Research Output
1 - 10 of 12
Publication
The near-UV: The true window on jet rotation
2017-01-01, Coffey, Deirdre, Bacciotti, Francesca, Podio, Linda, Erkal, Jessica
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.
Publication
Further indications of jet rotation in new ultraviolet and optical Hubble Space Telescope STIS spectra
2007-07-01, Coffey, Deirdre, Bacciotti, Francesca, Ray, Thomas P., Eislöffel, Jochen, Woitas, Jens
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.
Publication
ALMA Observations of Polarized Emission toward the CW Tau and DG Tau Protoplanetary Disks: Constraints on Dust Grain Growth and Settling
2018-10-01, Bacciotti, Francesca, Girart, Josep Miquel, Padovani, Marco, Coffey, Deirdre, et al.
We present polarimetric data of CW Tau and DG Tau, two well-known Class II disk/jet systems, obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) at 870 μm and 0.″2 average resolution. In CW Tau, the total and polarized emission are both smooth and symmetric, with polarization angles almost parallel to the minor axis of the projected disk. In contrast, DG Tau displays a structured polarized emission, with an elongated brighter region in the disk's near side and a belt-like feature beyond about 0.″3 from the source. At the same time, the total intensity is spatially smooth, with no features. The polarization pattern, almost parallel to the minor axis in the inner region, becomes azimuthal in the outer belt, possibly because of a drop in optical depth. The polarization fraction has average values of 1.2% in CW Tau and 0.4% in DG Tau. Our results are consistent with polarization from self-scattering of the dust thermal emission. In this hypothesis, the maximum size of the grains contributing to polarization is in the range 100-150 μm for CW Tau and 50-70 μm for DG Tau. The polarization maps combined with dust opacity estimates indicate that these grains are distributed in a geometrically thin layer in CW Tau, representing a settling in the disk midplane. Meanwhile, such settling is not yet apparent for DG Tau. These results advocate polarization studies as a fundamental complement to total emission observations, in investigations of the structure and the evolution of protoplanetary disks.
Publication
Hydrogen permitted lines in the first near-ir spectra of th 28 microjet: Accretion or ejection tracers?
2010-08-10, Coffey, Deirdre, Bacciotti, Francesca, Podio, Linda, Nisini, B.
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.
Publication
ALMA Polarimetric Studies of Rotating Jet/Disk Systems
2019-08-02, Bacciotti, Francesca, Girart, Josep Miquel, Padovani, Marco, Coffey, Deirdre, et al.
We have recently obtained polarimetric data at mm wavelengths with ALMA for the young systems DG Tau and CW Tau, for which the rotation properties of jet and disk have been investigated in previous high angular resolution studies. The motivation was to test the models of magneto-centrifugal launch of jets via the determination of the magnetic configuration at the disk surface. The analysis of these data, however, reveals that self-scattering of dust thermal radiation dominates the polarization pattern. It is shown that even if no information on the magnetic field can be derived in this case, the polarization data are a powerful tool for the diagnostics of the properties and the evolution of dust in protoplanetary disks.
Publication
Rotation of jets from young stars: New clues from the Hubble Space Telescope Imaging Spectrograph
2004-04-01, Coffey, Deirdre, Bacciotti, Francesca, Woitas, Jens, Ray, Thomas P., Eislöffel, Jochen
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.
Publication
T Tauri Jet Physics Resolved Near the Launching Region with the Hubble Space Telescope
2008-12-20, Coffey, Deirdre, Bacciotti, Francesca, Podio, Linda
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.
Publication
Physical properties of the jet from DG Tauri on sub-arcsecond scales with HST/STIS
2014-05, Maurri, L., Bacciotti, Francesca, Podio, Linda, Coffey, Deirdre
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.
Publication
Jet rotation: Launching region, angular momentum balance and magnetic properties in the bipolar outflow from RW Aur
2005-02-22, Woitas, Jens, Bacciotti, Francesca, Ray, Thomas P., Marconi, A., Coffey, Deirdre, Eislöffel, Jochen
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.
Publication
Searching for jet rotation in Class 0/I sources observed with GEMINI/GNIRS
2011-02, Coffey, Deirdre, Bacciotti, Francesca, Chrysostomou, A., Nisini, B., Davis, C.
Context. In recent years, there has been a number of detections of gradients in the radial velocity profile across jets from young stars. The significance of these results is considerable. They may be interpreted as a signature of jet rotation about its symmetry axis, thereby representing the only existing observational indications supporting the theory that jets extract angular momentum from star-disk systems. However, the possibility that we are indeed observing jet rotation in pre-main sequence systems is undergoing active debate. Aims. To test the validity of a rotation argument, we must extend the survey to a larger sample, including younger sources. Methods. We present the latest results of a radial velocity analysis on jets from Class 0 and I sources, using high resolution data from the infrared spectrograph GNIRS on GEMINI South. We obtained infrared spectra of protostellar jets HH34, HH 111-H, HH 212 NK1 and SK1. Results. The [Fe II] emission was unresolved in all cases and so Doppler shifts across the jet width could not be accessed. The H2 emission was resolved in all cases except HH 34. Doppler profiles across the molecular emission were obtained, and gradients in radial velocity of typically 3 km s-1 identified. Conclusions. Agreement with previous studies implies they may be interpreted as jet rotation, leading to toroidal velocity and angular momentum flux estimates of 1.5 km s-1 and 1 × 10-5 Ṁ yr-1 AU km s-1 respectively. However, caution is needed. For example, emission is asymmetric across the jets from HH 212 suggesting a more complex interpretation is warranted. Furthermore, observations for HH 212 and HH 111-H are conducted far from the source implying external influences are more likely to confuse the intrinsic flow kinematics. These observations demonstrate the difficulty of conducting this study from the ground, and highlight the necessity for high angular resolution via adaptive optics or space-based facilities.