# Formation pathway of Population III coalescing binary black holes through stable mass transfer

## TL;DR

This study explores how Population III stars can form binary black holes through stable mass transfer, providing a robust formation pathway that aligns with LIGO's observed merger rates and masses.

## Contribution

It introduces a new formation pathway for PopIII binary black holes via stable mass transfer, avoiding common envelope uncertainties, and estimates their merger rates consistent with LIGO observations.

## Key findings

- Approximately 10% of PopIII binaries form BBHs through stable mass transfer.
- About 10% of these BBHs merge within the Hubble time due to gravitational waves.
- The chirp mass distribution of merging BBHs is flat between 15 and 35 solar masses.

## Abstract

We study formation of stellar mass binary black holes (BBHs) originating from Population III (PopIII) stars, performing stellar evolution simulations for PopIII binaries with MESA. We find that a significant fraction of PopIII binaries form massive BBHs through stable mass transfer between two stars in a binary, without experiencing common envelope phases. We investigate necessary conditions required for PopIII binaries to form BBHs coalescing within the Hubble time with a semi-analytical model calibrated by the stellar evolution simulations. The formation efficiency of coalescing PopIII BBHs is estimated for two different initial conditions for PopIII binaries with large and small separations, respectively. Consequently, in both models, $\sim 10\%$ of the total PopIII binaries form BBHs only through stable mass transfer and $\sim 10\%$ of these BBHs merge due to gravitational wave emission within the Hubble time. Furthermore, the chirp mass of merging BBHs has a flat distribution over $15\lesssim M_{\rm chirp}/M_\odot \lesssim 35$. This formation pathway of PopIII BBHs is presumably robust because stable mass transfer is less uncertain than common envelope evolution, which is the main formation channel for Population II BBHs. We also test the hypothesis that the BBH mergers detected by LIGO originate from PopIII stars using our result and the total number of PopIII stars formed in the early universe as inferred from the optical depth measured by Planck. We conclude that the PopIII BBH formation scenario can explain the mass-weighted merger rate of the LIGO's O1 events with the maximal PopIII formation efficiency inferred from the Planck measurement, even without BBHs formed by unstable mass transfer or common envelope phases.