THERMAL STRUCTURE OF MIXING LAYERS IN BIPOLAR OUTFLOWS

Assuming that molecular outflows from young low-mass stars are driven by fast neutral atomic winds, we further explore the possibility that winds interact with the ambient cloud through entrainment of molecular material. We model the entrainment region as a mixing layer along the walls of a cavity within the cloud itself. We make use of the 21 cm H I spectral profiles observed in L1551 to specify a priori the overall geometry of the flow and the amount of entrainment at each point. Assu ming momentum conservation in the layer, we then determine -in a singl e parcel approximation- the radial temperature profile of the mixing l ayer. We find that, with a reasonable choice of assumptions, the tempe rature in these regions is 3000-5000 K. This results from heating due to the dissipation of wind kinetic energy as it decelerates outward du e to mass entrainment, and from cooling by roto-vibrational emission o f molecular hydrogen. We find in fact that, because of the low density , short crossing time, and replenishment from the cloud, the H-2 is no t dissociated and acts as the main coolant in the layer. In the case o f L1551 the H-2 emission from the mixing layer, e.g. in the v = 1 - 0 S(1) line, should be spread over most of the CO lobes and within the d etection limits of current (arcmin resolution) near-infrared spectrogr aphs.