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 bi-polar jets from the T Tauri stars TH28 and RW Aur, and the blue-shifted jet from T Tauri star LkHα321, using the Hubble Space Telescope Imaging Spectrograph (HST/STIS). Forbidden emission lines (FELs) 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 TH28, and 165 AU in the case of LkHα321. These velocity asymmetries are interpreted as rotation in the initial portion of the jet where it is accelerated and collimated. For the bi-polar jets, both lobes appear to rotate in the same direction. Values obtained were in agreement with the predictions of MHD disk-wind models Anderson et al. 2003;Dougados et al. 2003;Pesenti et al. 2003). 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 magneto-centrifugal launching (Anderson et al. 2003).

We present survey results which 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 wavelengths (NUV). 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 to 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 to 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.

We have observed the bipolar jet from RW Aur A with STIS on board the HST. After continuum subtraction, morphological and kinematic properties of this outflow can be traced to within 0. ′′ 1 from the source in forbidden emission lines. The jet appears well collimated, with typical FWHMs of 20 to 30 AU in the first 2 ′′ and surprisingly does not show a separate low-velocity component in contrast to earlier observations. The systemic radial outflow velocity of the blueshifted lobe is typically 50% larger than that of the redshifted one with a velocity difference of about 65 km s −1 . Although such asymmetries have been seen before on larger scales, our high spatial resolution observations suggest that they are intrinsic to the "central engine" rather than effects of the star's immediate environment. Temporal variations of the bipolar jet's outflow velocities appear to occur on timescales of a few years. They have combined to produce a 55% increase in the velocity asymmetry between the two lobes over the past decade. In the red lobe estimated mass fluxṀ j and momentum fluxṖ j values are around one half and one third of those for the blue lobe, respectively. The mass outflow to mass accretion rate is 0.05, the former being measured at a distance of 0. ′′ 35 from the source.

Abstract. 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 /r 0 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(Ṁ acc )/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 B φ /B p 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.

Context. We present the results of new spectral diagnostic investigations applied to high-resolution long-slit spectra obtained with the Hubble Space Telescope Imaging Spectrograph (HST/STIS) of the jet from the T Tauri star RW Aur. Aims. Our primary goal is to determine basic physical parameters (electron density n e and electron temperature T e , hydrogen ionisation fraction x e , total hydrogen density n H , radial velocity v r and the mass outflow rateṀ j ) along both the red-and blueshifted lobes of the RW Aur jet. Methods. The input dataset consists of seven long-slit spectra, of 0. 1 spatial resolution, taken with the STIS slit parallel to the jet, and stepped across it. We use the Bacciotti & Eislöffel (1999, A&A, 342, 717) Results. We were able to extract the parameters as far as 3. 9 in the red-and 2. 1 in the blueshifted beam. The electron density at the base of both lobes is close to the critical density for [S II] emission but then it decreases gradually with distance from the source. The range of electron temperatures derived for this jet (T e = 10 4 −2 × 10 4 K) is similar to those generally found in other outflows from young stars. The ionisation fraction x e varies between 0.04 and 0.4, increasing within the first few arcseconds and then decreasing in both lobes. The total hydrogen density, derived as n H = n e /x e , is on average 3.2 × 10 4 cm −3 and shows a gradual decrease along the beam. Variations of the above quantities along the jet lobes appear to be correlated with the position of knots. Combining the derived parameters with v r measured from the HST spectra and other characteristics available for this jet, we estimateṀ j following two different procedures. The mass-outflow rateṀ j is moderate and similar in the two lobes, despite the fact that the well-known asymmetry in the radial velocity persists close to the source. Using the results of the BE diagnostics we find averages along the first 2. 1 of both flows (a region presumably not yet affected by interaction with the jet environment) of 2.6 × 10 −9 M yr −1 for the red lobe and 2.0 × 10 −9 M yr −1 for the blueshifted flow, with an uncertainty of ±log M = 1.6. Conclusions. The fact that the derived mass outflow rate is similar in the two lobes appears to indicate that the central engine is constrained on the two sides of the system and that the observed asymmetries are due to environmental conditions. Possible suggestions for the origin of the differences are discussed. The RW Aur jet appears to be the second densest outflow from a T Tauri star studied so far, but its other properties are quite similar to those found in other jets from young stars, suggesting that a common acceleration mechanism operates in these sources.