Volume 565, May 2014
|Number of page(s)||15|
|Section||Interstellar and circumstellar matter|
|Published online||20 May 2014|
1 Universita di Firenze, Sez. di Astronomia, Largo E. Fermi 2, 50125 Florence, Italy
2 INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
3 Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
4 School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
5 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
6 Dipartimento di Matematica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
7 School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
Received: 17 June 2011
Accepted: 1 December 2013
Context. Stellar jets are believed to play a key role in star formation, but the question of how they originate is still being debated.
Aims. 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.
Methods. 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. In this way, we produced a 3D map (2D in space and 1D in velocity) of the jet’s physical parameters i.e. electron density ne, hydrogen ionisation fraction xe, and total hydrogen density nH. The method used is a new version of the BE-technique.
Results. A fundamental improvement is that the new diagnostic method allows us to overcome the upper density limit of the standard [S ii] diagnostics. As a result, we find at the base of the jet high electron density, ne ~ 105, and very low ionisation, xe ~ 0.02−0.05, which combine to give a total density up to nH ~ 3 × 106. 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 xe from 0.05 to a plateau at 0.7 downstream at 2′′ 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. No evidence was found of variations in the parameters across the jet, within a given velocity interval. The position-velocity diagrams indicate the presence of both fast accelerating gas and slower, less collimated material. We derive the mass outflow rate, Ṁj, in the blue-shifted lobe in different velocity channels, that contribute to a total of Ṁj ~ 8±4 × 10-9 M⊙ 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 L̇j ~ 2.9 ± 1.5 × 10-6 M⊙ yr-1 AU km s-1. We discuss implications of these findings for jet launch theories.
Conclusions. 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.
Key words: stars: pre-main sequence / Herbig-Haro objects / ISM: jets and outflows / stars: formation
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
Figures 16–18 are available in electronic form at http://www.aanda.org
© ESO, 2014
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