Most Black Holes are Born Very Slowly Rotating

Abstract: The age of gravitational wave (GW) astronomy has begun, and black hole (BH) mergers detected by LIGO are providing novel constraints on massive star evolution. A major uncertainty in stellar theory is the angular momentum (AM) transport within the star that determines its core rotation rate and the resulting BH's spin. Internal rotation rates of low-mass stars measured from asteroseismology prove that AM transport is efficient, suggesting that massive stellar cores may rotate slower than prior expectations. We investigate AM transport via the magnetic Tayler instability, which can largely explain the rotation rates of low-mass stars and white dwarfs. Implementing an updated AM transport prescription into models of high-mass stars, we compute the spins of their BH remnants. We predict that BHs born from single stars rotate very slowly, with $a \sim 10^{-2}$, regardless of initial rotation rate, possibly explaining the low $\chi_{\rm eff}$ of most BH binaries detected by LIGO thus far. A limited set of binary models suggests slow rotation for many binary scenarios as well, although homogeneous evolution and tidal spin-up of post-common envelope helium stars can create moderate or high BH spins. We make predictions for the values of $\chi_{\rm eff}$ in future LIGO events, and we discuss implications for engine-powered transients.

• The paper demonstrates that efficient angular momentum transport via the magnetic Tayler instability leads to nearly non-rotating black holes from single stellar collapse.
• Utilizing MESA simulations, the research incorporates factors like mass loss, convective overshooting, and metallicity to predict black hole spins on the order of 10⁻².
• The study also explores binary evolution scenarios, identifying tidal spin-up and homogeneous evolution as potential pathways for forming moderately or rapidly rotating black holes.

Summary of "Most Black Holes are Born Very Slowly Rotating"

The paper "Most Black Holes are Born Very Slowly Rotating" by Jim Fuller and Linhao Ma explores the angular momentum (AM) transport processes within high-mass stars, proposing that the majority of black holes (BHs) formed from single stellar collapse exhibit extremely slow natal spins. The research is pivotal as it correlates the low calculated spins of black holes with empirical data from gravitational wave observations, particularly those captured by LIGO, which largely indicate low effective spin parameters ($\chi_{\rm eff}$) in binary black hole mergers.

Core Findings

• Efficient AM Transport: Utilizing updated models of AM transport influenced by the magnetic Tayler instability, the study shows that massive stars lose much of their core angular momentum prior to collapse. This mechanism is akin to established models that have explained the slower-than-expected core rotation rates in low-mass stars.
• Stellar Models: The research employs sophisticated simulations via the MESA stellar evolution code, accounting for factors like mass loss due to stellar winds, convective overshooting, and metallicity, among others. These models consistently predict that single star remnants possess spins on the order of $a \sim 10^{-2}$, suggesting nearly non-rotating black holes as a result.
• Binary Star Scenarios: While single star models result in slow spins, the study also evaluates several binary evolution scenarios. The authors identify two principal paths to moderate or rapidly rotating black holes: tidal spin-up of helium stars in compact binaries and homogeneous evolution in massive, low-metallicity stars.

Theoretical and Observational Implications

• Gravitational Wave Astronomy: This study aligns theoretical predictions of BH spin with observed data from LIGO, which detects low effective spin parameters in a majority of events. Addressing the spin distribution helps refine models of stellar evolution and black hole formation.
• Engine-Powered Transients: The paper assesses pathways for creating rapidly rotating central objects necessary for phenomena like gamma-ray bursts or superluminous supernovae. They suggest binary interactions—not single star evolution—as vital mechanisms for enabling the requisite high spins of progenitor stars.
• Future Predictions: Given the findings, the authors speculate on potential distributions of $\chi_{\rm eff}$ in future gravitational wave detections. They anticipate a population skewing heavily towards zero, interrupted by distributions suggestive of binaries with specific spin-up processes.

Potential Developments

Looking forward, the hypotheses presented invite further speculative frameworks, especially regarding high-spin black holes evident in X-ray binaries, where observational complexities are not yet fully addressed. Exploring the role of fallback mechanisms in altering black hole spins post-collapse yields another dimension for future exploration. Similarly, expanding models to incorporate broader metallicity ranges could refine the understanding of homogeneous evolution scenarios and resultant black hole spins.

Overall, the paper contributes significantly to our comprehension of black hole formation and properties, offering substantial alignment with contemporary gravitational wave observations. This understanding aids the broader astrophysical discourse on stellar and binary evolution under disparate conditions—a vital component in decoding cosmic phenomena related to black holes.