Thomas P. Ray

Hubble Space Telescope/STIS Spectroscopy of the Optical Outflow from DG Tauri: Indications for Rotation in the Initial Jet Channel*

We have carried out a kinematical, high angular resolution (~01) study of the optical blueshifted flow from DG Tau within 05 from the source (i.e., 110 AU when deprojected along this flow). We analyzed 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, obtained by maintaining the slit parallel to the outflow axis while at the same time moving it transversely in steps of 007. 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 that the flow is axially symmetric, the velocity shifts are consistent with the southeastern side of the flow moving toward 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 toward the source and the derived toroidal velocities are in the range 6-15 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, of 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 magnetocentrifugal jet-launching models, although we cannot exclude the possibility that the observed velocity differences are due to some transverse outflow asymmetry other than rotation.

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 03 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 jets low-velocity component of ≈0.5-2 AU, consistent with the models of magnetocentrifugal launching.

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 ± 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.

Recipes for stellar jets: results of combined optical/infrared diagnostics

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 (ne and Te), the hydrogen ionisation fraction xe, and the total density nH 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, ne varies between 50 cm −3 and 3 × 10 3 cm −3 , xe ranges between 0.03 and 0.6, and the electron temperature Te is ∼1.3 × 10 4 Ki n 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 ne 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, u Mjet, in the flows using two independent methods. Taking into account the filling factor of the emitting gas, u –– –– –– –

A resolved outflow of matter from a brown dwarf.

The birth of stars involves not only accretion but also, counter-intuitively, the expulsion of matter in the form of highly supersonic outflows. Although this phenomenon has been seen in young stars, a fundamental question is whether it also occurs among newborn brown dwarfs: these are the so-called ‘failed stars’, with masses between stars and planets, that never manage to reach temperatures high enough for normal hydrogen fusion to occur. Recently, evidence for accretion in young brown dwarfs has mounted, and their spectra show lines that are suggestive of outflows. Here we report spectro-astrometric data that spatially resolve an outflow from a brown dwarf. The outflows characteristics appear similar to, but on a smaller scale than, outflows from normal young stars. This result suggests that the outflow mechanism is universal, and perhaps relevant even to the formation of planets.

A combined optical/infrared spectral diagnostic analysis of the HH1 jet

Complete flux-calibrated spectra covering the spectral range from 6000u to 2.5µm have been obtained along the HH1 jet and analysed in order to explore the potential of a combined optical/near-IR diagnostic applied to jets from young stellar objects. The main physical parameters (visual extinction, electron temperature and density, ionization fraction and total density) have been derived along the jet using various diagnostic line ratios. This multi-line analysis shows, in each spatially unresolved knot, the presence of zones at different excitation conditions, as expected from the cooling layers behind a shock front. In particular, a density stratification in the jet is evident from ratios of various lines of different critical density. We measure electron densities in the range 610 2 -310 3 cm −3 with the (S ii) optical doublet lines, 410 3 -10 4 cm −3 with the near-IR (Fe ii) lines, and 10 5 -10 6 cm −3 with optical (Fe ii) and CaII lines. The electron temperature also shows variations, with values between 8000-11000 K derived from optical/near-IR (Fe ii) lines and 11000-20000 K from a combined diagnostic employing optical (O i) and (N ii) lines. Thus (Fe ii) lines originate in a cooling layer located at larger distances from the shock front than that generating the optical lines, where the compression is higher and the temperature is declining. The derived parameters were used to measure the mass flux along the jet, adopting different procedures, the advantages and limitations of which are discussed. The (Fe ii)1.64µm line luminosity turns out to be more suitable to measure u Mjet than the optical lines, since it samples a fraction of the total mass flowing through a knot larger than the (O i) or (S ii) lines. u Mjet is high in the initial part of the flow (�2.210 −7 M⊙ yr −1 ) but decreases by about an order of magnitude further out. Conversely, the mass flux associated with the warm molecular material is low, u MH2�10 −9 M⊙ yr −1 , and does not show appreciable variations along the jet. We suggest that part of the mass flux in the external regions is not revealed in optical and IR lines because it is associated with a colder atomic component, which may be traced by the far-IR (O i)63µm line. Finally, we find that the gas-phase abundance of refractory species, such as Fe, C, Ca, and Ni, is lower than the solar value, with the lowest values (between 10 and 30% of solar) derived in the inner and densest regions. This suggests a significant fraction of dust grains may still be present in the jet beam, imposing constraints on the efficiency of grain destruction by multiple low-velocity shock events.

