OUTFLOW COLLIMATION IN YOUNG STELLAR OBJECTS

In this paper we explore the effect of radiative losses on purely hydr odynamic jet collimation models applicable to young stellar objects (Y SOs). In our models aspherical bubbles form from the interaction of a central YSO wind with an aspherical circumprotostellar density distrib ution. Building on our previous non-radiative study we demonstrate tha t supersonic jets are a natural and robust consequence of aspherical w ind-blown bubble evolution. The simulations show that the addition of radiative cooling makes the hydrodynamic collimation mechanisms studie d by Frank & Mellema more effective, We find a number of time-dependen t processes contributing to the collimation whose relative strength de pends on the age of the system and parameters characterizing the wind and the environment. As predicted by Frank & Mellema the flow focusing at an oblique inner shock becomes more effective when radiative cooli ng is included. An unexpected result of this is the production of cool (T < 10(4) K), dense (n approximate to 10(4) cm(-3)) jets forming thr ough 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 fo r the jet material become similar and the jet beam increases in temper ature (T approximate to 10(6) K). The duration of the cool jet phase d epends on the mass-loss rate, (m) over dot(w) , and velocity, V-w, of the wind. High values of (M) over dot(w) and low values of V-w produce longer cool jet phases. Since observations of YSO jets show considera ble variability in the jet beam we present a simple one-dimensional (1 D) model for the evolution of a variable wind interacting with an accr eting environment. We find that the accretion ram pressure can halt th e expansion of the bubble on time-scales comparable to the periodicity of the wind and length-scales les's than 100 au, the approximate obse rved scale for YSO jet collimation. 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.