Impact of common envelope development criteria on the formation of LIGO/Virgo sources

Abstract

The treatment and criteria for development of unstable Roche lobe overflow (RLOF) that leads to the common envelope (CE) phase have hindered the area of evolutionary predictions for decades. In particular, the formation of black hole-black hole (BH-BH), black hole-neutron star (BH-NS), and neutron star-neutron star (NS-NS) merging binaries depends sensitively on the CE phase in classical isolated binary evolution model. All these mergers are now reported as LIGO/Virgo sources or source candidates. CE is even considered by some as a mandatory phase in the formation of BH-BH, BH-NS, or NS-NS mergers in binary evolution models. At the moment, there is no full first-principles model for the development of the CE. We employed the StarTrack population synthesis code to test the current advancements in studies on the stability of RLOF for massive donors to assess their effect on the LIGO/Virgo source population. In particular, we allowed for more restrictive CE development criteria for massive donors (M > 18 M⊙). We also tested a modified condition for switching between different types of stable mass transfer and between the thermal or nuclear timescale. The implemented modifications significantly influence the basic properties of merging double compact objects, sometimes in non-intuitive ways. For one of the tested models, with restricted CE development criteria, the local merger rate density for BH-BH systems increased by a factor of 2–3 due to the emergence of a new dominant formation scenario without any CE phase. We find that the changes in highly uncertain assumptions on RLOF physics may significantly affect: (i) the local merger rate density; (ii) shape of the mass and mass ratio distributions; and (iii) dominant evolutionary formation (with and without CE) scenarios of LIGO/Virgo sources. Our results demonstrate that without sufficiently strong constraints on RLOF physics, it is not possible to draw fully reliable conclusions about the population of double compact object systems based on population synthesis studies.