Population III Binary Black Holes: Effects of Convective Overshooting on Formation of GW190521

Ataru Tanikawa, Tomoya Kinugawa, Takashi Yoshida, Kotaro Hijikawa, Hideyuki Umeda

Published 2020-10-15Version 1

GW190521 is a merger of two black holes (BHs), at least one of which is in so-called the pair-instability (PI) mass gap, and should be hard to form due to effects of PI supernovae (SNe) and pulsational PI (PPI). We study the formation of GW190521-like BH-BHs under Population (Pop.) III environments by binary population synthesis (BPS) calculations. We reveal that convective overshooting in stellar evolution strongly affects the formation of GW190521-like BH-BHs. A model with a small overshoot parameter (similar to GENEC) can form GW190521-like BH-BHs. The derived merger rate is $4 \times 10^{-2}$ yr$^{-1}$ Gpc$^{-3}$ at redshift of $\sim 0.82$, comparable to the merger rate of GW190521-like BH-BHs inferred by gravitational wave observations. In this model, a $\sim 90 M_\odot$ star can collapse to a $\sim 90 M_\odot$ BH avoiding PPI and PISN even if it is a member of a binary star. This is because it expands up to $\sim 10^2 R_\odot$, and lose little mass through binary evolution. However, a model with a large overshoot parameter (similar to Stern) cannot form GW190521-like BH-BHs at all. We cannot conclude that a Pop. III binary is the origin of GW190521, since the correct overshoot parameter is highly uncertain. If a Pop. III binary is the origin of GW190521, the merger rate of BH-BHs including a $100-135 M_\odot$ BH is much smaller than that of GW190521-like BH-BHs. This will be assessed by gravitational wave observations in the near future.