The [Ne III] Jet of DG Tau and its Ionization Scenarios
  Forbidden neon emission from jets of low-mass young stars can be used to
probe the underlying high-energy processes in these systems. We analyze spectra
of the jet of DG Tau obtained with the Very Large Telescope/X-Shooter
spectrograph in 2010. [Ne III] $\lambda$3869 is clearly detected in the
innermost 3" microjet and the outer knot located at $\sim$6".5. The velocity
structure of the inner microjet can be decomposed into the low-velocity
component (LVC) at $\sim -70$ km/s and the high-velocity component (HVC) at
$\sim -180$ km/s. Based on the observed [Ne III] flux and its spatial extent,
we suggest the origins of the [Ne III] emission regions and their relation with
known X-ray sources along the jet. The flares from the hard X-ray source close
to the star may be the main ionization source of the innermost microjet. The
fainter soft X-ray source at 0".2 from the star may provide sufficient heating
to help to sustain the ionization fraction against the recombination in the
flow. The outer knot may be reionized by shocks faster than 100 km/s such that
[Ne III] emission reappears and that the soft X-ray emission at 5".5 is
produced. Velocity decomposition of the archival Hubble Space Telescope spectra
obtained in 1999 shows that the HVC had been faster, with a velocity centroid
of $\sim -260$ km/s. Such a decrease in velocity may potentially be explained
by the expansion of the stellar magnetosphere, changing the truncation radius
and thus the launching speed of the jet. The energy released by magnetic
reconnections during relaxation of the transition can heat the gas up to
several tens of megakelvin and provide the explanation for on-source keV X-ray
flares that ionize the neon microjet.
