The ³³S(p,γ)³⁴Cl reaction in classical nova explosions

The analysis of microscopic grains within primitive meteorites has revealed isotopic ratios largely characteristic of the conditions thought to prevail in various astrophysical environments. Recently, several grains have been identified with isotopic signatures similar to those predicted within the ejecta of nova explosions on oxygen-neon white dwarfs. A possible smoking gun for a grain of nova origin is a large ³³S abundance: nucleosynthesis calculations predict as much as 150 times the solar abundance of ³³S in the ejecta of oxygen-neon novae. This overproduction factor may, however, vary by factors of at least 0.01 - 3 because of uncertainties in the ³³S(p,γ)³⁴Cl reaction rate over nova temperatures. In addition, better knowledge of this rate would help with the interpretation of nova observations over the S-Ca mass region, and contribute towards the firm establishment of a nucleosynthetic endpoint in these phenomena. Finally, constraining this rate may help to finally confirm or rule out the decay of an isomeric state of ³⁴Cl (Ex = 146 keV, t_1/2 =32 min) as a source for observable gamma-rays from novae. Direct examinations of the ³³S(p,γ)³⁴Cl reaction in the past have only identified resonances down to Er = 434 keV. At nova temperatures, lower-lying resonances could certainly play a dominant role. Several recent, complementary studies dedicated to improving our knowledge of the 33S(p,γ)34Cl rate, using both indirect methods (measurement of the ³⁴S( ³He,t)³⁴Cl and ³³S(³He,d) ³⁴Cl reactions with the Munich Q3D spectrograph) and direct methods (in normal kinematics at CENPA, University of Washington, and in inverse kinematics with the DRAGON recoil mass separator at TRIUMF) are presented here. Our results affect predictions of sulphur isotopic ratios in nova ejecta (e.g. ³²S/³³S) that may be used as diagnostic tools for the nova paternity of grains.