First measurement of the S 34 (p,γ) Cl 35 reaction rate through indirect methods for presolar nova grains

Abstract

Sulphur isotopic ratio measurements may help to establish the astrophysical sites in which certain presolar grains were formed. Nova model predictions of the S34/S32 ratio are, however, unreliable due to the lack of an experimental S34(p,γ)Cl35 reaction rate. To this end, we have measured the S34(He3,d)Cl35 reaction at 20 MeV using a high resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. Twenty-two levels over 6.2 MeV <7.4 MeV were identified, ten of which were previously unobserved. Proton-transfer spectroscopic factors have been measured for the first time over the energy range relevant for novae. With this new spectroscopic information a new S34(p,γ)Cl35 reaction rate has been determined using a Monte Carlo method. Hydrodynamic nova model calculations have been performed using this new reaction rate. These models show that remaining uncertainties in the S34(p,γ) rate affect nucleosynthesis predictions by less than a factor of 1.4, and predict a S34/S32 isotopic ratio of 0.014-0.017. Since recent type II supernova models predict S34/S32=0.026-0.053, the S34/S32 isotopic ratio may be used, in conjunction with other isotopic signatures, to distinguish presolar grains from oxygen-neon nova and type II supernova origin. Our results address a key nuclear physics uncertainty on which recent considerations discounting the nova origin of several grains depend.