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

Calculation of the thermonuclear P30(p,γ)S31 rate in oxygen-neon nova explosions depends critically upon nuclear structure information for states within ∼600 keV of the 30P+p threshold in S31. We have studied the P31(3He,t)S31 reaction at 25 MeV using a high-resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. Tritons corresponding to the states Ex(S31) ∼ 6.1–7.1 MeV were observed at ten angles between θlab = 10° and 55°. 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 P31(3He,t)S31 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 S31. Hydrodynamic nova simulations have been calculated in order to assess the impact on nova nucleosynthesis of remaining uncertainties in Jπ values of S31 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 P30(p,γ)S31 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.