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Evolution of the disk electron fraction and its impact on neutron star masses

Establish the time-dependent evolution of the electron fraction (Ye) in collapsar disks due to electron and positron capture reactions before and during gravitational collapse of clumps, and quantify how this evolution influences the Chandrasekhar mass and the masses of neutron stars formed.

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Background

Neutronization (Ye < 0.5) affects the Chandrasekhar mass (MCh ∝ Ye2) and the possibility of forming sub-solar mass neutron stars. While equilibrium and dynamical timescales are estimated, the detailed Ye evolution with collapse and its impact on final NS masses are stated as uncertain.

Resolving this will determine whether sub-solar mass neutron stars can plausibly form under the conditions described.

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., 10 Jul 2024) in Section 3, Summary