The Formation of Stellar Black Holes (1609.08411v2)

Abstract: It is believed that stellar black holes (BHs) can be formed in two different ways: Either a massive star collapses directly into a BH without a supernova (SN) explosion, or an explosion occurs in a proto-neutron star, but the energy is too low to completely unbind the stellar envelope, and a large fraction of it falls back onto the short-lived neutron star (NS), leading to the delayed formation of a BH. Theoretical models set progenitor masses for BH formation by implosion, namely, by complete or almost complete collapse, but observational evidences have been elusive. Here are reviewed the observational insights on BHs formed by implosion without large natal kicks from: (1) the kinematics in three dimensions of space of five Galactic BH X-ray binaries (BH-XRBs), (2) the diversity of optical and infrared observations of massive stars that collapse in the dark, with no luminous SN explosions, possibly leading to the formation of BHs, and (3) the sources of gravitational waves produced by mergers of stellar BHs so far detected with LIGO. Multiple indications of BH formation without ejection of a significant amount of matter and with no natal kicks obtained from these different areas of observational astrophysics, and the recent observational confirmation of the expected dependence of BH formation on metallicity and redshift, are qualitatively consistent with the high merger rates of binary black holes (BBHs) inferred from the first detections with LIGO.

• The paper reveals that direct collapse and failed supernovae are principal mechanisms in forming stellar black holes using multi-wavelength and gravitational evidence.
• The paper synthesizes observational data from Galactic X-ray binaries and disappearing massive stars to validate its theoretical models.
• The paper highlights metallicity's influence on black hole formation and its implications for binary systems, shaping future multi-messenger studies.

An Expert Overview of "The Formation of Stellar Black Holes"

The paper "The Formation of Stellar Black Holes" by I.F. Mirabel extensively examines the intricate processes associated with the formation of stellar mass black holes (BHs), with a particular focus on mechanisms involving direct collapse of massive stellar progenitors. It consolidates observational insights drawn from multiple empirical domains, including Galactic black hole X-ray binaries (BH-XRBs), optical and infrared observations of massive star collapses, and data from gravitational wave detections such as those provided by LIGO.

Mechanisms of Black Hole Formation

The paper outlines two primary formation pathways for stellar black holes. The first is via a direct collapse of massive stars without preceding supernova (SN) explosions, suggested to occur in stars with initial masses exceeding a threshold that varies with metallicity. The second involves a failed SN, where insufficient energy is imparted to completely unbind the stellar envelope, leading to the fallback of matter onto a nascent neutron star, subsequently forming a black hole.

Observational Evidence

Mirabel synthesizes data from Galactic BH-XRBs to support claims of BHs formed by implosion without natal kicks, a premise bolstered by kinematic studies. Specifically, Cygnus X-1 and GRS 1915+105 are highlighted as cases where progenitor stars likely underwent direct collapse, corroborated by kinematic fidelity with parent stellar associations and negligible peculiar velocities perpendicular to the Galactic plane. The peculiar motion of these systems, or lack thereof, suggests BH formation devoid of explosive mass ejections.

The search for disappearing massive stars without luminous SNe also reinforces models predicting BH formation by "failed SN," as suggested by the detection of likely BH birth events such as N6946-BH1. Observations demonstrate these events to be consistent with models proposing weak, low-energy outbursts in lieu of traditional SNe.

Theoretical Models and Metallicity Dependency

Theoretical models presented in the paper elucidate a dependency on progenitor stars' metallicity and compactness, impacting BH formation likelihood and evolution pathways. The predictions have been ostensibly verified by observational data showing an enhanced occurrence of high mass X-ray binaries in low metallicity environments, an expected consequence of metallicity's influence on massive star wind mass-loss rates.

Implications for Binary Black Hole Systems

The formation of binary black hole (BBH) systems emerges as a salient topic considering recent gravitational wave detections by LIGO. The paper deliberates over potential evolutionary pathways leading to BBH formation, incorporating models ranging from isolated binary evolution, chemically homogeneous evolution in massive overcontact binaries, to dynamical capture in globular clusters. These scenarios assume BH formation without significant natal kicks, a hypothesis supported by the relative directional stability of several BH-XRB systems.

Broader Astrophysical Context

The findings suggest ramifications for understanding the early universe's BH population dynamics, particularly at high redshifts where metallicity effects would predominate. Furthermore, the implications extend to the observational strategy, advocating for enhanced surveillance through the Athena X-ray satellite and upgraded radio observatories like the VLA to identify BH formations and address outstanding theoretical questions.

Conclusion

This paper offers critical insights into the formation of stellar BHs, highlighting the nuanced interaction between empirical observation and theoretical prediction. While the analysis is constrained by the sample size and uncertainty in key variables, the conclusions drawn underline critical pathways of stellar evolution and compact object formation. Future observational data, particularly from multi-messenger astronomy, will be pivotal in refining these models, offering a clearer understanding of the life cycles of the universe's most enigmatic constituents.