  We suggest that the abrupt switch, from hierarchical clustering on scales
larger than 0.04 pc, to binary (and occasionally higher multiple) systems on
smaller scales, which Larson has deduced from his analysis of the grouping of
pre-Main-Sequence stars in Taurus, arises because pre-protostellar gas becomes
thermally coupled to dust at sufficiently high densities. The resulting change
from gas-cooling by molecular lines at low densities to gas-cooling by dust at
high densities enables the matter to radiate much more efficiently, and hence
to undergo dynamical fragmentation.
  We derive the domain where gas-cooling by dust facilitates dynamical
fragmentation. Low-mass (i.e. solar mass) clumps - those supported mainly by
thermal pressure - can probably access this domain spontaneously, albeit rather
quasistatically, provided they exist in a region where external perturbations
are few and far between. More massive clumps probably require an impulsive
external perturbation, for instance a supersonic collision with another clump,
in order for the gas to reach sufficiently high density to couple thermally to
the dust. Impulsive external perturbations should promote fragmentation, by
generating highly non-line ar substructures which can then be amplified by
gravity during the subsequent collapse.
