Confirmation of a new resonance in ${}^{26}\mathrm{Si}$ and contribution of classical novae to the galactic abundance of ${}^{26}\mathrm{Al}$

The ${}^{25}\mathrm{Al}(p,\gamma )$ reaction has long been highlighted as a possible means to bypass the production of ${}^{26}\mathrm{Al}$ cosmic $\gamma$ rays in classical nova explosions. However, uncertainties in the properties of key resonant states in ${}^{26}\mathrm{Si}$ have hindered our ability to accurately model the influence of this reaction in such environments. We report on a detailed $\gamma$-ray spectroscopy study of ${}^{26}\mathrm{Si}$ and present evidence for the existence of a new, likely $\ell =1$, resonance in the ${}^{25}\mathrm{Al}$ $+$ $p$ system at $E_r=153.9\left(15\right)$ keV. This state is now expected to provide the dominant contribution to the ${}^{25}\mathrm{Al}(p,\gamma )$ stellar reaction rate over the temperature range, $T\approx 0.1-0.2$ GK. Despite a significant increase in the rate at low temperatures, we find that the final ejected abundance of ${}^{26}\mathrm{Al}$ from classical novae remains largely unaffected even if the reaction rate is artificially increased by a factor of 10. Based on new, galactic chemical evolution calculations, we estimate that the maximum contribution of novae to the observed galactic abundance of ${}^{26}\mathrm{Al}$ is $\approx 0.2M_{\odot }$. Finally, we briefly highlight the important role that super-asymptotic giant branch stars may play in the production of ${}^{26}\mathrm{Al}$.

Figure 1

A typical $\mathrm{\Delta }E\text{−}E$ ionization chamber histogram used for $Z$ selectivity. The distinct regions associated with various atomic numbers are labeled. Example software particle identification gates are shown on the histogram.

Figure 2

Tracked and Doppler-corrected $\gamma$-ray energy spectrum measured in coincidence with ${}^{26}\mathrm{Si}$ recoils at the FMA focal plane. The inset shows the energy region of interest for high-energy direct-to-ground state decays.

Figure 3

A portion of the $\gamma$-ray spectrum gated on the 989-keV transition in ${}^{26}\mathrm{Si}$. The energies of the $\gamma$ rays are given in keV.

Figure 4

A GRETINA energy spectrum simulated with the geant4 toolkit, displaying the response to an isolated 5667-keV $\gamma$ ray. The insets show portions of the spectrum zoomed in on the single- and double-escape peaks and on region around 511 keV.

Figure 5

Favored mirror assignments for excited states in ${}^{26}\mathrm{Si}$ (left) and ${}^{26}\mathrm{Mg}$ (right) between 4.5 and 6.4 MeV. The $3_1^{+}$ state, which is labeled in red, is not observed in the current study but the mirror assignment has been well established in previous work.

Figure 6

The ${}^{25}\mathrm{Al}(p,\gamma ){}^{26}\mathrm{Si}$ reaction rate estimated for temperatures up to 0.5 GK assuming the resonance properties presented in Table 2 and discussed in the text. The contributions of individual resonances to the total rate are also shown.