Aluminium-26 production in low- and intermediate-mass binary systems

Aluminium-26 is a radioactive isotope which can be synthesized within asymptotic giant branch (AGB) stars, primarily through hot bottom burning. Studies exploring Al-26 production within AGB stars typically focus on single-stars; however, observations show that low- and intermediate-mass stars commonly exist in binaries. We use the binary population synthesis code binary_c to explore the impact of binary evolution on Al-26 yields at solar metallicity both within individual AGB stars and a low/intermediate-mass stellar population. We find the key stellar structural condition achieving most Al-26 overproduction is for stars to enter the thermally pulsing AGB (TP-AGB) phase with small cores relative to their total masses, allowing those stars to spend abnormally long times on the TP-AGB compared to single-stars of identical mass. Our population with a binary fraction of 0.75 has an Al-26 weighted population yield increase of 25 per cent compared to our population of only single-stars. Stellar-models calculated from the Mt Stromlo/Monash Stellar Structure Program, which we use to test our results from binary_c and closely examine the interior structure of the overproducing stars, support our binary_c results only when the stellar envelope gains mass after core-He depletion. Stars which gain mass before core-He depletion still overproduce Al-26, but to a lesser extent. This introduces some physical uncertainty into our conclusions as 55 per cent of our Al-26 overproducing stars gain envelope mass through stellar wind accretion onto pre-AGB objects. Our work highlights the need to consider binary influence on the production of Al-26.

Automated summary

In this study, the impact of binary evolution on Al-26 yields is explored. It is found that the key condition for achieving the most overproduction of Al-26 is for stars to have small cores relative to their total masses when entering the TP-AGB phase. The population with a binary fraction of 0.75 shows a 25% increase in Al-26 weighted population yield compared to the population of single-stars only. Further analysis of the stellar interior structure supports these results.