The optical spectra of atomic ionic mixing layers in outflows from young stars

It has been previously suggested on kinematical grounds that at least part of the emission from Herbig-Haro OIH) objects might be produced in turbulen t mixing layers associated with high velocity outflows. In order to explore this possibility, we compute 1D models of the temperature and ionization c ross sections of mixing layers between a high velocity flow and a stationar y environment (both the jet beam and the environment being atomic). The mix ing layer models are very simple from the dynamical point of view (being ba sed 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 enviro nment pressure P-env. These models are to some extent comparable to a famil y of plane-parallel, steady shock models computed for different pre-shock v elocities and densities. We find that the temperature cross-section changes from parabolic to flat-topped as the layer width increases, as predicted b y earlier analytical results. The ionization fraction does not show this be havior and is always strongly but of coronal equilibrium, because of turbul ent diffusion of neutral gas-from the jet and external medium into the laye r. Above a minimum jet velocity, we find that the predicted temperature and op tical spectra of the mixing-layer depend mostly on the combination hP(env)/ (alpha nu(j)(2)) = (-)Sigma [where alpha (similar or equal to 0.007) is the adopted turbulent viscosity parameter and nu j the velocity of the jet]. T his contrasts with shock models, where line ratios are strongly dependent u pon the shock velocity. The [O I]6300/H alpha ratio predicted by mixing-lay er 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 ratio s in the intermediate velocity component of T Tauri jets, which have been s uggested to trace jet mixing layers.