Mass spectrum of bound clumps and binary neutron star formation mechanisms in collapsar disks

Characterize the mass spectrum of bound clumps formed in gravitationally unstable collapsar disks and determine whether clump fission or gas-aided capture processes lead to the formation of binary neutron stars within the disk.

Background

Initial estimates suggest clump masses of ~0.03–1 solar masses, and gap opening is unlikely, allowing potential growth via mergers or accretion. However, the resulting mass distribution of clumps and whether dynamical pathways such as fission or gas-assisted capture produce binary neutron stars remain unresolved.

This issue is central to predicting in-disk hierarchical mergers and the observational signatures proposed by the authors.

References

Although our estimates paint a plausible story, a number of uncertainties remain, particularly with regards to: (a) whether the stripped progenitor stars of collapsars can possess sufficient angular momentum to create massive ≳ M ⊙ disks at large radii ≳ 100 R g around the central black hole; (b) whether the criterion for forming gravitationally-bound objects is in fact satisfied by a combination of neutrino and alpha particle dissociation cooling in a full multi-dimensional turbulent disk environment; (c) the resulting mass spectrum of the bound clumps, and whether clump-fissioning or gas-aided capture leads to binary NS formation; (d) the evolution of the disk electron fraction due to pair captures prior and during gravitational collapse, and how this impacts the masses of the NSs that form; (e) feedback effects on the disk mass and energy budget from accretion onto the collapsed remnants.

— Fragmentation in Gravitationally-Unstable Collapsar Disks and Sub-Solar Neutron Star Mergers  (2407.07955 - Metzger et al., 2024) in Section 3, Summary