Merging binary black holes formed through chemically homogeneous evolution in short-period stellar binaries (1601.00007v3)

Abstract: We explore a newly proposed channel to create binary black holes of stellar origin. This scenario applies to massive, tight binaries where mixing induced by rotation and tides transports the products of hydrogen burning throughout the stellar envelopes. This slowly enriches the entire star with helium, preventing the build-up of an internal chemical gradient. The stars remain compact as they evolve nearly chemically homogeneously, eventually forming two black holes, which, we estimate, typically merge 4--11 Gyr after formation. Like other proposed channels, this evolutionary pathway suffers from significant theoretical uncertainties, but could be constrained in the near future by data from advanced ground-based gravitational-wave detectors. We perform Monte Carlo simulations of the expected merger rate over cosmic time to explore the implications and uncertainties. Our default model for this channel yields a local binary black hole merger rate of about $10$ Gpc$^{-3}$ yr$^{-1}$ at redshift $z=0$, peaking at twice this rate at $z=0.5$. This means that this channel is competitive, in terms of expected rates, with the conventional formation scenarios that involve a common-envelope phase during isolated binary evolution or dynamical interaction in a dense cluster. The events from this channel may be distinguished by the preference for nearly equal-mass components and high masses, with typical total masses between 50 and 110 $\textrm{M}_\odot$. Unlike the conventional isolated binary evolution scenario that involves shrinkage of the orbit during a common-envelope phase, short time delays are unlikely for this channel, implying that we do not expect mergers at high redshift.

• The paper demonstrates that chemically homogeneous evolution in tight stellar binaries forms massive black hole pairs without undergoing a common-envelope phase.
• It uses Monte Carlo simulations to estimate local merger rates around 10 Gpc^-3 yr^-1, peaking near 20 Gpc^-3 yr^-1 at redshift 0.5.
• The findings imply that rotational mixing efficiency, wind mass loss, and metallicity thresholds are crucial for accurately predicting binary black hole merger rates.

Overview of Binary Black Holes from Chemically Homogeneous Evolution in Close Binaries

This paper by Mandel and de Mink explores an alternative evolutionary channel for forming binary black holes (BBH) from massive stars in close binary systems. The proposed scenario hinges on chemically homogeneous evolution owing to rapid stellar rotation in tight binaries, preventing typical large-scale justifications such as mass transfer and common-envelope evolution phases. Key insights and implications from the paper, including its numerical results and assumptions, contribute to the understanding of BBH formation and gravitational-wave astronomy.

Chemically Homogeneous Evolution Pathway

Chemically homogeneous evolution arises in stars that rotate rapidly enough for their internal mixing to prevent build-up of a hydrogen-helium gradient. Such stars evolve quasi-homogeneously and remain compact without typical envelope expansion. This evolution significantly affects mass transfer scenarios, especially in binaries with synchronized orbits due to tidal locking, producing massive helium stars that may directly collapse into black holes.

Implications and Uncertainties

The paper's Monte Carlo simulations indicate a significant contribution of this channel to cosmic BBH merger rates, with estimated local merger rates of 10 Gpc$^{-3}$ yr$^{-1}$ and peaking at 20 Gpc$^{-3}$ yr$^{-1}$ around $z=0.5$. This makes the chemically homogeneous channel competitive with traditional formation routes involving dynamic interactions or common-envelope evolution. The paper also predicts distinctive BBH characteristics from this channel, such as high masses (50--110 $M_\odot$) and nearly equal mass ratios due to homogeneous mixing effects, setting venues for observational distinction via gravitational-wave detections.

However, several critical uncertainties influence these results. Key factors include efficiency of rotational mixing, effects of wind-driven mass loss, and metallicity thresholds for homogeneous evolution. Wind interaction theories, for instance, may underestimate binary tightening effects for stars in close proximity. The paper presents variations by adjusting these parameters, showing the model’s sensitivity, which spans several orders of magnitude in merger rate predictions.

Future Outlook and Observational Prospects

The evolutionary models are contingent on specific metallicity thresholds ($Z \leq 0.004$), shifting the channel’s prominence in varying cosmic conditions. Consequently, the role of metallicity in dissimilar spatial and temporal star formation may lead to different evolutionary outcomes. The predicted distinct gravitational-wave signatures could provide frameworks not only for tracing cosmic star-formation history but also for constraining stellar astrophysics principles.

Given these promising prospects, further theoretical refinements and numerically enhanced simulations are warranted, particularly focusing on multidimensional models for turbulent mixing and observational efforts to detect intermediate phases, such as high-mass X-ray binaries arising during BH pre-formation. The adaptive nature of gravitational-wave astrophysics and emerging detector technologies will be instrumental in constraining BBH population synthesis and advancing stellar evolution paradigms.

Overall, this paper underscores the importance of novel evolutionary channels in massive stellar binaries, expanding understanding of BBH formation and offering a comprehensive exploration into chemically homogeneous evolution within the framework of modern gravitational-wave astronomy.