The optical spectra of atomic/ionic mixing layers in outflows from young stars
L. Binette 1,
S. Cabrit 2,
A. Raga 1 and
J. Cantó 1
Received 16 December 1998 / Accepted 1 March 1999
It has been previously suggested on kinematical grounds that at least part of the emission from Herbig-Haro (HH) objects might be produced in turbulent mixing layers associated with high velocity outflows. In order to explore this possibility, we compute 1D models of the temperature and ionization cross sections of mixing layers between a high velocity flow and a stationary environment (both the jet beam and the environment being atomic). The mixing layer models are very simple from the dynamical point of view (being based on a "turbulent viscosity" approach), but include a detailed treatment of the relevant atomic/ionic processes and radiative cooling for the 8 most important elements (H, He, C, N, O, Ne, S, Fe).
In this way, we produce a grid of 1D models with three free parameters: the velocity of the jet, the local width of the mixing layer h, and the environment pressure . These models are to some extent comparable to a family of plane-parallel, steady shock models computed for different pre-shock velocities and densities. We find that the temperature cross-section changes from parabolic to flat-topped as the layer width increases, as predicted by earlier analytical results. The ionization fraction does not show this behavior and is always strongly out of coronal equilibrium, because of turbulent diffusion of neutral gas from the jet and external medium into the layer.
Above a minimum jet velocity, we find that the predicted temperature and optical spectra of the mixing-layer depend mostly on the combination [where () is the adopted turbulent viscosity parameter and the velocity of the jet]. This contrasts with shock models, where line ratios are strongly dependent upon the shock velocity. The [O I]6300/H ratio predicted by mixing-layer models appears too high by a factor of two compared with the line ratios of low-excitation HH objects. However, it may be compatible with line ratios in the intermediate velocity component of T Tauri jets, which have been suggested to trace jet mixing layers.
Key words: hydrodynamics shock waves stars: pre-main sequence stars: winds, outflows ISM: Herbig-Haro objects ISM: jets and outflows
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© European Southern Observatory (ESO) 1999
Online publication: May 6, 1999