  It is suggested that the thermal physics of star-forming clouds may play a
more important role than has usually been recognized in the origin of the
stellar IMF and in determining a characteristic mass scale. The importance of
the thermal physics has been clearly demonstrated for the formation of the
first stars in the universe, where it is well understood and results in cooling
to a characteristic minimum temperature at a preferred density, and hence in a
characteristic scale for fragmentation. In present-day star-forming clouds, an
analogous situation may exist in that at low densities the temperature is
expected to decrease with increasing density, reaching a minimum when the gas
becomes thermally coupled to the dust and then rising slowly at higher
densities. A minimum temperature of about 5 K is predicted to occur at a
density of the order of 10^(-18) g cm^(-3), and at this point the Jeans mass is
about 0.3 solar masses, similar to the mass at which the IMF peaks. If most of
the fragmentation in star-forming clouds occurs in filaments, as is suggested
by many simulations as well as by observations, fragmentation seems likely to
occur preferentially at the density where the temperature reaches a minimum,
and the Jeans mass at this point may then determine a characteristic scale for
fragmentation and hence a preferred stellar mass.
