arXiv subjects

Francesca Bacciotti

Publications and source records attributed to Francesca Bacciotti.

35 records · Page 2Linked to original sources

arXiv2010

Searching for Jet Rotation in Class 0/I Sources observed with GEMINI/GNIRS

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. To test the validity of a rotation argument, we must extend the survey to a larger sample, including younger sources. 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 protostellar jets HH 34, HH 111-H, HH 212 NK1 and SK1. The [Fe II] emission was unresolved in all cases and so Doppler shifts across the jet width could not be accessed. The H_2 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 identified. 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 and 1x10^-5 M_odot/yr AU km/s 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.

astro-ph.SR↗

arXiv2010

Brown Dwarf Jets: Investigating the Universality of Jet Launching Mechanisms at the Lowest Masses

Recently it has become apparent that proto-stellar-like outflow activity extends to the brown dwarf (BD) mass regime. While the presence of accretion appears to be the common ingredient in all objects known to drive jets fundamental questions remain unanswered. The more prominent being the exact mechanism by which jets are launched, and whether this mechanism remains universal among such a diversity of sources and scales. To address these questions we have been investigating outflow activity in a sample of protostellar objects that differ considerably in mass and mass accretion rate. Central to this is our study of brown dwarf jets. To date Classical T Tauri stars (CTTS) have offered us the best touchstone for decoding the launching mechanism. Here we shall summarise what is understood so far of BD jets and the important constraints observations can place on models. We will focus on the comparison between jets driven by objects with central mass < 0.1M \odot and those driven by CTTSs. In particular we wish to understand how the the ratio of the mass outflow to accretion rate compares to what has been measured for CTTSs.

astro-ph.SR↗

arXiv2010

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 TTauri star ThA 15-28 (aka Th 28). Spectra were obtained with 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 both molecular and atomic lines. The H_2 component was found to be entirely blueshifted around the base of the bipolar jet. It shows that only the blue lobe is emitting in H_2 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 Br gamma and Pa beta 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] 2.13 micron line which is of similarly high excitation; H I velocities which match the jet [Fe II] velocities in both the blue and red lobe; and high electron density close to the source of >6x10^4 cm^-3 derived from the [Fe II] 1.64,1.60 micron ratio. These near-infrared data complement 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 H alpha emission is included here along side the other H I lines. Identifying Br gamma and Pa beta 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.

physics.space-ph↗

arXiv2009

IR diagnostics of embedded jets: kinematics and physical characteristics of the HH46-47 jet

We present an analysis of the kinematics and physical properties of the Class I driven jet HH46-47 based on IR medium and low resolution spectroscopy obtained with ISAAC on VLT. Our aim is to study the gas physics as a function of the velocity and distance from the source and compare the results with similar studies performed on other Class I and CTTS jets, as well as with existing models for the jet formation and excitation. The ratios and luminosities of several important diagnostic lines have been used to derive physical parameters. [FeII] and H2 Position Velocity Diagrams (PVDs) have been in addition constructed to study the kinematics. Within 1000-2000 AU from the source the atomic gas presents a wide range of radial velocities. Only the gas component at the highest velocity (HVC) survives at large distances. The H2, shows, close to the source, only a single velocity component at almost zero velocity, while it reaches higer velocities further downstream. Electron densities (ne) and mass ejection fluxes (Mjet) have been measured separately for the HVC and LVC from the [FeII] lines. ne increases with decreasing velocities, while the opposite occurs for Mjet. The mass flux carried out by the molecular component, measured from the H2 lines flux, is ~4 10^-9 Msun/yr. We have estimated that the Fe gas phase abundance is significantly lower than the solar value, with ~88% of iron still depleted onto dust grains in the internal jet region. The derived densities and mass flux values are typical of Class I objects or very active TTauri stars. However, the spatial extent of the LVC and the velocity dependence of the electron density have been so far observed only in another Class I jet, the HH34 jet, and are not explained by the current models of jet launching.

astro-ph.SR↗

arXiv2009

HST/STIS observations of the RW Aurigae bipolar jet: mapping the physical parameters close to the source

We present the results of new spectral diagnostic investigations applied to high-resolution long-slit spectra of the RW Aur bipolar jet obtained with HST/STIS. The spectra include the forbidden doublets [O I] 6300,6363 Å, [S II] 6716,6731 Å, and [N II] 6548, 6583 Åthat we utilized to determine electron density, electron temperature, hydrogen ionisation fraction, total hydrogen density, radial velocity and the mass outflow rate. We were able to extract the parameters as far as 3".9 in the red- and 2".1 in the blueshifted beam. 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. The overall trend of the physical parameters along the first few arcseconds of the RW Aur jet is similar to that of HH 30 and DG Tau and this can reflect analogies in the mechanisms operating in that region, suggesting the same engine is accelerating the jets in the T Tauri stars with outflows. Our study of the RW Aur jet indicates for the first time that, despite the detected marked asymmetries in physical and kinematic properties between the two lobes, the mass outflow rates in the two lobes are similar. This appears to indicate that the central engine has constraining symmetries on both sides of the system, and that the observed asymmetries are probably due to different environmental conditions.

