Now showing 1 - 5 of 5
  • Publication
    The near-UV: The true window on jet rotation
    (Società Astronomica Italiana, 2017-01-01) ; ; ;
    High resolution observations of jet rotation in newly forming stars have the potential to support theories of magneto-centrifugal jet launching. We report a detection of a radial velocity difference across the blue-shifted jet from RY Tau, the direction of which matches the CO disk rotation sense. Now, in 3 of 3 cases, the sense of the near-UV jet gradient matches the disk rotation sense, implying that we are indeed observing jet rotation. It seems the jet core, probed at near-UV wavelengths, is protected by the outer jet layers from kinematic contaminations, and thus represents the only true window on jet rotation.
      49
  • Publication
    T Tauri Jet Physics Resolved Near the Launching Region with the Hubble Space Telescope
    We present an analysis of the gas physics at the base of jets from five T Tauri stars based on high angular resolution optical spectra, using the Hubble Space Telescope Imaging Spectrograph (HST STIS). The spectra refer to a region within 100 AU of the star, i.e., where the collimation of the jet has just taken place. We form position-velocity (PV) images of the line ratios to get a global picture of the flow excitation. We then apply a specialized diagnostic technique to find the electron density, ionization fraction, electron temperature, and total density. Our results are in the form of PV maps of the obtained quantities, in which the gas behavior is resolved as a function of both radial velocity and distance from the jet axis. They highlight a number of interesting physical features of the jet collimation region, including regions of extremely high density, asymmetries with respect to the axis, and possible shock signatures. Finally, we estimate the jet mass and angular momentum outflow rates, both of which are fundamental parameters in constraining models of accretion-ejection structures, particularly if the parameters can be determined close to the jet foot point. Comparing mass flow rates for cases where the mass accretion rate is available in the literature (i.e., for DG Tau, RW Aur, and CW Tau) reveals a mass ejection-to-accretion ratio of 0.01-0.07. Finally, where possible (i.e., for DG Tau and CW Tau), both mass and angular momentum outflow rates are resolved into higher and lower velocity jet material. For the clearer case of DG Tau, this reveals that the more collimated higher velocity component plays a dominant role in mass and angular momentum transport.
      155Scopus© Citations 69
  • 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.
      201Scopus© Citations 9
  • 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.
      186Scopus© Citations 30
  • Publication
    Organic molecules in the protoplanetary disk of DG Tauri revealed by ALMA
    Context. Planets form in protoplanetary disks and inherit their chemical compositions. Aims. It is thus crucial to map the distribution and investigate the formation of simple organics, such as formaldehyde and methanol, in protoplanetary disks. Methods. We analyze ALMA observations of the nearby disk-jet system around the T Tauri star DG Tau in the o-H2CO 31, 2-21, 1 and CH3OH 3-2, 2-4-1, 4 E, 50, 5-40, 4 A transitions at an unprecedented resolution of $ ∼0.15 $, i.e., ∼18 au at a distance of 121 pc. Results. The H2CO emission originates from a rotating ring extending from ∼40 au with a peak at ∼62 au, i.e., at the edge of the 1.3 mm dust continuum. CH3OH emission is not detected down to an rms of 3 mJy beam-1 in the 0.162 km s-1 channel. Assuming an ortho-to-para ratio of 1.8-2.8 the ring-and disk-height-averaged H2CO column density is ∼0.3-4 × 1014 cm-2, while that of CH3OH is < 0.04-0.7 × 1014 cm-2. In the inner 40 au no o-H2CO emission is detected with an upper limit on its beam-averaged column density of ∼0.5-6 × 1013 cm-2. Conclusions. The H2CO ring in the disk of DG Tau is located beyond the CO iceline (RCO ∼ 30 au). This suggests that the H2CO abundance is enhanced in the outer disk due to formation on grain surfaces by the hydrogenation of CO ice. The emission peak at the edge of the mm dust continuum may be due to enhanced desorption of H2CO in the gas phase caused by increased UV penetration and/or temperature inversion. The CH3OH/H2CO abundance ratio is < 1, in agreement with disk chemistry models. The inner edge of the H2CO ring coincides with the radius where the polarization of the dust continuum changes orientation, hinting at a tight link between the H2CO chemistry and the dust properties in the outer disk and at the possible presence of substructures in the dust distribution.
      179Scopus© Citations 27