Evaluating chemically homogeneous evolution in stellar binaries: Electromagnetic implications -- Ionizing photons, SLSN-I, GRB, Ic-BL

TL;DR

This study explores how rapid rotation and mass transfer in binary stars can lead to chemically homogeneous evolution, impacting the formation of energetic transients and ionizing photon emission, with implications for understanding stellar evolution and transient phenomena.

Contribution

It generalizes the angular momentum accretion threshold for CHE, demonstrating accretion CHE as a dominant formation channel for homogeneous stars and related transients.

Findings

Accretion CHE can occur at as low as 2% AM accretion efficiency.

Accretion CHE stars retain more angular momentum till core collapse.

Ionizing photon emission decreases more rapidly at higher metallicities.

Abstract

We 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 (1981) to calculate the minimum angular momentum (AM) accretion required by a secondary star to experience accretion-induced CHE. Contrary to traditionally assumed 5-10 percent accretion of initial mass ($Z \lesssim 0.004$, $M \gtrsim$ 20 M$_{\odot}$) for spinning up the accretor (resulting in CHE) this value can drop to $\sim$ 2 percent for efficient AM accretion while for certain systems it could be substantially larger. We conduct a population study using \textsc{bpass} by 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 percent 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 GRBs, 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.