Molecular Hydrogen in the Outflow From CEP E

We present images of the outflow from Cep E in the 1-0 S(1), the 2-1 S(1), and, for the first time in any outflow, the 3-2 S(3) line of molecular hydrogen. These are supplemented by [Fe II]1.64 I¼m and narrow-band continuum images. We find two almost perpendicular outflows emanating from Cep E. From these and from the spectral energy distribution the Cep E source appears to be a Class 0 binary. Wiggles and sideways positional offsets of bows in the flow are interpreted as due to precession. A crude precession model then yields estimates for the precession angle, precession period, and possibly also for the binary separation. Line ratio maps, built from the molecular hydrogen images, yield the excitational state of the H2 gas. Surprisingly, the 2-1 S(1)/1-0 S(1) and the 3-2 S(3)/1-0 S(1) line ratios are largely constant everywhere in the outflow. Models of slabs of gas at one single or two constant temperatures fail to account for our data, as do fluorescence, and planar J- and C-shock models. Moreover, J-type bow shocks are a possible, though not very probable, explanation for our data, since they work only if the bows are extremely wide. C-type bow shocks, on the other hand, explain our data well.

Optical jets from the high-luminosity young stars LkH-alpha 234 and AFGL 4029

The discovery of Herbig-Haro (HH)-like jets emanating from the high-luminosity young stars LkH-alpha 234 and AFGL 4029 is reported. Both jets are associated with high-velocity molecular outflows. The LkH-alpha 234 jet lies along the symmetry axis of a recently discovered molecular cavity with LkH-alpha 234 at its apex. The AFGL 4029 jet has at least two radial velocity components, one of which reaches velocities of up to - 500 km/s. A proper motion study of the HH objects in NGC 7129 is also reported. It is found that the collinearity of the HH objects with LkH-alpha 234 is merely fortuitous. Alternative sources are suggested. 27 refs.

VLT integral field spectroscopy of embedded protostars: using near-infrared emission lines as tracers of accretion and outflow

Aims. We present near-infrared spectroscopy of the forbidden emission line (FEL) and molecular hydrogen emission line (MHEL) regions at the bases of Herbig-Haro (HH) jets from seven embedded protostars: SVS 13 (the HH 7-11 progenitor), HH 26-IRS, HH 34-IRS, HH 72-IRS, HH 83-IRS, HH 300-IRS (IRAS 04239+2436) and HH 999-IRS (IRAS 06047-1117) Methods. The integral field spectrograph, SINFONI, on the European Southern Observatory’s Very Large Telescope (VLT) was used to characterise jet parameters in these formative regions, where the jets are collimated and accelerated. Results. We find considerable differences in the spectra of HH 83-IRS when compared to the other six sources; CO bandhead and atomic permitted lines from Ca i ,N ai ,M gi and Al i are observed in emission in all but HH 83-IRS, where they are detected in absorption. It is likely that this source is more evolved than the others (or at the very least considerably less active). Strong CO bandhead emission is also detected in emission in the other six sources, while extended H2 ro-vibrational and [Fe ii] forbidden emission lines trace the outflows (only the HH jet from HH 83-IRS is undetected). CO bandhead and Brγ emission peaks are in most cases coincident with the jet source continuum position, consistent with excitation in an accretion disk or accretion flow. However, in the closest source, HH 300-IRS, we do find evidence for excitation in the outflow: here the emission peak is offset by 3.6(±0.7) AU along the flow axis. We also note a correlation between CO and Mg i ,N ai and Ca i intensities, which supports the idea that these atomic permitted lines are associated with accretion disks. From H2 and [Fe ii] images we measure jet widths and derive upper limits to flow component opening angles. Although we do not find that the ionised [Fe ii] component is consistently narrower than the H2 flow component, we do find that narrower H2 and/or [Fe ii] flow components are associated with higher radial velocities (as reported in the literature). Flow opening angles, over the first few hundred AU in each source, are measured to be in the range 21 ◦ –42 ◦ in both H2 and [Fe ii]. Finally, from our 3-D data we are also able to map the extinction and electron density at the base of the outflows from some of our targets: within a few hundred AU, both decrease sharply with distance from the source. Conclusions. It seems clear that collimated atomic and molecular jets, which may initially exhibit a wide opening angle, are a feature of outflows from Class I protostars, Class II T Tauri stars, and possibly even Class 0 sources, and that these jets can be traced to within a few hundred AU of the driving source. A common jet collimation and acceleration mechanism seems inescapable for all stages of low mass star formation.

Proper Motions And Variability Of The H2 Emission In The HH 46/47 System

We report here on the first proper-motion measurements of molecular hydrogen emission features in the Herbig-Haro 46/47 outflow. Assuming a distance of 350 pc to this flow, the inferred tangential velocities range from a few tens to almost 500 km s-1. The highest velocities are observed for H2 knots either in, or close to, the jet/counterjet axes. Knots constituting the wings of the large-scale H2 bow are found to move much more slowly. These results appear to be in agreement with recent numerical simulations of H2 emission from pulsed jets. We also report the first detection of variability in H2 features for a young stellar object (YSO) outflow. It was found that several H2 knots significantly changed their luminosity over the 4 yr time base used to conduct our study. This is in line with current estimates for the cooling time of gas radiating shocked H2 emission in YSO environments.