The role of mass transfer and common envelope evolution in the formation of merging binary black holes

Abstract

As the number of merging binary black holes observed with ground-based gravitational-wave detectors grows, increasingly accurate theoretical models are required to compare them to the observed sample and disentangle contributions from multiple channels. In formation models involving isolated binary stars, important uncertainties remain regarding the stability of mass transfer and common-envelope evolution. To study some of these uncertainties, we have computed binary simulations using the MESA code consisting of a 30 M⊙ star in a low metallicity (Z⊙/10) environment with a black-hole companion. We have developed an updated prescription to compute mass transfer rates including the possibility of outflows from outer Lagrangian points, as well as a method to self-consistently determine the core-envelope boundary in cases where there is common-envelope evolution. We find that binaries survive common-envelope evolution only if unstable mass transfer happens after the formation of a deep convective envelope, resulting in a narrow range (0.2 dex) in period for successful envelope ejection. All cases where binary interaction is initiated with a radiative envelope have large binding energies (∼1050 erg), and they result in mergers during the common-envelope phase even under the assumption that all the internal and recombination energy of the envelope, as well as the energy from an inspiral, is used to eject the envelope. This is independent of whether or not helium is ignited in the core of the donor, conditions under which various rapid-population synthesis calculations assume a successful envelope ejection is possible. Moreover, we find that the critical mass ratio for instability is such that across a large range in initial orbital periods (∼1−1000 days), merging binary black holes can be formed via stable mass transfer. A large fraction of these systems undergo overflow of their L2 equipotential, in which case we find that stable mass transfer produces merging binary black holes even under extreme assumptions of mass and angular momentum outflows. Our conclusions are limited to the study of one donor mass at a single metallicity, but they suggest that population synthesis calculations overestimate the formation rate of merging binary black holes produced by common-envelope evolution and that stable mass transfer could dominate the formation rate from isolated binaries. This is in agreement with a few other recent studies. Further work is required to extend these results to different masses and metallicities as well as to understand how they can be incorporated into rapid population synthesis calculations.

Impact of Massive Binary Star and Cosmic Evolution on Gravitational Wave Observations II: Double Compact Object Rates and Properties

“…The situation is even more complex when considering that our larger suite of models (560 realizations) still only represents a subset of the overall uncertainties in population synthesis studies. For example, even our broader set of models does not account for uncertainties in the stellar evolution tracks (e.g., Laplace et al 2020;Agrawal et al 2020), internal mixing (e.g., Schootemeijer et al 2019), stellar rotation (e.g., de Mink & Mandel 2016Mapelli et al 2020), the more complex physics of the CE phase (e.g., Klencki et al 2021;Ivanova et al 2020;Marchant et al 2021;Olejak et al 2021), the additional possible remnant mass prescriptions (e.g. Dabrowny et al 2021;, the initial conditions of binary systems (e.g., de Mink & Belczynski 2015;Moe & Di Stefano 2017;Klencki et al 2018), and the possible contributions from other formation channels (e.g., Zevin et al 2021).…”

The spins of compact objects born from helium stars in binary systems

“…We have generated a suite of detailed binary stellar evolution models, examining angular momentum (AM) transport and core rotation rates of massive helium stars in close binary systems. Such binaries could possibly be formed via common envelope evolution (e.g., Belczynski et al 2002), or via stable mass transfer (e.g., Marchant et al 2021), which may be more likely for massive helium stars (Klencki et al 2020). Our models improve upon prior efforts by implementing an updated AM transport prescription based on magnetic torques associated with the Tayler instability (Fuller et al 2019), which has been calibrated with asteroseismic core rotation rates of low-mass red giant stars.…”

“…The discrepancy is even worse when one considers that the ejecta mass of our massive models would be far larger than 5 M , meaning that the ejecta velocity would be smaller than 15, 000 km/s and those SNe would not appear broad-lined. Another problem with this scenario is that such massive He stars may be less likely to form via a common envelope event (Klencki et al 2020;Marchant et al 2021;van Son et al 2021), and less likely to explode (O'Connor & Ott 2011;Zapartas et al 2021). This may disfavor magnetars as the power source of most Ic-BL SNe.…”

“…Several recent works (e.g., Pavlovskii et al 2017;Klencki et al 2020;van Son et al 2021;Marchant et al 2021) have argued that many merging BHs may form via orbital decay during stable mass transfer rather than common envelope evolution. Our models are agnostic to the progenitor channel as long as it produces a nearly hydrogen-free progenitor star in a short-period orbit, and as long as the envelope stripping occurs on a very short time scale.…”

A statistical view on stable and unstable Roche lobe overflow of a tertiary star onto the inner binary in triple systems

“…Furthermore, the boundary between stable and unstable mass transfer when a star fills its Roche lobe around a companion is poorly understood, especially in triples. Here, we adopted a strongly simplified criterion (based on Hurley et al 2002), whereas it is known that such criteria based on population synthesis methods can over-predict the occurrence rate of CE in binaries (e.g., Woods & Ivanova 2011;Ge et al 2015;Marchant et al 2021). Evidently, these uncertainties are even more severe in the triple case.…”