astro-ph.SR↗

arXiv2008

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 PV images of the line ratios to get a global picture of the flow excitation. We then apply a specialised diagnostic technique to find the electron density, ionisation fraction, electron temperature and total density. Our results are in the form of PV maps of the obtained quantities, in which the gas behaviour 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, particularily if the parameters can be determined close to the jet footpoint. Comparing mass flow rates for cases where the latter is available in the literature (i.e. DG Tau, RW Aur and CW Tau) reveals a mass ejection-to-accretion ratio of 0.01 - 0.07. Finally, where possible (i.e. DG Tau and CW Tau), both mass and angular momentum outflow rates have been resolved into higher and lower velocity jet material. For the clearer case of DG Tau, this revealed that the more collimated higher velocity component plays a dominant role in mass and angular momentum transport.

astro-ph↗

arXiv2008

Three-fluid plasmas in star formation I. Magneto-hydrodynamic equations

Interstellar magnetic fields influence all stages of the process of star formation, from the collapse of molecular cloud cores to the formation of protostellar jets. This requires us to have a full understanding of the physical properties of magnetized plasmas of different degrees of ionization for a wide range of densities and temperatures. We derive general equations governing the magneto-hydrodynamic evolution of a three-fluid medium of arbitrary ionization, also including the possibility of charged dust grains as the main charge carriers. In a companion paper (Pinto & Galli 2007), we complement this analysis computing accurate expressions of the collisional coupling coefficients. Over spatial and temporal scales larger than the so-called large-scale plasma limit and the collision-dominated plasma limit, and for non-relativistic fluid speeds, we obtain an advection-diffusion for the magnetic field. We derive the general expressions for the resistivities, the diffusion time scales and the heating rates in a three-fluid medium and we use them to estimate the evolution of the magnetic field in molecular clouds and protostellar jets. Collisions between charged particles significantly increase the value of the Ohmic resistivity during the process of cloud collapse, affecting in particular the decoupling of matter and magnetic field and enhancing the rate of energy dissipation. The Hall resistivity can take larger values than previously found when the negative charge is mostly carried by dust grains. In weakly-or mildy-ionized protostellar jets, ambipolar diffusion is found to occur on a time scale comparable to the dynamical time scale, limiting the validity of steady-state and nondissipative models to study the jet's structure.

astro-ph↗

arXiv2008

A HST study of the environment of the Herbig Ae/Be star LkHa 233 and its bipolar jet

We present the results of HST/STIS and WFPC2 observations of LkHa 233 and its environment. LkHa233 is a Herbig Ae/Be star with a collimated bipolar jet. We investigate optical forbidden lines along the LkHa 233 jet to determine physical parameters of this jet (electron density n_e, hydrogen ionisation fraction x_e, electron temperature T_e, and mass density n_H). The knowledge of these parameters allows us a direct comparison of a jet from a Herbig star with those from T Tauri stars. The WFPC2 images in broad-band filters clearly show a dark lane caused either by a circumstellar disk or a dust torus. In the blueshifted lobe, n_e is close to or above the critical density for [SII] lines (2.5x10^4 cm^-3) in the first arcsecond and decreases with distance from the source. The ionisation x_e~0.2-0.6 gently rises for the first 500 AU of the flow and shows two re-ionisation events further away from the origin. The T_e varies along the flow between 10^4 K and 3x10^4 K. The (radial) outflow velocities are ~ 80-160 km s^(-1), and they appear to increase with distance from the source. In the redshifted lobe, T_e, n_e, x_e, and n_H are all lower than in the blueshifted lobe, but have the same order of magnitude. All these derived parameters are just beyond or at the upper limits of those observed for classical T Tauri star jets. This may indicate that the flows from the higher mass Herbig stars are indeed scaled-up examples of the same phenomenon as in T Tauri stars.

astro-ph↗

arXiv2007

Further Indications of Jet Rotation in New Ultraviolet and Optical HST/STIS Spectra

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, while differences in the NUV range were consistently lower at typically 10 (+/-5) km/s. 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.

astro-ph↗

arXiv2006

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

We examine the conditions of the plasma along a sample of 'classical' Herbig-Haro 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 at moderate spatial/spectral resolution. The obtained spectra cover a wide wavelength range from 0.6-2.5 um, 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. The derived physical parameters (such as the extinction, the electron density and temperature, the ionisation fraction, and the total density) are used to estimate the depletion onto dust grains of Calcium and Iron with respect to solar abundances. This turns out to be between 70% and 0% for Ca and ~90% for Fe, suggesting that the weak shocks present in the beams are not capable of completely destroying the dust grains. We then derive the mass flux rates (Mdot_jet is on average 5 10^-8 M_solar yr^-1) and the associated linear momentum fluxes. The latter are higher than, or of the same order as, those measured in the coaxial molecular flows, suggesting that the flows are jet driven. Finally, we discuss differences between jets in our sample.

