  The thermal properties of star-forming clouds have an important influence on
how they fragment into stars, and it is suggested in this paper that the
low-mass stellar IMF, which appears to be almost universal, is determined
largely by the thermal physics of these clouds. In particular, it is suggested
that the characteristic stellar mass, a little below one solar mass, is
determined by the transition from an initial cooling phase of collapse to a
later phase of slowly rising temperature that occurs when the gas becomes
thermally coupled to the dust. Numerical simulations support the hypothesis
that the Jeans mass at this transition point plays an important role in
determining the peak mass of the IMF. A filamentary geometry may also play a
key role in the fragmentation process because the isothermal case is a critical
one for the collapse of a cylinder: the collapse and fragmentation of a
cylinder can continue freely as long as the temperature continues to decrease,
but not if it begins to increase. The limited available results on the
dependence of the thermal properties of clouds on metallicity do not suggest a
strong dependence of the IMF on metallicity, but the far-infrared background
radiation in starburst regions and in the early universe may significantly
shift the peak mass to higher masses in these situations.
