  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").