astro-ph↗

arXiv2006

Toward Resolving the Outflow Engine: An Observational Perspective

Jets from young stars represent one of the most striking signposts of star formation. The phenomenon has been researched for over two decades and there is now general agreement that such jets are generated as a by-product of accretion; most likely by the accretion disk itself. Thus they mimic what occurs in more exotic objects such as active galactic nuclei and micro-quasars. The precise mechanism for their production however remains a mystery. To a large degree, progress is hampered observationally by the embedded nature of many jet sources as well as a lack of spatial resolution: Crude estimates, as well as more sophisticated models, nevertheless suggest that jets are accelerated and focused on scales of a few AU at most. It is only in the past few years however that we have begun to probe such scales in detail using classical T Tauri stars as touchstones. Application of adaptive optics, data provided by the HST, use of specialised techniques such as spectro-astrometry, and the development of spectral diagnostic tools, are beginning to reveal conditions in the jet launch zone. This has helped enormously to constrain models. Further improvements in the quality of the observational data are expected when the new generation of interferometers come on-line. Here we review some of the most dramatic findings in this area since Protostars and Planets~IV including indications for jet rotation, i.e. that they transport angular momentum. We will also show how measurements, such as those of width and the velocity field close to the source, suggest jets are initially launched as warm magneto-centrifugal disk winds. (abridged)

astro-ph↗

arXiv2005

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

Complete flux-calibrated spectra covering the spectral range from 6000 A to 2.5 um 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. Important physical parameters 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. In particular, [FeII] 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. dM/dt is high in the initial part of the flow but decreases by about an order of magnitude further out. Conversely, the mass flux associated with the warm molecular material is low 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]63 um line. Finally, we find that the gas-phase abundance of refractory species is lower than the solar value suggesting that a significant fraction of dust grains may still be present in the jet beam.

astro-ph↗

arXiv2005

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 amongst 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 outflow's 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.

astro-ph↗

arXiv2004

Jet rotation: launching region, angular momentum balance and magnetic properties in the bipolar outflow from RW Aur

Using STIS on board the HST we have obtained a spectroscopic map of the bipolar jet from RW Aur. We find signatures of rotation within the first 300 AU of the jet (1.''5 from RW Aur). Both lobes rotate in the same direction (i.e. with different helicities). Toroidal velocities are in the range 5 - 30 km/s at 20 (30) AU from the symmetry axis in the blueshifted (redshifted) lobe, in line with other STIS observations and with theoretical simulations. The sense of rotation is anti-clockwise looking from the tip of the blue lobe down to the star. Rotation is more evident in the [OI] and [NII] lines and at the largest sampled distance from the axis. We derive that the mass ejected in the observed part of the outflow is accelerated from a region in the disk within about 0.5 (1.6) AU from the star for the blue (red) lobe. We estimate that the angular momentum transport rate of the jet can be two thirds or more of the estimated rate transported through the relevant portion of the disk. The magnetic lever arm (ratio rA/r0 between the Alfv`en and footpoint radii) is between 3.5 and 4.6 (accuracy 20-25%), or, the ejection index $ξ$ = d ln($\dot{M}_acc$) / d r is in the range 0.025 - 0.046 (same accuracy).These values are in the range predicted by the models, and suggest that some heating must be provided at the base of the flow. We also derive the ratio B_phi / B_p of the toroidal and poloidal components of the magnetic field at the observed location. B_phi / B_p = 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. The toroidal component is dominant, as predicted by magnetic collimation models.

astro-ph↗

arXiv2003

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 bi-polar jets from the T Tauri stars TH28 and RW Aur, and the blue-shifted jet from T Tauri star LkH$α$321, using the Hubble Space Telescope Imaging Spectrograph (HST/STIS). Forbidden emission lines (FELs) show distinct and systematic velocity asymmetries of 10 -- 25 (+/- 5) km/s at a distance of 0".3 from the source, representing a (projected) distance of ~ 40 AU along the jet in the case of RW Aur, ~ 50 AU for TH28, and 165 AU in the case of LkH$α$321. 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 (Bacciotti et al 2002, 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).

astro-ph↗

arXiv2002

HST/STIS Observations of the Bipolar Jet from RW Aurigae: Tracing Outflow Asymmetries Close to the Source

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. 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 flux and momentum flux 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.

astro-ph↗

arXiv2000

HST/STIS spectroscopy of the optical outflow from DG Tau: structure and kinematics on sub-arcsecond scales

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 Ha. Here we present high spatial resolution synthetic line images close to the star in distinct radial velocity intervals (from ~ +50 km/s to ~ -450 km/s in four bins, each ~ 125 km/s wide). The lowest velocity emission is also examined in finer detail (from +60 km/s to -70 km/s in five bins ~ 25 km/s 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 the density increases longitudinally with proximity to the source and also 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.

astro-ph↗