First measurement of the S34(p,γ)Cl35 reaction rate through indirect methods for presolar nova grains

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Sulphur isotopic ratio measurements may help to establish the astrophysical sites in which certain presolar grains were formed. Nova model predictions of the S-34/S-32 ratio are, however, unreliable due to the lack of an experimental S-34(p,gamma)Cl-35 reaction rate. To this end, we have measured the S-34(He-3,d)Cl-35 reaction at 20 MeV using a high resolution quadrupole-dipole-dipole-dipolemagnetic spectrograph. Twenty-two levels over 6.2 MeV < E-x (Cl-35) < 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 S-34(p,gamma)Cl-35 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 S-34(p,gamma) rate affect nucleosynthesis predictions by less than a factor of 1.4, and predict a S-34/S-32 isotopic ratio of 0.014-0.017. Since recent type II supernova models predict S-34/S-32 = 0.026-0.053, the S-34/S-32 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.