  We report the results of a numerical study on the initial formation stages of
low-mass protostellar binary systems. We determine the separation of
protostellar binaries formed as a function of the initial thermal state by
varying the initial temperature in a slightly modified version of the Burkert
and Bodenheimer collapse test. We find that the outcome is highly sensitive to
both the initial temperature of the cloud and the initial amplitude of
azimuthal density perturbation A. For A=10 %, variations of only 1 unit Kelvin
below 10 K lead to changes of up to 100 AU ( i.e. of order 30 %) in the
instantaneous separation, whereas for this small A the initial temperatures
above 10 K yield, instead of a binary, a single low-mass fragment that never
reaches protostellar densities. Protostellar binaries, however, do emerge when
the perturbation amplitude is increased from 10 % to 25 %. We also investigate
the impact of the critical density which governs the transition from isothermal
to adiabatic thermodynamic behaviour of the collapsing gas. We find that the
critical density not only affects the overall structural evolution of the gas
envelope, but also the size of the rotating disk structures formed during
collapse as well as the number of protostellar fragments resulting from the
final fragmentation of the disks. This mechanism can give rise to young
protostellar objects constituting bound multiple stellar systems.
