2021MNRAS.501.4514C

Pair-instability (PI) is expected to open a gap in the mass spectrum of black holes (BHs) between ≃40-65 and ≃120 M_☉. The existence of the mass gap is currently being challenged by the detection of GW190521, with a primary component mass of 85⁺²¹_–14 M_☉. Here, we investigate the main uncertainties on the PI mass gap: the ¹²C(α, γ)¹⁶O reaction rate and the H-rich envelope collapse. With the standard ¹²C(α, γ)¹⁶O rate, the lower edge of the mass gap can be 70 M_☉ if we allow for the collapse of the residual H-rich envelope at metallicity Z <= 0.0003. Adopting the uncertainties given by the STARLIB database, for models computed with the ¹²C(α, γ)¹⁶O rate -1 σ, we find that the PI mass gap ranges between ≃80 and ≃150 M_☉. Stars with M_ZAMS > 110 M_☉ may experience a deep dredge-up episode during the core helium-burning phase, that extracts matter from the core enriching the envelope. As a consequence of the He-core mass reduction, a star with M_ZAMS = 160 M_☉ may avoid the PI and produce a BH of 150 M_☉. In the -2 σ case, the PI mass gap ranges from 92 to 110 M_☉. Finally, in models computed with ¹²C(α, γ)¹⁶O -3 σ, the mass gap is completely removed by the dredge-up effect. The onset of this dredge-up is particularly sensitive to the assumed model for convection and mixing. The combined effect of H-rich envelope collapse and low ¹²C(α, γ)¹⁶O rate can lead to the formation of BHs with masses consistent with the primary component of GW190521.