section of different datasets at energies Ec.m. The stellar reaction rate still contains significant uncertainties

Objectives

Methods

Results

Conclusion