2013A&A...550L...1S

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 50AU of the jet. We present new Hubble Space Telescope (HST) far-ultraviolet (FUV) long-slit spectra spatially resolving the CIV emission (T∼10⁵K) from the jet for the first time, in addition to quasi-simultaneous HST observations of optical forbidden emission lines ([OI], [NII], [SII], and [OIII]) and fluorescent H₂ lines. The CIV emission peaks at ≃42AU from the stellar position and has a FWHM of ≃52AU along the jet. Its deprojected velocity of around 200km/s 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∼10⁵K jet of ∼10^–9M_☉/yr, i.e., between the values for the lower temperature jet (T≃10⁴K) and the hotter X-ray emitting part (T>10⁶K). However, a simple initially hot wind requires a large launching region (∼1AU), and we therefore favor local heating.