Nucleosynthesis of $^{26}$Al in Classical Novae: Past, Present and Future

The ground state of the unstable nucleus $^{26}$Al ($t_{1/2}=7.2\times 10^5$ yr) decays through the first excited state of $^{26}$Mg $99.7\%$ of the time resulting in the emission of a 1.809-MeV $\gamma$ ray. The distribution of this $\gamma$-ray line, first observed in 1979 [1], has been measured along the Galactic plane by several balloon-borne and satellite experiments, confirming the on-going nucleosynthesis of $^{26}$Al in the Galaxy. The stellar source of this isotope has been the subject of debate, but current estimates indicate the majority is produced in massive stars, while $20-30\%$ is synthesized in classical ONe novae. The situation is further complicated by the existence of an isomeric state at 228 keV ($^{26}$Al$^m$: $J^{\pi}=0^+$, $t_{1/2}=6.3$ s), which $\beta$ decays directly to the ground state of $^{26}$Mg, bypassing the emission of the 1.809-MeV $\gamma$ ray. There are three reaction sequences that produce $^{26}$Al$^{g,m}$ in classical novae, and a variety of studies have been performed to determine the rates of the various reactions involved (e.g. [2-4]). A survey of these measurements will be given, as well as the current status of $^{26}$Al production in novae. Future plans to accurately determine the reaction rates that dominate the remaining uncertainties in $^{26}$Al nucleosynthesis will also be discussed. \\[4pt] [1] W. A. Mahoney $et$ $al.$, Astrophys. J. {\bf 286}, 578 (1984).\\[0pt] [2] C. M. Deibel $et$ $al.$, Phys. Rev. C {\bf 80}, 035806 (2009).\\[0pt] [3] G. Lotay $et$ $al.$, Phys. Rev. C {\bf 80}, 055802 (2009).\\[0pt] [4] M. B. Bennett $et$ $al.$, Phys. Rev. Lett. {\bf 111}, 232503 (2013).