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We study formation of stellar mass binary black holes (BBHs) originating fromPopulation III (PopIII) stars, performing stellar evolution simulations forPopIII binaries with MESA. We find that a significant fraction of PopIIIbinaries form massive BBHs through stable mass transfer between two stars in abinary, without experiencing common envelope phases. We investigate necessaryconditions required for PopIII binaries to form BBHs coalescing within theHubble time with a semi-analytical model calibrated by the stellar evolutionsimulations. The formation efficiency of coalescing PopIII BBHs is estimatedfor two different initial conditions for PopIII binaries with large and smallseparations, respectively. Consequently, in both models, $\sim 10\%$ of thetotal PopIII binaries form BBHs only through stable mass transfer and $\sim10\%$ of these BBHs merge due to gravitational wave emission within the Hubbletime. Furthermore, the chirp mass of merging BBHs has a flat distribution over$15\lesssim M_{\rm chirp}/M_\odot \lesssim 35$. This formation pathway ofPopIII BBHs is presumably robust because stable mass transfer is less uncertainthan common envelope evolution, which is the main formation channel forPopulation II BBHs. We also test the hypothesis that the BBH mergers detectedby LIGO originate from PopIII stars using our result and the total number ofPopIII stars formed in the early universe as inferred from the optical depthmeasured by Planck. We conclude that the PopIII BBH formation scenario canexplain the mass-weighted merger rate of the LIGO's O1 events with the maximalPopIII formation efficiency inferred from the Planck measurement, even withoutBBHs formed by unstable mass transfer or common envelope phases.

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