We present forward modelling from the bpass code suite of the population of observed gravitational wave (GW) transients reported by the LIGO/VIRGO consortium (LVC) during their third observing run, O3(a+b). Specifically, we predict the expected chirp mass and mass ratio distributions for GW transients, taking account of detector sensitivity to determine how many events should have been detected by the current detector network in O3(a+b). We investigate how these predictions change by alternating between four different remnant mass estimation schemes and two supernovae (SNe) kick prescriptions. We find that none of the model populations resulting from these variations accurately match the whole O3(a+b) GW transient catalog. However, agreement from some models to part of the catalog suggests ways to achieve a more complete fit. These include reducing the number of low mass black holes (BHs) close to the mass gap, while also increasing the number of higher mass BHs below the pair-instability SN limit. Finally, we find that the interaction between the value of the remnant mass from a stellar model and the choice of SN kick is complex and different kick prescriptions may be required depending on whether a neutron star or BH is formed.

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 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 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 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 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.

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The recent discovery of an unambiguous quiescent BH and main sequence O star companion in VFTS 243 opens the door to new constraints on theoretical stellar evolution and population models looking to reproduce the progenitors of black hole - black hole binaries. Here we show that the Binary Population and Spectral Synthesis fiducial models (BPASSv2.2.1) natively predict VFTS 243-like systems: We find that VFTS 243 likely originated from a binary system in a ∼15 day orbit with primary mass ranging from 40 to 50  M⊙ and secondary star with initial mass 24–25 M⊙. Additionally we find that the death of the primary star must have resulted in a low energy explosion E < 1050 ergs. With a uniform prior we find that the kick velocity of the new-born black hole was ≤10  km s−1. The very low eccentricity reported for VFTS 243 and the subsequent conclusion by the authors that the SN kick must have been very small is in line with the peak in the posterior distribution between 0 and 5  km s−1 found from our numerical simulations performed with a uniform prior. Finally, the reduced Hobbs kick distribution commonly used in black hole population synthesis is strongly disfavoured, whereas the Bray kick with the most recent parameter calibration predicts 4.2 ± 3.4  km s−1, which is very consistent with the posterior velocity distributions obtained for our matching VFTS 243-like models using a uniform kick prior.

We investigate the occurrence of rapid rotation induced Chemically Homogeneous Evolution (CHE) caused by strong tides and mass accretion in binary systems. Using , we derive a relation for the minimum initial angular frequency required by a single star to experience CHE. This is then extended to derive a similar relation for accretion induced CHE in binaries by generalizing the analytical relation in Packet (1981). In contrast to traditionally assumed 5-10 per cent accretion of initial mass for spinning up an accretor (resulting in CHE) with 𝑍 0.004 and 𝑀 20M , this value can drop to ∼ 2 per cent for efficient angular momentum accretion. On the other hand, for certain systems, one might overestimate the efficiency of accretion induced spin-up. We conduct a population study using by evolving stars under the influence of strong tides in short-period binaries and also account for the updated effect of accretion induced rapid rotation. We find accretion CHE (compared to tidal CHE) to be the dominant means of producing homogeneous stars even at 10 per cent angular momentum 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 angular momentum till core-collapse. Thus we show that accretion CHE could be an important formation channel for 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.