Formation of GW190521 from stellar evolution: the impact of the hydrogen-rich envelope, dredge-up, and C-12(alpha, gamma)O-16 rate on the pair-instability black hole mass gap

Pair-instability (PI) is expected to open a gap in the mass spectrum of black holes (BHs) between ≈40-65 and ≈120 MO. The existence of the mass gap is currently being challenged by the detection of GW190521, with a primary component mass of 85^{+21}_{-14} MO. 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 MO 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\, \sigma, we find that the PI mass gap ranges between ≈80 and ≈150 MO. Stars with MZAMS > 110 MO 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 MZAMS = 160 MO may avoid the PI and produce a BH of 150 MO. In the -2\, {}\sigma {} case, the PI mass gap ranges from 92 to 110 MO. Finally, in models computed with 12C(α, γ)16O -3\, {}\sigma {}, 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.