Fragmentation in Gravitationally Unstable Collapsar Disks and Subsolar Neutron Star Mergers

Abstract

Although stable neutron stars (NSs) can in principle exist down to masses Mns ≈ 0.1 M⊙, standard models of stellar core-collapse predict a robust lower limit Mns ≳ 1.2 M⊙, roughly commensurate with the Chandrasekhar mass MCh of the progenitor’s iron core (electron fraction Ye ≈ 0.5). However, this limit may be circumvented in sufficiently dense neutron-rich environments (Ye < 0.5) for which is reduced to ≲1 M⊙. Such physical conditions could arise in the black hole accretion disks formed from the collapse of rapidly rotating stars (“collapsars”), as a result of gravitational instabilities and cooling-induced fragmentation, similar to models for planet formation in protostellar disks. We confirm that the conditions to form subsolar-mass NS (ssNS) may be marginally satisfied in the outer regions of massive neutrino-cooled collapsar disks. If the disk fragments into multiple ssNSs, their subsequent coalescence offers a channel for precipitating subsolar mass LIGO/Virgo gravitational-wave mergers that does not implicate primordial black holes. The model makes several additional predictions: (1) ∼Hz frequency Doppler modulation of the ssNS-merger gravitational-wave signals due to the binary’s orbital motion in the disk; (2) at least one additional gravitational-wave event (coincident within ≲hours), from the coalescence of the ssNS-merger remnant(s) with the central black hole; (3) an associated gamma-ray burst and supernova counterpart, the latter boosted in energy and enriched with r-process elements from the NS merger(s) embedded within the exploding stellar envelope (“kilonovae inside a supernova”).