We have carried out a kinematical, high angular resolution (∼ 0. ′′ 1) study of the optical blueshifted flow from DG Tau within 0. ′′ 5 from the source (i.e. 110 AU when de-projected along this flow). We analysed optical emission line profiles extracted from a set of seven long-slit spectra taken with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST), obtained by maintaining the slit parallel to the outflow axis while at the same time moving it transversely in steps of 0. ′′ 07. For the spatially resolved flow of moderate velocity (peaking at -70 km s −1 ), we have found systematic differences in the radial velocities of lines from opposing slit positions i.e. on alternate sides of the jet axis. The results, obtained using two independent techniques, are corrected for the spurious wavelength shift due to the uneven illumination of the STIS slit. Other instrumental effects are shown to be either absent or unimportant. The derived relative Doppler shifts range from 5 to 20 km s −1 . Assuming the flow is axially symmetric, the velocity shifts are consistent with the southeastern side of the flow moving towards the observer faster than the corresponding northwestern side. If this finding is interpreted as rotation, the flow is then rotating clockwise looking from the jet towards the source and the derived toroidal velocities are in 1 Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association -2the range 6 to 15 km s −1 , depending on position. Combining these values with recent estimates of the mass loss rate, one would obtain an angular momentum flux, for the low to moderate velocity regime of the flow, ofJ w,lm ∼ 3.8 10 −5 M ⊙ yr −1 AU km s −1 . Our findings may constitute the first detection of rotation in the initial channel of a jet flow. The derived values appear to be consistent with the predictions of popular magneto-centrifugal jet-launching models, although we cannot exclude the possibility that the observed velocity differences are due to some transverse outflow asymmetry other than rotation.

We have carried out a spatio-kinematic study of the outflow from the classical T Tauri star DG Tau using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). A series of seven spatially offset long-slit spectra spaced by 0.07 ′′ were obtained along the axis of the outflow to build up a 3-D intensity-velocity "cube" in various forbidden emission lines (FELs) and Hα. Here we present high spatial resolution synthetic line images close to the star in distinct radial velocity intervals (from ∼ +50 km s −1 to ∼ -450 km s −1 in four bins, each ∼ 125 km s −1 wide). The lowest velocity emission is also examined in finer detail (from +60 km s −1 to -70 km s −1 in five bins ∼ 25 km s −1 wide). We have found that the highest velocity and most highly collimated component, i.e. the jet, can be traced from DG Tau to a distance D∼0.7 ′′ . The jet is on the axis of a pear-shaped limb-brightened bubble which extends between 0.4 ′′ and 1.5 ′′ from the source and which we interpret as a bow shock. Other condensations are seen close to the star indicating ongoing temporal variations in the flow. The low-velocity component of the outflow is found to be spatially wide close to the source (∼0.2 ′′ at D=0.2 ′′ ), in contrast to the high velocity jet (width < ∼ 0.1 ′′ ). We have also found evidence to suggest that not only does the density in the outflow increase longitudinally with proximity to the source but that it also increases laterally towards the flow axis. Thus, at least in the case of DG Tau, the flow becomes gradually denser as it increases in velocity and becomes more collimated. Our observations show a continous bracketing of the higher speed central flow within the lower speed, less collimated, broader flow, down to the lowest velocity scales. This suggests that the low and high velocity FELs in the highly active T Tauri star DG Tau are intimately related. Implications of these observations for FEL models will be considered in a future paper (Bacciotti et al. 2000).

We examine the conditions of the plasma along a sample of "classical" Herbig-Haro (HH) jets located in the Orion and Vela star forming regions, through combined optical-infrared spectral diagnostics. Our sample includes HH 111, HH 34, HH 83, HH 73, HH 24 C/E, HH 24 J, observed quasi-simultaneously and in the same manner at moderate spatial/spectral resolution. Once intercalibrated, the obtained spectra cover a wide wavelength range from 0.6−2.5 µm, including many transitions from regions of different excitation conditions. This allows us to probe the density and temperature stratification which characterises the cooling zones behind the shock fronts along the jet. From the line ratios we derive the variation of the visual extinction along the flow, the electron density and temperature (n e and T e ), the hydrogen ionisation fraction x e , and the total density n H in the emission region of different lines. The knowledge of such parameters is essential for testing existing jet models and for planning follow-up high-angular resolution observations. From the diagnostics of optical forbidden lines we find, on average, that in the examined jets, in the region of optical emission, n e varies between 50 cm −3 and 3 × 10 3 cm −3 , x e ranges between 0.03 and 0.6, and the electron temperature T e is ∼1.3 × 10 4 K in the HH 111 and HH 34 jets, while it appears to be higher (1.8 × 10 4 K on average) in the other examined jets. The electron density and temperature derived from [Fe ii] lines, turn out to be, respectively, higher and lower in comparison to those determined from optical lines, in agreement with the fact that the [Fe ii] lines arise in the more compressed gas located further from the shock front. An even denser component in the jets, with values of n e up to 10 6 cm −3 is detected using the ratio of calcium lines. The derived physical parameters are used to estimate the depletion onto dust grains of calcium and iron with respect to solar abundances. This turns out to be quite substantial, being between 70% and 0% for Ca and ∼90% for Fe. This leads us to suggest that the weak shocks present in the beams are not capable of completely destroying the ambient dust grains, confirming previous theoretical studies. We then derive the mass flux rates,Ṁ jet , in the flows using two independent methods. Taking into account the filling factor of the emitting gas,Ṁ jet is on average 5 × 10 −8 M yr −1 . The associated linear momentum fluxes (Ṗ jet = v jetṀjet ) are higher than, or of the same order as, those measured in the coaxial molecular flows, where present, suggesting that the flows are jet driven. Finally, we discuss differences between jets in our sample. In general, we find that higher ionisation and electron temperatures are associated with less dense jets. The comparison suggests that the shock mechanism exciting the knots along the flows has the same efficiency in all the examined objects, and the observed differences are consistent with the different densities, and hence cooling rates, found in the vario...

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).