Spectroscopy of ${}^{30}\mathrm{P}$ and the abundance of ${}^{29}\mathrm{Si}$ in presolar grains

The astrophysical ${}^{29}\mathrm{Si}(p,\gamma$) reaction is expected to play a key role in determining the final ${}^{29}\mathrm{Si}$ 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 light-ion ${}^{28}\mathrm{Si}({}^3\mathrm{He},p$) fusion-evaporation reaction was used to populate low-spin proton-unbound excited states in the nucleus ${}^{30}\mathrm{P}$ that govern the rate of the astrophysical ${}^{29}\mathrm{Si}(p,\gamma$) reaction. In particular, $\gamma$ decays were observed from resonances up to $E_r=500\mathrm{keV}$, 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 ${}^{29}\mathrm{Si}(p,\gamma$) reaction over the entire peak temperature range of oxygen-neon novae, is higher than previously expected. As such, the abundance of ${}^{29}\mathrm{Si}$ ejected during nova explosions is likely to be less than that predicted by the most recent theoretical models.

Figure 1

(a) $\gamma \text{−}\gamma$ coincidence spectrum with a gate placed on the 709-keV $1_2^{+}\to 1_1^{+}$ transition in ${}^{30}\mathrm{P}$. Numbers in parentheses represent resonance energies. (b) $\gamma \text{−}\gamma$ coincidence spectrum with a gate placed on the 1973-keV $3_1^{+}\to 1_1^{+}$ transition. (c) $\gamma \text{−}\gamma$ coincidence spectrum with a gate placed on the 677-keV $0_1^{+}\to 1_1^{+}$ transition and (d) $\gamma \text{−}\gamma \text{−}\gamma$ coincidence spectrum with gates placed on the 677-keV $0_1^{+}\to 1_1^{+}$ and 2260-keV $2_3^{+}\to 0_1^{+}$ transitions.

Figure 2

(Top) The ${}^{29}\mathrm{Si}(p,\gamma ){}^{30}\mathrm{P}$ stellar reaction rate as a function of temperature. The solid blue line represents the present rate as per the resonant parameters presented in Table 2 (the direct capture component is negligible for temperatures $T\ge 0.03\mathrm{GK}$ [25]), whereas the dashed red line represents the rate reported in Refs. [19, 26]. (Bottom) The solid blue line shows the ratio of the present rate to the rate of Refs. [19, 26]. A dashed red line has been included to highlight what would be expected for a ratio of 1:1.