Elsevier

New Astronomy Reviews

Volume 78, August 2017, Pages 1-15
New Astronomy Reviews

The formation of stellar black holes

https://doi.org/10.1016/j.newar.2017.04.002Get rights and content

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 (GWs) 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.

Introduction

The formation of stellar BHs is of topical interest for several areas of astrophysics. Stellar BHs are remnants of massive stars, possible seeds for the formation of supermassive BHs, and sources of the most energetic phenomena in the universe, such as the gravitational waves produced by fusion of BHs.

BHs and NSs are the fossils of stars with masses above ∼8 M⦿. It is known that some fraction of NSs have large runaway motions, probably due to strong natal kicks (NKs) imparted to the compact object. NKs have also been invoked in models of the core collapse of massive stars that lead to the formation of BHs. Such models predict in addition, that under specific conditions, BHs can also be formed by implosion with no energetic kicks (Fryer and Kalogera, 2001; Fryer, 1999), depending on mass, binarity, metallicity, angular momentum, and magnetic fields, among others properties of the progenitor star. NKs are of interest in Gravitational Wave Astrophysics since from population synthesis models of isolated binary evolution it is inferred that the merger rate of BBHs increases by a factor of ∼20 when BH NKs are decreased from a kick distribution typical of NSs to zero (Dominik et al., 2012).

It is believed that the runaway velocity of a BH-XRB can be due to the following mechanisms. (1) The sudden baryonic mass-loss in the SN explosion of the primary star of a binary (Blaauw, 1961) (Blaauw kick). In this case the ejected matter will continue to move with the orbital velocity of the progenitor, and to conserve momentum the resulting compact binary will move in the opposite direction (Nelemans et al., 1999). A sudden mass loss would unbind the binary only when more than half the binary's total mass is instantaneously lost, which is not expected (Nelemans et al., 1999). (2) NKs can also be imparted to the compact object, by anisotropic emission of neutrinos (Fryer and Kusenko, 2006) and GWs (Bonnell and Pringle, 1995) during core-collapse. If formed in a dense stellar cluster, other possible causes for the runaway velocity of a compact BH-XRB could be either one of several possible dynamical interactions in the stellar cluster (Poveda et al., 1967; Allen, 2011; Rodriguez et al., 2016), or the explosion of a massive star that before its collapse formed a multiple bound system with the runaway compact binary (Section 2.3).

Section snippets

Kinematics of Galactic black hole X-ray binaries

The kinematics of BH-XRBs can provide clues on the formation of BHs. If a compact object is accompanied by a mass-donor star in an X-ray binary, it is possible to determine the distance, radial velocity, and proper motion of the system's barycenter, from which can be derived the velocity in three dimensions of space, and in some cases the path to the site of birth may be tracked.

Among the estimated 3 × 108 stellar BHs in the Galaxy (Remillard and McClintock, 2016), about 20 BH-XRBs have been

BH formation as a function of metallicity and redshift

Theoretical models on the evolution of single massive stars (Belczynski et al., 2016; Heger et al., 2003; Georgy et al., 2009; Fragos et al., 2013) predict mass and metallicity dependence of stellar progenitors for BH formation (e.g. Fig. 1). For stellar clusters of a given mass it has been proposed that the numbers and orbital period distributions of HMXBs should also depend on metallicity (Linden et al., 2010). More recent models (Spera et al., 2015) predict with respect to previous models,

Progenitors of BHs: massive stars that collapse in the dark?

Models of the evolution of massive stars had predicted that stars between ∼25 and 140 M⦿ and up to solar metallicity end as BHs by direct or failed supernova collapse (Fryer and Kalogera, 2001; Heger et al., 2003; Fryer et al., 2012) (Fig. 1). More recent models predict that zero age main sequence (ZAMS) stars of solar metallicity with masses between 15 and 80 M⦿ may end as BHs, but in some intervals of ZAMS masses, stars may explode as SNe and end as NSs (Lovergrove and Woosley, 2013; Sukhbold

The formation of binary stellar black holes

The first sources of gravitational waves detected by LIGO were mergers of stellar BHs (Abbott et al., 2016c) and the question on how these BBHs may be formed is of topical interest. In previous sections have been presented observational results that are consistent with the idea that stellar black holes may be formed by the implosion of massive stars in the dark, without luminous natal SNe, with no ejection of a significant amount of baryonic matter, and with no energetic kicks.

Summary and conclusions

  • •

    From the kinematics of Galactic BH X-ray binaries in three dimensions of space it is found that stars of solar metallicity and >40 M⦿ may collapse directly to form BHs by implosion, without energetic SN explosions and Natal Kicks (NKs). In fact, from the kinematics of Cygnus X-1 it is inferred that the BH of ∼15 M⦿ was formed in situ with no NK, by the implosion of a progenitor of ∼40 M⦿, that probably went through a Wolf Rayet phase with a total mass loss of ∼25 M⦿.The kinematics of GRS 1915 + 105

Acknowledgements

Thierry Foglizzo, Luis F. Rodríguez, Françoise Combes and James Miller-Jones made insightful comments to the first draft of this review. An anonymous referee made important remarks to the submitted version that helped to provide a more appropriate balance to the presentation of this review. Selma de Mink, Tassos Fragos, Garik Israelian, Smadar Naoz, Jerome Orosz, Jorge Melnick and Jochen Greiner provided useful information. Irapuan Rodrigues and Stéphane Schanne kindly helped with software to

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