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 approximate to 40-65 and approximate to 120 M-circle dot. The existence of the mass gap is currently being challenged by the detection of GW190521, with a primary component mass of 85(-14)(+21) M-circle dot. Here, we investigate the main uncertainties on the PI mass gap: the C-12(alpha, gamma)O-16 reaction rate and the H-rich envelope collapse. With the standard C-12(alpha, gamma)O-16 rate, the lower edge of the mass gap can be 70 M-circle dot 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 C-12(alpha, gamma)O-16 rate -1 sigma, we find that the PI mass gap ranges between approximate to 80 and approximate to 150 M-circle dot. Stars with M-ZAMS > 110 M-circle dot 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-circle dot may avoid the PI and produce a BH of 150 M-circle dot. In the -2 sigma case, the PI mass gap ranges from 92 to 110 M-circle dot. Finally, in models computed with C-12(alpha, gamma)O-16 -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 C-12(alpha, gamma)O-16 rate can lead to the formation of BHs with masses consistent with the primary component of GW190521.

Automated summary

Pair-instability is expected to create a mass gap in the black hole spectrum, but uncertainties in factors like reaction rates and envelope collapse can cause variations in the gap boundaries. The variation in star mass and helium core mass can lead to different black hole masses.