Ambipolar Diffusion in YSO Jets

We address the issue of ambipolar diffusion in Herbig-Haro jets. The current consensus holds that these jets are launched and collimated via MHD forces. Observations have, however, shown that the jets can be mildly to weakly ionized. Beginning with a simple model for cylindrical equilibrium between neutral, plasma and magnetic pressures we calculate the characteristic time-scale for ambipolar diffusion. Our results show that a significant fraction of HH jets will have ambipolar diffusion time-scales equivalent to, or less than the dynamical time-scales. This implies that MHD equilibria established at the base of a HH jet may not be maintained as the jet propagates far from its source. For typical jet parameters one finds that the length scale where ambipolar diffusion should become significant corresponds to the typical size of large (parsec) scale jets. We discuss the significance of these results for the issue of magnetic fields in parsec-scale jets.