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.