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Table D.1.
Primary physical assumption varied for each BNS population synthesis models from Iorio et al. (2023).
| Model | Physics Varied | Description & Impact on Rates |
|---|---|---|
| F | Fiducial | The baseline model using standard assumptions for stellar winds and mass transfer, using a mass-dependent supernova kick model that allows for relatively high survival rates (Giacobbo & Mapelli 2020). |
| Supernova Natal Kicks | ||
| K265 | High Kicks | Neutron stars receive strong birth kicks (Maxwellian σ = 265 km/s). Significantly reduces the merger rate (by factor ∼4 vs F) as binaries are disrupted. |
| K150 | Low Kicks | Neutron stars receive weaker birth kicks (σ = 150 km/s), increasing the rate compared to K265, but still yields fewer mergers than F. |
| Mass Transfer Stability | ||
| QCBSE | Standard Stability | Uses restrictive criteria to decide if mass transfer is stable. Drastically suppresses the BNS rate (by ∼7) as giant donors undergo unstable transfer and merge prematurely. |
| QCBB | Alternative Stability | Assumes mass transfer from pure-Helium stars is always stable. This avoids for some stars a final, often fatal CE phase where they would otherwise merge prematurely, allowing more systems to survive and eventually merge as BNSs. |
| RBSE | Accretion Efficiency | Assumes the companion star accretes matter less efficiently during mass transfer, giving a moderate reduction in merger rates compared to F. |
| Common Envelope Physics | ||
| LK, LC, LK | High Binding Energy | LK and LC assume tightly bound envelopes. This suppresses BNS formation as binaries fail to eject the envelope and merge as stars. Similarly, LX uses the binding energy prescriptions from Xu & Li (2010) which also reduces merger rates. |
| OPT | Optimistic CE | Allows Hertzsprung gap donors to survive a CE phase. While it allows survival where standard models fail, this model is built upon the restrictive QCBSE physics. |
| Other Assumptions | ||
| F19 | Pair Instability | Uses Pair Instability prescriptions from Farmer et al. (2019). This mainly affects the black hole mass spectrum and has negligible impact on BNS merger rates. |
| F5M | Sampling Check | A high-resolution run of F. Used to verify that merger rates are not artefacts of statistical sampling noise. |
| SND | Supernova Engine | Uses a delayed supernova mechanism. This yield comparable total rates to F but alters the mass distribution of the compact objects by allowing some progenitors to not directly collapse into a black-hole. |
| QHE | Homogeneous Evolution | Allows rapidly rotating stars to mix chemically. Total merger rates are similar to F, though formation channels differ as these stars do not form Red Giants and remain compact. |
| NT | No Tides | Disables tidal forces. This results in a minor reduction in merger rates compared to F. |
| NTC | Circularization | Allows binaries to merger sooner during Roche-lobe overflow. Increases the merger rate (∼2) by preventing collisions in eccentric orbits. |
Notes. All models listed above are combined with four variations of the CE efficiency parameter, αCE ∈ {0.5, 1.0, 3.0, 5.0}, resulting in the 64 total populations analysed in this work.
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