Fragmentation in Gravitationally-Unstable Collapsar Disks and Sub-Solar Neutron Star Mergers

Although stable neutron stars (NS) can in principle exist down to masses Mns ~ 0.1Msun, standard models of stellar core-collapse predict a robust lower limit Mns >~ 1.2Msun, 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 Mch ~ Ye^2 is reduced to < Msun. 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 sub-solar mass NS (ssNS) may be marginally satisfied in the outer regions of massive neutrino-cooled collapsar disks. If the disk fragments into multiple ssNS, their subsequent coalescence offers a channel for precipitating sub-solar 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").