  Partially-ionized plasmas consist of charged and neutral particles whose
mutual collisions modify magnetic reconnection compared with the fully-ionized
case. The collisions alter the rate and locations of the magnetic dissipation
heating and the distribution of energies among the particles accelerated into
the non-thermal tail. We examine the collisional regimes for the onset of fast
reconnection in two environments: the partially-ionized layers of the solar
atmosphere and the protoplanetary disks that are the birthplaces for planets
around young stars. In both these environments, magnetic nulls readily develop
into resistive current sheets in the regime where the charged and neutral
particles are fully coupled by collisions, but the current sheets quickly break
down under the ideal tearing instability. The current sheets collapse
repeatedly, forming magnetic islands at successively smaller scales, till they
enter a collisionally-decoupled regime where the magnetic energy is rapidly
turned into heat and charged-particle kinetic energy. Small-scale, decoupled
fast reconnection in the solar atmosphere may lead to preferential heating and
energization of ions and electrons that escape into the corona. In
protoplanetary disks such reconnection causes localized heating in the
atmospheric layers that produce much of the infrared atomic and molecular line
emission observed with the Spitzer and James Webb Space Telescopes.
