SIMBAD references

We discuss the spatial evolution into biconical shocks of axisymmetric perturbations in supersonic jets subject to non-equilibrium radiative losses. Our aim is to interpret the origin of the knots, observed along many jets in Young Stellar Objects (YSOs), in terms of shocks originated by Kelvin-Helmholtz modes. In this scheme, we find that the [S II]/H_α line intensity ratios and the mean intra-knot spacing result connected to each other, and we show the evolution of the shock morphologies as a consequence of shock merging effects. From the numerical calculations we also obtain the knot proper motion velocities and the line flux behavior along the jet and make comparisons with the observations.