MIXING LAYERS IN STELLAR OUTFLOWS

It has been suggested in the past that at least part of the emission observed in high-velocity outflows from young stars could be formed in turbulent mixing layers. We present a simplified mixing layer model based on a simple "turbulent viscosity" description of the dissipation and transport associated with the fully developed turbulence. Predictions from these models agree very well with results from laboratory flows with Mach numbers M = 1 --> 20, showing that our theoretical approach is indeed valid for the M approximately 10 regime that is relevant for stellar outflows. We discuss the properties of adiabatic, isothermal, and nonadiabatic (i.e., with an energy loss term appropriate for the regime found in stellar jets) mixing layers, and describe the transition from a laminar stellar outflow to a fully turbulent flow. We find that mixing layers have a sizeable radiative luminosity, so that in principle they could substantially contribute to the total emission from stellar jets or from Herbig-Haro objects. The theoretical framework presented in this paper is quite general, and could be applied to mixing layers with detailed atomic and/or molecular processes (as would be necessary for modeling molecular outflows), or to mixing layers with different geometries (e.g., the accretion disk/stellar photosphere boundary layer).