The evolutionary roads leading to low effective spins, high black hole masses, and O1/O2 rates of LIGO/Virgo binary black holes (1706.07053v5)

Abstract: All ten LIGO/Virgo binary black hole (BH-BH) coalescences reported from the O1/O2 runs have near zero effective spins. There are only three potential explanations of this fact. If the BH spin magnitudes are large then (i) either both BH spin vectors must be nearly in the orbital plane or (ii) the spin angular momenta of the BHs must be oppositely directed and similar in magnitude. Or, (iii) the BH spin magnitudes are small. We test the third hypothesis within the framework of the classical isolated binary evolution scenario of the BH-BH merger formation. We test three models of angular momentum transport in massive stars: a mildly efficient transport by meridional currents (as employed in the Geneva code), an efficient transport by the Tayler-Spruit magnetic dynamo (as implemented in the MESA code), and a very-efficient transport (as proposed by Fuller et al.) to calculate natal BH spins. We allow for binary evolution to increase the BH spins through accretion and account for the potential spin-up of stars through tidal interactions. Additionally, we update the calculations of the stellar-origin BH masses, include revisions to the history of star formation and to the chemical evolution across cosmic time. We find that we can match simultaneously the observed BH-BH merger rate density, BH masses, and effective spins. Models with efficient angular momentum transport are favored. The updated stellar-mass weighted gas-phase metallicity evolution now used in our models appears to be a key in better reproducing the LIGO/Virgo merger rate estimate. Mass losses during the pair-instability pulsation supernova phase are likely overestimated if the merger GW170729 hosts a BH more massive than 50 Msun. We also estimate rate of BH-NS mergers from recent LIGO/Virgo observations. Our updated models of BH-BH, BH-NS and NS-NS mergers are now publicly available at www.syntheticuniverse.org.

• The paper demonstrates that efficient angular momentum transport in isolated binary evolution produces low effective spins in merging black holes.
• It employs three distinct stellar angular momentum models to correlate binary evolution processes with the observed merger rate densities.
• The study highlights that high-mass, low-spin black holes dominate LIGO/Virgo observations, refining our understanding of massive star evolution.

The Evolutionary Pathways to Low Effective Spins, High Black Hole Masses, and Observed LIGO/Virgo BH-BH Merger Rates

The paper of gravitational-wave sources, notably binary black hole (BH-BH) mergers observed by LIGO/Virgo, presents a revolutionary means to probe the characteristics and origins of massive stellar remnants in the universe. In the paper titled "The evolutionary roads leading to low effective spins, high black hole masses, and O1/O2 rates of LIGO/Virgo binary black holes," the authors explore the plausible evolutionary scenarios that could account for the observed properties of these mergers.

The paper addresses a significant observation from the LIGO/Virgo O1/O2 runs: the effective spins of the detected BH-BH mergers are close to zero. The authors propose three primary scenarios to explain this: (i) BH spins are large, but the spin vectors are aligned in the orbital plane, (ii) the spins are oppositely directed but of comparable magnitude, or (iii) BH spins are intrinsically small. The paper focuses on the last possibility, exploring its implications for stellar evolution models and merger rates.

Methodological Approach

The paper adopts the classical isolated binary evolution scenario to test the hypothesis of small natal BH spins. It utilizes three angular momentum transport models in massive stars: the meridional circulation-driven transport as employed in the Geneva code, the efficient Tayler-Spruit magnetic dynamo as modeled in MESA, and a very efficient transport model proposed by Fuller et al. These models aid in estimating natal BH spins and determining their impacts on BH-BH merger properties.

Binary evolution, including mass transfer in binaries and the effects of pair-instability pulsations, is comprehensively modeled to predict BH masses and spins at the time of merger. The authors also take into account the cosmological history of star formation and metallicity evolution, which are critical in shaping the star-forming environments that lead to BH-BH mergers.

Results and Interpretation

The results indicate a strong preference for scenarios involving efficient angular momentum transport. Models incorporating efficient or very efficient angular momentum transport better match the observed LIGO/Virgo BH-BH merger rate densities and effective spin distributions. These models suggest that large-mass BHs with low spins are the most common progenitors of gravitational-wave sources.

Additionally, the paper provides estimates for the merger rate density of BH-NS systems, predicting outcomes consistent with recent observations. This emphasizes the potential significance of tidal interactions in producing a subset of BH-BH mergers with non-negligible effective spins.

Conclusion and Implications

The findings from this paper have broad implications for our understanding of both gravitational-wave astronomy and stellar evolution. The results support the idea that efficient angular momentum transport processes are necessary to produce the low-spin, high-mass BHs observed by LIGO/Virgo. This work also highlights the importance of accurate modeling of stellar physics in predicting merger rates and spins, which in turn are pivotal for constraining stellar evolution theories and massive star formation.

The paper opens pathways for future explorations into the progenitor systems of gravitational-wave sources. It suggests the need for further theoretical and computational advances in stellar modelings, such as accounting for binary interactions that may influence angular momentum distribution. Additionally, with upcoming data from future gravitational-wave observing runs, the models presented here will be key in refining our understanding of stellar dynamics and compact object formation. The increasing depth of gravitational-wave observations will also allow for more stringent tests of these evolutionary models, potentially revealing more about the mechanisms driving the birth and evolution of massive BHs.