Fluid Dynamics of Stellar Jets in Real Time: Third Epoch HST Images of
  HH 1, HH 34, and HH 47
  We present new, third-epoch HST H-alpha and [S II] images of three HH jets
(HH 1 and 2, HH 34, and HH 47) and compare these images with those from the
previous epochs. The high-spatial resolution, coupled with a time-series whose
cadence is of order both the hydrodynamical and radiative cooling timescales of
the flow allows us to follow the hydrodynamical/magnetohydrodynamical evolution
of an astrophysical plasma system in which ionization and radiative cooling
play significant roles. Cooling zones behind the shocks are resolved, so it is
possible to identify which way material flows through a given shock wave. The
images show that heterogeneity is paramount in these jets, with clumps
dominating the morphologies of both bow shocks and their Mach disks. This
clumpiness exists on scales smaller than the jet widths and determines the
behavior of many of the features in the jets. Evidence also exists for
considerable shear as jets interact with their surrounding molecular clouds,
and in several cases we observe shock waves as they form and fade where
material emerges from the source and as it proceeds along the beam of the jet.
Fine-structure within two extended bow shocks may result from Mach stems that
form at the intersection points of oblique shocks within these clumpy objects.
Taken together, these observations represent the most significant foray thus
far into the time domain for stellar jets, and comprise one of the richest data
sets in existence for comparing the behavior of a complex astrophysical plasma
flows with numerical simulations and laboratory experiments.
