[Paper Review] Importance of stable mass transfer and stellar winds for the formation of gravitational wave sources

Andris Dorozsmai, Silvia Toonen|arXiv (Cornell University)|Jul 18, 2022

Pulsars and Gravitational Waves Research4 citations

TL;DR

This study investigates how uncertainties in mass transfer stability and stellar wind prescriptions affect the formation of gravitational wave-emitting binary black holes via population synthesis. It finds that mass transfer efficiency and angular momentum loss critically determine whether stable mass transfer or common-envelope evolution dominates, while stellar winds have minimal impact on merging binary masses unless combined with extreme assumptions.

ABSTRACT

The large number of gravitational wave (GW) detections have revealed the properties of the merging black hole binary population, but how such systems are formed is still heavily debated. Understanding the imprint of stellar physics on the observable GW population will shed light on how we can use the gravitational wave data, along with other observations, to constrain the poorly understood evolution of massive binaries. We perform a parameter study on the classical isolated binary formation channel with the population synthesis code SeBa to investigate how sensitive the properties of the coalescing binary black hole population are on the uncertainties related to first phase of mass transfer and stellar winds. We vary five assumptions: 1 and 2) the mass transfer efficiency and the angular momentum loss during the first mass transfer phase, 3) the mass transfer stability criteria for giant donors with radiative envelopes, 4) the effective temperature at which an evolved star develops a deep convective envelope, and 5) the mass loss rates of stellar winds. We find that current uncertainties related to first phase of mass transfer have a huge impact on the relative importance of different dominant channels, while the observable demographics of GW sources are not significantly affected. Our varied parameters have a complex, interrelated effect on the population properties of GW sources. Therefore, inference of massive binary physics from GW data alone remains extremely challenging, given the large uncertainties in our current models.

Motivation & Objective

• To assess the impact of uncertain stellar physics—particularly mass transfer stability and stellar wind prescriptions—on the formation of merging binary black holes via isolated binary evolution.
• To determine how variations in key physical parameters affect the relative dominance of different formation channels (e.g., stable mass transfer vs. common-envelope evolution).
• To evaluate whether current gravitational wave observations can constrain poorly understood binary evolution physics, given model uncertainties.
• To identify which observational diagnostics (e.g., WR-O/B binaries) could improve constraints on binary physics from gravitational wave data.

Proposed method

• Employed the SeBa population synthesis code to simulate isolated binary evolution with varying assumptions on mass transfer and stellar wind physics.
• Systematically varied five key parameters: mass transfer efficiency, angular momentum loss during first mass transfer, stability criteria for giant donors with radiative envelopes, effective temperature for deep convective envelope formation, and stellar wind mass loss rates.
• Tracked the evolution of binary systems through mass transfer phases, common envelope ejection, and final black hole merger via gravitational wave emission.
• Compared resulting merger rates and black hole mass distributions across different model variants to assess sensitivity to input uncertainties.
• Used observational constraints from known WR-O/B binaries to validate model predictions and inform parameter space exploration.
• Quantified the impact of each parameter on the relative contribution of stable mass transfer versus common-envelope evolution channels.

Experimental results

Research questions

• RQ1 How does the choice of angular momentum loss mode during the first mass transfer phase affect the relative dominance of stable mass transfer versus common-envelope evolution channels?
• RQ2 To what extent does the stability criterion for mass transfer in giant donors (defined by ζ_ad,rad) influence the merger rate of binary black holes?
• RQ3 How sensitive are the masses and merger rates of gravitational wave sources to variations in stellar wind mass loss rates, particularly for line-driven and LBV winds?
• RQ4 Can the observed properties of WR-O/B binaries help constrain the physics of mass transfer in massive binary systems relevant to gravitational wave source formation?
• RQ5 What is the combined effect of multiple uncertain parameters on the observable demographics of merging binary black holes?

Key findings

• The choice of angular momentum loss mode (γ = 1 vs. 2.5) dramatically alters the dominant formation channel: γ = 2.5 favors the stable mass transfer channel, while γ = 1 suppresses it and enhances the common-envelope evolution channel.
• Increasing the stability criterion ζ_ad,rad from 4 to 7.5 significantly boosts the merger rate of low-mass binary black holes (M_BH ≤ 20 M☉), but has minimal effect on higher-mass systems due to mass ratio limits at low metallicities.
• Reducing line-driven wind mass loss rates by a factor of three increases the masses of isolated black holes but has negligible impact on merging binary black hole masses, due to compensatory increases in Wolf-Rayet phase mass loss after envelope stripping.
• LBV wind mass loss rates above ∼10⁻³ M☉ yr⁻¹ suppress the common-envelope channel entirely, as mass loss prevents the formation of tight enough binaries for efficient CEE, while the stable channel remains unaffected due to rapid envelope stripping.
• The stable mass transfer channel becomes dominant only under specific combinations of high mass transfer efficiency and high angular momentum loss (γ = 2.5), highlighting the strong interdependence of model parameters.
• Inference of massive binary physics from gravitational wave data alone remains highly challenging due to the complex, non-linear interplay of uncertainties in mass transfer and wind prescriptions.