The time-dependent ejection velocity histories of HH 34 and HH 111

The southern lobe of HH 34 and the western lobe of HH 111 show remarkable similarities. Both objects have a chain of well aligned knots, ending in well defined bow shocks ( HH 34S and HH 111V, respectively). In this paper, we derive the past ejection velocity histories from previously published radial velocity measurements of the HH 34 and HH 111 jets. This is done under the assumption that the flows are ballistic. From these reconstructed ejection velocity variabilities, we compute axisymmetric gasdynamic simulations of the two flows, and find that we do obtain large working surfaces at the positions of the HH 34S and HH 111V bow shocks for the time at which these objects are being observed. This appears to be quite definite proof that these bow shocks are indeed the result of an ejection velocity time-variability. Also, there is the observational fact that the HH 34S bow shock is huge, being wider than HH 111V by a factor of similar to3. We find that we can reproduce this difference in size by choosing an appropriate value for the density of the ambient medium. Finally, from our small sample of two reconstructed ejection velocity variability histories, we attempt to make some statements about the general nature of these variabilites, and their implications on the possible ejection/collimation mechanisms.