Isotopic S-32/S-33 ratio as a diagnostic of presolar grains from novae
Measurements of sulphur isotopes in presolar grains can help to identify the astrophysical sites in which these grains were formed. A more precise thermonuclear rate of the S-33(p, gamma)Cl-34 reaction is required, however, to assess the diagnostic ability of sulphur isotopic ratios. We have studied the S-33(He-3,d)Cl-34 proton-transfer reaction at 25 MeV using a high-resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. Deuteron spectra were measured at ten scattering angles between 10 degrees and 55 degrees. Twenty-four levels in Cl-34 over E-x = 4.6-5.9 MeV were observed, including three levels for the first time. Proton spectroscopic factors were extracted for the first time for levels above the S-33 + p threshold, spanning the energy range required for calculations of the thermonuclear S-33(p, gamma)Cl-34 rate in classical nova explosions. We have determined a new S-33(p, gamma)Cl-34 rate using a Monte Carlo method and have performed new hydrodynamic nova simulations to determine the impact on nova nucleosynthesis of remaining nuclear physics uncertainties in the reaction rate. We find that these uncertainties lead to a factor of <= 5 variation in the S-33(p, gamma)Cl-34 rate over typical nova peak temperatures, and variation in the ejected nova yields of S-Ca isotopes by <= 20%. In particular, the predicted S-32/S-33 ratio is 110-130 for the nova model considered, compared to 110-440 with previous rate uncertainties. As recent type II supernova models predict ratios of 130-200, the S-32/S-33 ratio may be used to distinguish between grains of nova and supernova origin. (C) 2014 The Authors. Published by Elsevier B.V.