The Stellar Merger Scenario for Black Holes in the Pair-instability Gap
Abstract The recent detection of GW190521 stimulated ideas on how to populate the predicted black hole (BH) pair-instability (PI) mass gap. One proposal is the dynamical merger of two stars below the PI regime forming a star with a small core and an oversized envelope. We outline the main challenges this scenario faces to form one BH in the gap. In particular, the core needs to avoid growing during the merger, and the merger product needs to retain enough mass, including in the subsequent evolution, and at core collapse (CC). We explore this scenario with detailed stellar evolution calculations, starting with ad hoc initial conditions enforcing no core growth during the merger. We find that these massive merger products are likely to be helium-rich and spend most of their remaining lifetime within regions of instabilities in the Herzsprung–Russell diagram, such as luminous blue variable eruptions. An energetic estimate of the amount of mass loss neglecting the back reaction of the star suggests that the total amount of mass that can be removed at low metallicity is ≲1 M ⊙ . This is small enough that at CC our models are retaining sufficient mass to form BHs in the PI gap more »
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Award ID(s):
Publication Date:
NSF-PAR ID:
10342460
Journal Name:
The Astrophysical Journal Letters
Volume:
904
Issue:
2
Page Range or eLocation-ID:
L13
ISSN:
2041-8205