New Cl34 proton-threshold states and the thermonuclear S33(p,γ)Cl34 rate in ONe novae

Analysis of presolar grains in primitive meteorites has shown isotopic ratios largely characteristic of the conditions thought to prevail in various astrophysical environments. A possible indicator for a grain of ONe nova origin is a large S33 abundance: nucleosynthesis calculations predict as much as 150 times the solar abundance of S33 in the ejecta of nova explosions on massive ONe white dwarfs. This overproduction factor may, however, vary by factors of at least 0.01-3 because of uncertainties of several orders of magnitude in the S33(p,γ)Cl34 reaction rate at nova peak temperatures (Tpeak~0.1-0.4 GK). These uncertainties arise due to the lack of nuclear physics information for states within ~600 keV of the S33+p threshold in Cl34 (Sp(Cl34) = 5143 keV). To better constrain this rate we have measured, for the first time, the S34(He3,t)Cl34 reaction over the region Ex(Cl34) = 4.9-6 MeV. We confirm previous states and find 15 new states in this energy region. New S33(p,γ)Cl34 resonances at ER=281(2), 301(2), and 342(2) keV may dominate this rate at relevant nova temperatures. Our results could affect predictions of sulphur isotopic ratios in nova ejecta (e.g., S32/S33) that may be used as diagnostic tools for the nova paternity of grains.