Physical properties of the jet from DG Tauri on sub-arcsecond scales with HST/STIS

Abstract

We derive the physical properties at the base of the jet from DG Tau both along and across the flow and as a function of velocity. We analysed seven optical spectra of the DG Tau jet, taken with the Hubble Space Telescope Imaging Spectrograph. The spectra were obtained by placing a long-slit parallel to the jet axis and stepping it across the jet width. The resulting position-velocity diagrams in optical forbidden emission lines allowed access to plasma conditions via calculation of emission line ratios. We find at the base of the jet high electron density, $n_e\sim$ 10${}^5$, and very low ionisation, $x_e\sim 0.02-0.05$, which combine to give a total density up to $n_H\sim$ 3 10${}^6$. This analysis confirms previous reports of variations in plasma parameters along the jet, (i.e. decrease in density by several orders of magnitude, increase of $x_e$ from 0.05 to a plateau at 0.7 downstream at 2${}^{\prime \prime }$ from the star). Furthermore, a spatial coincidence is revealed between sharp gradients in the total density and supersonic velocity jumps. This strongly suggests that the emission is caused by shock excitation. The position-velocity diagrams indicate the presence of both fast accelerating gas and slower, less collimated material. We derive the mass outflow rate, ${\dot{M}}_j$, in the blue-shifted lobe in different velocity channels, that contribute to a total of ${\dot{M}}_j\sim$ 8 $\pm$ 4 10${}^{-9}$ M${}_{\odot }$ yr${}^{-1}$. We estimate that a symmetric bipolar jet would transport at the low and intermediate velocities probed by rotation measurements, an angular momentum flux of ${\dot{L}}_j\sim$ 2.9 $\pm$ 1.5 10${}^{-6}$ M${}_{\odot }$ yr${}^{-1}$ AU km s${}^{-1}$. The derived properties of the DG Tau jet are demonstrated to be consistent with magneto-centrifugal theory. However, non-stationary modelling is required in order to explain all of the features revealed at high resolution.

16 pages, 18 figures

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