Now showing 1 - 6 of 6
  • Publication
    Physical properties of the jet from DG Tauri on sub-arcsecond scales with HST/STIS
    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 n H ~ 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, Mj, in the blue-shifted lobe in different velocity channels, that contribute to a total of Mj ~ 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 L̇ j ~ 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.
      193Scopus© Citations 30
  • Publication
    Further indications of jet rotation in new ultraviolet and optical Hubble Space Telescope STIS spectra
    We present survey results that suggest rotation signatures at the base of T Tauri jets. Observations were conducted with the Hubble Space Telescope Imaging Spectrograph at optical and near-ultraviolet (NUV) wavelengths. Results are presented for the approaching jet from DG Tau, CW Tau, HH 30, and the bipolar jet from TH 28. Systematic asymmetries in Doppler shift were detected across the jet, within 100 AU from the star. At optical wavelengths, radial velocity differences were typically (10-25) ± 5 km s-1, while differences in the NUV range were consistently lower, at typically 10 ± 5 km s-1. Results are interpreted as possible rotation signatures. Importantly, there is agreement between the optical and NUV results for DG Tau. Under the assumption of steady magnetocentrifugal acceleration, the survey results lead to estimates for the distance of the jet footpoint from the star, and give values consistent with earlier studies. In the case of DG Tau, for example, we see that the higher velocity component appears to be launched from a distance of 0.2-0.5 AU from the star along the disk plane, while the lower velocity component appears to trace a wider part of the jet launched from as far as 1.9 AU. The results for the other targets are similar. Therefore, if indeed the detected Doppler gradients trace rotation within the jet, then under the assumption of steady MHD ejection, the derived footpoint radii support the existence of magnetized disk winds. However, since we do not resolved the innermost layers of the flow, we cannot exclude the possibility that there also exists an X-wind or stellar wind component. © 2007, The American Astronomical Society, All rights reserved.
      197Scopus© Citations 105
  • Publication
    Hydrogen permitted lines in the first near-ir spectra of th 28 microjet: Accretion or ejection tracers?
    We report the first near-infrared detection of the bipolar microjet from T Tauri star ThA 15-28 (hereafter Th 28). Spectra were obtained with Very Large Telescope (VLT)/ISAAC for the slit both perpendicular and parallel to the flow to examine jet kinematics and gas physics within the first arcsecond from the star. The jet was successfully detected in bothmolecular and atomic lines. The H2 component was found to be entirely blueshifted around the base of the bipolar jet. It shows that only the blue lobe is emitting in H2 while light is scattered in the direction of the red lobe, highlighting an asymmetric extinction and/or excitation between the two lobes. Consistent with this view, the red lobe is brighter in all atomic lines. Interestingly, the jet was detected not only in [Fe ii], but also in Brγ and Paβ lines. Though considered tracers mainly of accretion, we find that these high excitation hydrogen permitted lines trace the jet as far as 150 AU from the star. This is confirmed in a number of ways: the presence of the [Fe ii] 2.13μm line which is of similarly high excitation; Hi velocities which match the jet [Fe ii] velocities in both the blue and red lobe; and high electron density close to the source of >6 × 104 cm-3 derived from the [Fe ii] 1.64, 1.60μm ratio. These near-infrared data complement Hubble Space Telescope Imaging Spectrograph (HST/STIS) optical and near-ultraviolet data for the same target which were used in a jet rotation study, although no rotation signature could be identified here due to insufficient angular resolution. The unpublished HST/STIS Hα emission is included here alongside the other Hi lines. Identifying Brγ and Paβ as tracers of ejection is significant because of the importance of finding strong near-infrared probes close to the star, where forbidden lines are quenched, which will help understand accretion ejection when observed with high spatial resolution instruments such as VLTI/AMBER. © 2010. The American Astronomical Society. All rights reserved.
