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

Abstract

Aluminium-26 is a radioactive isotope which can be synthesized within asymptotic giant branch (AGB) stars, primarily through hot bottom burning. Studies exploring26Al 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 on26Al yields at solar metallicity both within individual AGB stars and a low/intermediate-mass stellar population. We find the key stellar structural condition achieving most26Al 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 an26Al 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 overproduce26Al, but to a lesser extent. This introduces some physical uncertainty into our conclusions as 55 per cent of our26Al 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 of26Al.