Parsec-scale jets from young stars

We present deep, large-scale [S Il] lambda lambda 6716,6731 images of Herbig-Haro (HH) flows and jets in HH 34, RNO 43, HH 38/43, and the HH 24 region. In all of these regions new emission knots and filaments are seen to connect previously known HH objects. We find three bipolar outflows which have total projected lengths of more than 1.5 pc, and two more objects in which a blueshifted lobe of more than 0.6 pc is observed together with a shorter redshifted lobe. The large extent of these outflows consequently implies dynamical ages of about 10(4) years on average. With such a life time HH jets are perfectly able to transfer enough momentum to the surrounding medium to drive CO outflows. Most of the jets studied here do nut show very large gaps of emission between the source and the flow end, indicating that long phases of total inactivity of the source (i.e., with durations greater than or similar to 30% of the dynamical age) usually do not occur. We argue that the rather strong changes in how direction over a relatively short part of the total outflow length observed in some of the outflows is difficult to explain by precession of the jet beam. (C) 1997 American Astronomical Society.