Improving the P30(p,gamma)S31 rate in oxygen-neon novae: Constraints on Jpi values for proton-threshold states in S31

Calculation of the thermonuclear P-30(p,gamma)S-31 rate in oxygen-neon nova explosions depends critically upon nuclear structure information for states within similar to 600 keV of the P-30 + p threshold in S-31. We have studied the P-31(He-3, t)S-31 reaction at 25 MeV using a high-resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. Tritons corresponding to the states E-x(S-31) similar to 6.1-7.1 MeV were observed at ten angles between theta(lab) = 10 degrees and 55 degrees. States that were only tentatively identified in past studies have been observed unambiguously. For the first time, we have measured and analyzed angular distributions of the P-31(He-3, t)S-31 reaction. We present, also for the first time, a consistent set of experimental spin constraints for all except one of the critical proton-threshold states in S-31. Hydrodynamic nova simulations have been calculated in order to assess the impact on nova nucleosynthesis of remaining uncertainties in J(pi) values of S-31 states and the unknown relevant proton spectroscopic factors. We find that these uncertainties may lead to a factor of up to 20 variation in the P-30(p,gamma)S-31 rate over typical nova peak temperatures, which may then lead to a factor of up to 4 variation in the nova yields of Si-Ar isotopes.