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

Guglielmo Costa; Alessandro Bressan; Michela Mapelli; Paola Marigo; Giuliano Iorio; Mario Spera

doi:10.1093/mnras/staa3916

paper

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

arXiv:2010.02242 · doi:10.1093/mnras/staa3916

Abstract

Pair-instability (PI) is expected to open a gap in the mass spectrum of black holes (BHs) between $\approx 40 - 65$ M $_{⊙}$ and $\approx 120$ M $_{⊙}$ . The existence of the mass gap is currently being challenged by the detection of GW190521, with a primary component mass of $8 5_{- 14}^{+ 21}$ M $_{⊙}$ . Here, we investigate the main uncertainties on the PI mass gap: the $^{12}$ C( $α$ , $γ$ ) $^{16}$ O reaction rate and the H-rich envelope collapse. With the standard $^{12}$ C( $α$ , $γ$ ) $^{16}$ O 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 $Z \leq 0.0003$ . Adopting the uncertainties given by the STARLIB database, for models computed with the $^{12}$ C( $α$ , $γ$ ) $^{16}$ O rate $- 1 σ$ , we find that the PI mass gap ranges between $\approx 80$ M $_{⊙}$ and $\approx 150$ 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 M $_{⊙}$ to 110 M $_{⊙}$ . Finally, in models computed with $^{12}$ C( $α$ , $γ$ ) $^{16}$ O $- 3 σ$ , 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 $^{12}$ C( $α$ , $γ$ ) $^{16}$ O rate can lead to the formation of BHs with masses consistent with the primary component of GW190521.

20 pages, 16 figures, accepted for publication in MNRAS

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

Abstract

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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

Abstract

References in corpus (38)

Cited by in corpus (105)

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