      204Scopus© Citations 9
  • Publication
    Jet rotation investigated in the near-ultraviolet with the Hubble Space Telescope imaging spectrograph
    We present results of the second phase of our near-ultraviolet investigation into protostellar jet rotation using the Hubble Space Telescope Imaging Spectrograph. We obtain long-slit spectra at the base of five T Tauri jets to determine if there is a difference in radial velocity between the jet borders which may be interpreted as a rotation signature. These observations are extremely challenging and push the limits of current instrumentation, but have the potential to provide long-awaited observational support for the magnetocentrifugal mechanism of jet launching in which jets remove angular momentum from protostellar systems. We successfully detect all five jet targets (from RW Aur, HN Tau, DP Tau, and CW Tau) in several near-ultraviolet emission lines, including the strong Mg II doublet. However, only RW Aur's bipolar jet presents a sufficiently high signal-to-noise ratio to allow for analysis. The approaching jet lobe shows a difference of 10kms-1 in a direction which agrees with the disk rotation sense, but is opposite to previously published optical measurements for the receding jet. The near-ultraviolet difference is not found six months later, nor is it found in the fainter receding jet. Overall, in the case of RW Aur, differences are not consistent with a simple jet rotation interpretation. Indeed, given the renowned complexity and variability of this system, it now seems likely that any rotation signature is confused by other influences, with the inevitable conclusion that RW Aur is not suited to a jet rotation study. © 2012. The American Astronomical Society. All rights reserved.
      163Scopus© Citations 27
  • Publication
    Jet rotation: Launching region, angular momentum balance and magnetic properties in the bipolar outflow from RW Aur
    Using STIS on board the HST we have obtained a spectroscopic map of the bipolar jet from RW Aur with the slit parallel to the jet axis and moved across the jet in steps of 0″.07. After applying a velocity correction due to uneven slit illumination we find signatures of rotation within the first 300 AU of the jet (1″.5 at the distance of RW Aur). Both lobes rotate in the same direction (i.e. with different helicities), with toroidal velocities in the range 5-30 km s-1 at 20 and 30 AU from the symmetry axis in the blueshifted and redshifted lobes, respectively. The sense of rotation is anti-clockwise looking from the tip of the blue lobe (PA 130° north to east) down to the star. Rotation is more evident in the [OI] and [NII] lines and at the largest sampled distance from the axis. These results are consistent with other STIS observations carried out with the slit perpendicular to the jet axis, and with theoretical simulations. Using current magneto-hydrodynamic models for the launch of the jets, we find that the mass ejected in the observed part of the outflow is accelerated from a region in the disk within about 0.5 AU from the star for the blue lobe, and within 1.6 AU from the star for the red lobe. Using also previous results we estimate upper and lower limits for the angular momentum transport rate of the jet. We find that this can be a large fraction (two thirds or more) of the estimated rate transported through the relevant portion of the disk. The magnetic lever arm (defined as the ratio r A/r0 between the Alfvèn and footpoint radii) is in the range 3.5-4.6 (with an accuracy of 20-25%), or, alternatively, the ejection index ξ = d ln(Ṁacc)/dr is in the range 0.025-0.046 (with similar uncertainties). The derived values are in the range predicted by the models, but they also suggest that some heating must be provided at the base of the flow. Finally, using the general disk wind theory we derive the ratio Bφ/Bp of the toroidal and poloidal components of the magnetic field at the observed location (i.e. about 80-100 AU above the disk). We find this quantity to be 3.8 ± 1.1 at 30 AU from the axis in the red lobe and -8.9 ± 2.7 at 20 AU from the axis in the blue lobe (assuming cylindrical coordinates centred on the star and with positive z along the blue lobe). The toroidal component appears to be dominant, which would be consistent with magnetic collimation of the jet. The field appears to be more tightly wrapped on the blue side. © ESO 2005.
      169Scopus© Citations 85
  • Publication
    Rotation of jets from young stars: New clues from the Hubble Space Telescope Imaging Spectrograph
    We report findings from the first set of data in a current survey to establish conclusively whether jets from young stars rotate. We observed the bipolar jets from the T Tauri stars TH 28 and RW Aur and the blueshifted jet from T Tauri star LkHα 321, using the Hubble Space Telescope Imaging Spectrograph. Forbidden emission lines show distinct and systematic velocity asymmetries of 10-25 (±5) km s-1 at a distance of 0″.3 from the source, representing a (projected) distance of ≈40 AU along the jet in the case of RW Aur, ≈50 AU for TH 28, and 165 AU in the case of LkHα 321. These velocity asymmetries are interpreted as rotation in the initial portion of the jet where it is accelerated and collimated. For the bipolar jets, both lobes appear to rotate in the same direction. Values obtained were in agreement with the predictions of MHD disk-wind models. Finally, we determine, from derived toroidal and poloidal velocities, values for the distance from the central axis of the footpoint for the jet's low-velocity component of ≈0.5-2 AU, consistent with the models of magnetocentrifugal launching.
      151Scopus© Citations 145