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

TL;DR

This study explores how uncertainties in nuclear reaction rates and stellar processes affect the predicted black hole mass gap, potentially explaining the formation of massive black holes like GW190521.

Contribution

It demonstrates that variations in the $^{12}$C($eta$, $ extgamma$)$^{16}$O rate and envelope collapse can significantly alter the pair-instability black hole mass gap, providing new insights into massive black hole formation.

Findings

The lower edge of the mass gap can be as high as 70 M$_ ext{\odot}$ with standard reaction rates.

The mass gap range extends up to 150 M$_ ext{\odot}$ when considering reaction rate uncertainties.

Dredge-up episodes can reduce core mass and allow stars to produce black holes within the observed mass of GW190521.

Abstract

Pair-instability (PI) is expected to open a gap in the mass spectrum of black holes (BHs) between $\approx{}40-65$ M$_\odot$ and $\approx{}120$ M$_\odot$. 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$_{\odot}$. Here, we investigate the main uncertainties on the PI mass gap: the $^{12}$C($\alpha$, $\gamma$)$^{16}$O reaction rate and the H-rich envelope collapse. With the standard $^{12}$C($\alpha$, $\gamma$)$^{16}$O rate, the lower edge of the mass gap can be 70 M$_\odot$ if we allow for the collapse of the residual H-rich envelope at metallicity $Z\leq{}0.0003$. Adopting the uncertainties given by the STARLIB database, for models computed with the $^{12}$C($\alpha$, $\gamma$)$^{16}$O rate $-1\, \sigma$, we find that the PI mass gap ranges between $\approx{}80$ M$_\odot$ and $\approx{}150$ M$_\odot$. Stars with $M_{\rm ZAMS}>110$ M$_\odot$ 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 $M_{\rm ZAMS} =160$ M$_\odot$ may avoid the PI and produce a BH of 150 M$_\odot$. In the $-2\,{}\sigma{}$ case, the PI mass gap ranges from 92 M$_\odot$ to 110 M$_\odot$. Finally, in models computed with $^{12}$C($\alpha$, $\gamma$)$^{16}$O $-3\,{}\sigma{}$, 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 $^{12}$C($\alpha$, $\gamma$)$^{16}$O rate can lead to the formation of BHs with masses consistent with the primary component of GW190521.