Spectroscopy of P30 and the abundance of Si29 in presolar grains

The astrophysical Si-29(p, gamma) reaction is expected to play a key role in determining the final Si-29 yields ejected in nova explosions. Such yields are used to accurately identify the stellar origins of meteoritic stardust and recently, distinctive silicon isotopic ratios have been extracted from a number of presolar grains. Here, the lightion Si-28(He-3, p) fusion-evaporation reaction was used to populate low-spin proton-unbound excited states in the nucleus P-30 that govern the rate of the astrophysical Si-29(p, gamma) reaction. In particular, gamma decays were observed from resonances up to E-r = 500 keV, and key resonances at 217 and 315 keV have now been identified as 2(+) and 2(-) levels, respectively. The present paper provides the first estimate of the 217-keV resonance strength and indicates that the strength of the 315-keV resonance, which dominates the rate of the Si-29(p, gamma) reaction over the entire peak temperature range of oxygen-neon novae, is higher than previously expected. As such, the abundance of Si-29 ejected during nova explosions is likely to be less than that predicted by the most recent theoretical models.