Outflow collimation in young stellar objects

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 circumprotostellar 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. We find a number of time-dependent processes contributing to the collimation whose relative strength depends on the age of the system and parameters characterising the wind and the environment. As predicted by Frank & Mellema (1996) the flow-focusing at an oblique inner shock becomes more effective when radiative cooling is included. An unexpected result of this is the production of cool (T < 10 K), dense (n ≈ 10 cm) 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 and the jet beam increases in temperature (T ≈ 10 K). The duration of the cool jet phase depends on the mass loss rate, Ṁw, and velocity, Vw, of the wind. High values of Ṁw and low values of Vw produce longer cool jet phases. 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. We find that the accretion ram pressure can halt the expansion of the bubble on time scales comparable to the periodicity of the wind and length scales less than 100 AU, the approximate observed 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.