Formation of GW190521 from stellar evolution: the impact of the hydrogen-rich envelope, dredge-up, and 12C(α, γ)16O 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 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 $-1\, \sigma$ , 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 $-2\, {}\sigma {}$ case, the PI mass gap ranges from 92 to 110 M_⊙. 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.