| Abstract
| - Protostellar jets are tightly connected to the accretion process and regulate the angular momentum balance of accreting star-disk systems. The DG Tau jet is one of the best-studied protostellar jets and contains plasma with temperatures ranging over three orders of magnitude within the innermost 50 AU of the jet. We present new Hubble Space Telescope (HST) far-ultraviolet (FUV) long-slit spectra spatially resolving the C iv emission ( T ~ 10 5 K) from the jet for the first time, in addition to quasi-simultaneous HST observations of optical forbidden emission lines ([O i], [N ii], [S ii], and [O iii]) and fluorescent H 2 lines. The C iv emission peaks at ≈ 42 AU from the stellar position and has a FWHM of ≈ 52 AU along the jet. Its deprojected velocity of around 200 km s -1 decreases monotonically away from the driving source. In addition, we compare our HST data with the X-ray emission from the DG Tau jet. We investigate the requirements to explain the data by an initially hot jet compared to local heating. Both scenarios indicate a mass loss by the T ~ 10 5 K jet of ~10 -9M⊙ yr -1, i.e., between the values for the lower temperature jet ( T ≈ 10 4K) and the hotter X-ray emitting part ( T ≳ 10 6 K). However, a simple initially hot wind requires a large launching region (~1 AU), and we therefore favor local heating.
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