Implications of the low frequency turn-over in the spectrum of radio
  knot C in DG Tau
  The synchrotron spectrum of radio knot C in the protostellar object DG Tau
has a low frequency turn-over. This is used to show that its magnetic field
strength is likely to be at least 10 mG, which is roughly two orders of
magnitude larger than previously estimated. The earlier, lower value is due to
an overestimate of the emission volume together with an omission of the
dependence of the minimum magnetic field on the synchrotron spectral index.
Since the source is partially resolved, this implies a low volume filling
factor for the synchrotron emission. It is argued that the high pressure needed
to account for the observations is due to shocks. In addition, cooling of the
thermal gas is probably necessary in order to further enhance the magnetic
field strength as well as the density of relativistic electrons. It is
suggested that the observed spectral index implies that the energy of the radio
emitting electrons is below that needed to take part in first order Fermi
acceleration. Hence, the radio emission gives insights to the properties of its
pre-acceleration phase. Attention is also drawn to the similarities between the
properties of radio knot C and the shock induced radio emission in supernovae.
