The first gravitational-wave source from the isolated evolution of two 40-100 Msun stars (1602.04531v2)

Abstract: The merger of two massive 30 Msun black holes has been detected in gravitational waves (1,GW150914). This discovery validates recent predictions (2-4) that massive binary black holes would constitute the first detection. However, previous calculations have not sampled the relevant binary black hole progenitors---massive, low-metallicity binary stars---with sufficient accuracy and input physics to enable robust predictions to better than several orders of magnitude (5-10). Here we report a suite of high-precision numerical simulations of binary black hole formation via the evolution of isolated binary stars, providing a framework to interpret GW150914 and predict the properties of subsequent binary black hole gravitational-wave events. Our models imply that these events form in an environment where the metallicity is less than 10 percent of solar; have initial masses of 40-100 Msun; and interact through mass transfer and a common envelope phase. Their progenitors likely form either at 2 Gyr, or somewhat less likely, at 11 Gyr after the Big Bang. Most binary black holes form without supernova explosions, and their spins are nearly unchanged since birth, but do not have to be parallel. The classical field formation of binary black holes proposed in this study, with low natal kicks and restricted common envelope evolution, produces 40 times more binary black holes than dynamical formation channels involving globular clusters (11) and is comparable to the rate from homogeneous evolution channels (12-15). Our calculations predict detections of about 1,000 black hole mergers per year with total mass of 20-80 Msun once second generation ground-based gravitational wave observatories reach full sensitivity.

• The paper shows that isolated binary evolution in low-metallicity environments can form massive BH-BH mergers detectable as gravitational-wave sources.
• It uses high-precision simulations across 32 metallicities and compares three models to address uncertainties in common envelope phases and natal kicks.
• The findings predict nearly 1,000 black hole mergers per year, matching LIGO observations and advancing our understanding of stellar evolution and cosmic chemical enrichment.

Interpretation of the First Gravitational-Wave Source from Isolated Binary Massive Stars

The paper under discussion presents a comprehensive paper on the evolutionary pathways leading to the formation of massive black hole binaries (BH-BH), particularly focusing on low-metallicity environments and their significance for gravitational-wave sources. The paper's primary focus is on constrained models that can predict BH-BH merger events such as GW150914, which involves two massive black holes of approximately 30 solar masses each.

Numerical Simulations and Models

The research employs a suite of high-precision numerical simulations utilizing the StarTrack population synthesis code. The authors advance previous models by incorporating a detailed observationally-supported star formation rate, chemical enrichment history, and revised initial conditions for the evolution of binary stars. They simulate around 20 million binaries for each of 32 metallicities to adequately sample environments conducive to massive BH-BH formation and leverage these simulations to predict gravitational-wave event properties and formation rates.

Three models are discussed to embody major uncertainties:

1. Model 1 (M1), the "standard" model, simulates classical double compact object formation.
2. Model 2 (M2), the "optimistic" model, allows Hertzsprung gap stars to survive the common envelope phase, inflating the predicted number of BH-BH binaries.
3. Model 3 (M3), the "pessimistic" model, assumes a high natal kick for black holes, reducing the survivability of BH-BH progenitors.

Strong Numerical Results and Implications

The models predict that massive BH-BH systems, such as those comparable to GW150914, form in low-metallicity (less than 10% of solar) environments. The simulations suggest these binaries typically arise either early in the Universe's history (about 2 billion years after the Big Bang) or more recently (around 11 billion years after). Interestingly, contrary to the dynamical formation in globular clusters, the classical isolated binary evolution with restricted common envelope ejection predicts approximately 40 times more BH-BH binaries.

The authors predict around 1,000 black hole mergers per year for observatories at full sensitivity, with a mass range of 20–80 solar masses. For BH-BH binaries detectable by the first observing run of the Laser Interferometer Gravitational-Wave Observatory (LIGO), the estimated detection rate is in good agreement with actual observations, particularly for their standard model (M1).

Implications and Future Prospects

The findings suggest that massive black hole mergers are primarily a consequence of isolated binary evolution without significant influence from supernova explosions, indicated by near conservation of spin alignment since birth. The research implies low-metallicity star-forming regions in the modern Universe will significantly contribute to BH-BH merger rates. Furthermore, the paper contrasts isolated binary and dynamical formation pathways, which may have implications for understanding the environmental conditions under which these massive black hole systems emerge.

As LIGO and other gravitational-wave observatories enhance their sensitivity and increase their collected data, the predicted event rate and mass distributions serve as a cornerstone for comparison. Such observations could validate or challenge these models, offering pathways to refine assumptions about stellar evolution, supernova physics, and cosmic chemical enrichment.

Conclusion

This paper's intricate simulations and models offer valuable insights into the formation of gravitational-wave-generating massive BH-BH binaries, asserting the importance of low-metallicity environments and isolated binary evolution pathways. The results emphasize the necessity for future investigations to integrate observational data to fine-tune model parameters and extend the theoretical understanding of black hole astrophysics across cosmological timescales. The work sets a foundation for anticipating and interpreting ongoing and future gravitational-wave detections, marking a pivotal step in astrophysical research and the paper of cosmic evolution.