Session S03: Astrophysical implications of GW190521

Mass-gap black holes and the nursery of stars

Young star clusters and open clusters are the nursery of massive stars in the local Universe. Their initial central density is sufficiently high to foster gravitational encounters and collisions, but -unlike globular clusters- they are relatively quickly disrupted by the tidal field of their host galaxies. In this talk, I will review the main dynamical processes leading to the formation of massive binary black holes in young star clusters. Stellar exchanges and collisions are particularly effective in building up massive binary black holes, with primary component mass higher than 40 Msun. Star-star collisions and hierarchical mergers might even give birth to black holes in the pair instability mass gap (~60-120 Msun) or even in the intermediate-mass regime (~100-10'000 Msun). Recent studies predict that ~1% of binary black hole mergers in young star clusters have mass in the pair-instability gap. This result has a variety of implications for current and next-generation ground-based gravitational-wave interferometers.   

Mind the gap: What can GW190521 tell us about stellar astrophysics?

With the detection of binary black hole (BH) mergers from LIGO/Virgo we have opened up the field of gravitational wave astronomy creating a new window into the Universe. These discoveries bring new and independent information about how very massive stars end their life, and the final remnants they leave behind. GW190521 however currently presents an enigma as both black holes in the merger where above the theoretical maximum for stellar mass black holes, set by the pair instability mass gap. Massive stars are expected to undergo pulsational pair instabilities leading to a core collapse supernovae (PPISN) leaving behind BH’s in the mass range detected by LIGO/Virgo. More massive stars are expected to be completely disrupted in a pair instability supernovae (PISN), leaving behind no remnant. This boundary between the fates sets the location of the PISN black hole mass gap. I will discuss the physics that sets the location of the PISN mass gap, how robust is its prediction, and what this means for the possible progenitors of GW190521. One of the key pieces of physics that sets the location of the PISN mass gap, is the nuclear reaction rate ¹²C(α,γ)¹⁶O which can be constrained if we know the location of the PISN mass gap. I will show what constraints we get from the other gravitational wave detections and what the detection of GW190521 might imply for this critical reaction rate.   

Dynamical formation of GW190521 in stellar clusters

In 2020, the LIGO/Virgo Collaboration announced the discovery of GW190521 in which a 142 solar mass black hole (BH) formed from the merger of 85 solar mass and 66 solar mass BHs. This discovery provides the first concrete evidence for the existence of an intermediate-mass BH (IMBH) larger than a 100 solar mass. BHs with these masses had not been observed before and theoretical models show that they are difficult to form through isolated stellar/binary evolution of massive stars. Many stars are born in stellar clusters where the density of stars can be up to a million times higher than the density of stars in the solar neighbourhood (e.g., globular clusters (GCs), nuclear stellar clusters (NSCs). Close gravitational encounters between binary stars and BHs are conducive to dynamically forming merging BBHs in these dense stellar environments. If the merged BH is retained in the cluster, then additional dynamical encounters may further increase its mass. Therefore, it is possible that GW190521 type BBHs can be formed in dense stellar clusters. In this talk, I will summarize results from state-of-the-art Monte Carlo and direct N-body simulations of stellar clusters to highlight the different pathways that can lead to the dynamical formation of merging binary BHs similar to GW190521.