Evaluating chemically homogeneous evolution in stellar binaries: electromagnetic implications – ionizing photons, SLSN-I, GRB, Ic-BL

Abstract

ABSTRACTWe investigate the occurrence of rapid-rotation-induced chemically homogeneous evolution (CHE) due to strong tides and mass accretion in binaries. To this end, we generalize the relation in Packet to calculate the minimum angular momentum (AM) accretion required by a secondary star to experience accretion-induced CHE. Contrary to traditionally assumed 5–10 per cent accretion of initial mass (Z ≲ 0.004, M ≳ 20 M⊙) for spinning up the accretor (resulting in CHE), this value can drop to ∼2 per cent for efficient AM accretion, while for certain systems it could be substantially larger. We conduct a population study using bpass of evolving stars under the influence of strong tides in short-period binaries and also account for the updated effect of accretion-induced spin-up. We find accretion CHE (compared to tidal CHE) to be the dominant means of producing homogeneous stars even at 10 per cent AM accretion efficiency during mass transfer. Unlike tidal CHE, it is seen that CH stars arising due to accretion can retain a larger fraction of their AM till core collapse. Thus, we show that accretion CHE could be an important formation channel for energetic electromagnetic transients like gamma-ray bursts, Ic-BL (SLSN-I, Ic-BL) under the collapsar (magnetar) formalism, and a single CH star could lead to both the transients under their respective formation scenario. Lastly, we show that under the current treatment of CHE, the emission rate of ionizing photons by such stars decreases more rapidly at higher metallicities than previously predicted.