Population III binary black holes: effects of convective overshooting on formation of GW190521

• Published in: Monthly notices of the Royal Astronomical Society Vol. 505; no. 2; pp. 2170 - 2176
• Main Authors: Tanikawa, Ataru, Kinugawa, Tomoya, Yoshida, Takashi, Hijikawa, Kotaro, Umeda, Hideyuki
• Format: Journal Article
• Language: English
• Published: Oxford University Press 01.08.2021
• Subjects: binaries: close; black hole mergers; gravitational waves; stars: Population III; stars: black holes
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stab1421

ABSTRACT GW190521 is a merger of two black holes (BHs), wherein at least one BH lies within the pair-instability (PI) mass gap, and it is difficult to form because of the effects of PI supernovae and pulsational PI (PPI). In this study, we examined the formation of GW190521-like BH-BHs under Population (Pop) III environments by binary population synthesis 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 × 10−2 yr−1 Gpc−3 at a redshift of ∼0.82, which is comparable to the merger rate of GW190521-like BH-BHs inferred by gravitational wave (GW) observations. In this model, a ∼90 M⊙ star collapses to form a ∼90 M⊙ BH by avoiding PPI and PISN even if it is a member of a binary star. This is because it expands up to 102 R⊙, and lose only little mass through binary evolution. However, a model with a large overshoot parameter (similar to Stern) cannot form GW190521-like BH-BHs at all. Thus, we cannot conclude that a Pop III binary system is the origin of GW190521 because determination of the overshoot parameter involves highly uncertain. If a Pop III binary system is the origin of GW190521, the merger rate of BH-BHs including a 100–135 M⊙ BH is substantially smaller than that of GW190521-like BH-BHs. This will be assessed by GW observations in the near future.