  In this paper we explore the effect of radiative losses on purely
hydrodynamic jet collimation models applicable to Young Stellar Objects (YSOs).
In our models aspherical bubbles form from the interaction of a central YSO
wind with an aspherical circum-protostellar density distribution. Building on a
previous non-radiative study (Frank & Mellema 1996) we demonstrate that
supersonic jets are a natural and robust consequence of aspherical wind-blown
bubble evolution. The simulations show that the addition of radiative cooling
makes the hydrodynamic collimation mechanisms studied by Frank & Mellema (1996)
more effective. An unexpected result is the production of cool, dense jets
forming through conical converging flows at the poles of the bubbles. For
steady winds the formation of these jets occurs early in the bubble evolution.
At later times we find that the dynamical and cooling time scales for the jet
material become similar. Since observations of YSO jets show considerable
variability in the jet beam we present a simple one-dimensional (1-D) model for
the evolution of a variable wind interacting with an accreting environment.
These models indicate that, in the presence of a varying protostellar wind, the
hydrodynamic collimation processes studied in our simulations can produce cool
jets with sizes and time scales consistent with observations.
