T_merger (colored dots) as a function of χ_eff and M_chirp. The three panels in first row correspond to different initial metallicities (left panel: 1.0 Z_⊙; middle panel: 0.1 Z_⊙; right panel: 0.01 Z_⊙) of He-rich stars. The second row refers to the models including inefficient AM transport mechanism (TS off). The dotted lines refer to the specific initial orbital periods and are marked with numbers in units of days. The observed high-confidence BBH events (69 BBHs with false alarm rate of less than one per year) reported from the LVK are shown with the error bars at 90% credibility in each panel. Black triangles represent BBHs whose T_merger is longer than a Hubble time. Two BBH events are highlighted: GW190517 (square) and GW190521 (star).