The impermanent fate of massive stars in AGN discs

Abstract

ABSTRACT Stars are likely to form or to be captured in active galactic nucleus (AGN) discs. Their mass reaches an equilibrium when their rate of accretion is balanced by that of wind. If the exchanged gas is well mixed with the stellar core, this metabolic process would indefinitely sustain an ‘immortal’ state on the main sequence (MS) and pollute the disc with He byproducts. This theoretical extrapolation is inconsistent with the super-solar α element and Fe abundances inferred from the broad emission lines in AGNs with modest He concentration. We show this paradox can be resolved with a highly efficient retention of the He ashes or the suppression of chemical blending. The latter mechanism is robust in the geometrically thin dense sub-pc regions of the disc where the embedded-stars’ mass is limited by the gap-formation condition. These stars contain a radiative zone between their mass-exchange stellar surface and the nuclear-burning core. Insulation of the core lead to the gradual decrease of its H fuel and the stars’ equilibrium masses. These stars transition to their post-MS (PostMS) tracks on a chemical evolution time-scale of a few Myr. Subsequently, the triple-α and α-chain reactions generate α and Fe byproducts which are released into their natal discs. These PostMS stars also undergo core collapse, set off type II supernova, and leave behind a few solar-mass residual black holes or neutron stars.