262 research outputs found
Numerical simulations of radiative magnetized Herbig-Haro jets: the influence of pre-ionization from X-rays on emission lines
We investigate supersonic, axisymmetric magnetohydrodynamic (MHD) jets with a
time-dependent injection velocity by numerical simulations with the PLUTO code.
Using a comprehensive set of parameters, we explore different jet
configurations in the attempt to construct models that can be directly compared
to observational data of microjets. In particular, we focus our attention on
the emitting properties of traveling knots and construct, at the same time,
accurate line intensity ratios and surface brightness maps. Direct comparison
of the resulting brightness and line intensity ratios distributions with
observational data of microjets shows that a closer match can be obtained only
when the jet material is pre-ionized to some degree. A very likely source for a
pre-ionized medium is photoionization by X-ray flux coming from the central
object.Comment: Accepted for publication in Ap
Tomographic reconstruction of the three-dimensional structure of the HH30 jet
The physical parameters of Herbig-Haro jets are usually determined from
emission line ratios, obtained from spectroscopy or narrow band imaging,
assuming that the emitting region is homogeneous along the line of sight. Under
the more general hypothesis of axisymmetry, we apply tomographic reconstruction
techniques to the analysis of Herbig-Haro jets. We use data of the HH30 jet
taken by Hartigan & Morse (2007) with the Hubble space telescope using the
slitless spectroscopy technique. Using a non-parametric Tikhonov regularization
technique, we determine the volumetric emission line intensities of the
[SII]6716,6731, [OI]6300 and [NII]6583 forbidden emission lines. From our
tomographic analysis of the corresponding line ratios, we produce
"three-dimensional" images of the physical parameters. The reconstructed
density, temperature and ionization fraction present much steeper profiles than
those inferred using the assumption of homogeneity. Our technique reveals that
the reconstructed jet is much more collimated than the observed one close to
the source (a width ~ 5 AU vs. ~ 20 AU at a distance of 10 AU from the star),
while they have similar widths at larger distances. In addition, our results
show a much more fragmented and irregular jet structure than the classical
analysis, suggesting that the the ejection history of the jet from the
star-disk system has a shorter timescale component (~ some months) superimposed
on a longer, previously observed timescale (of a few years). Finally, we
discuss the possible application of the same technique to other stellar jets
and planetary nebulae.Comment: 13 pages, 9 figures, accepted by Ap
Solving the excitation and chemical abundances in shocks: the case of HH1
We present deep spectroscopic (3600 - 24700 A) X-shooter observations of the
bright Herbig-Haro object HH1, one of the best laboratories to study the
chemical and physical modifications caused by protostellar shocks on the natal
cloud. We observe atomic fine structure lines, HI, and He, recombination lines
and H_2, ro-vibrational lines (more than 500 detections in total). Line
emission was analyzed by means of Non Local Thermal Equilibiurm codes to derive
the electron temperature and density, and, for the first time, we are able to
accurately probe different physical regimes behind a dissociative shock. We
find a temperature stratification in the range 4000 - 80000 K, and a
significant correlation between temperature and ionization energy. Two density
regimes are identified for the ionized gas, a more tenuous, spatially broad
component (density about 10^3 cm^-3), and a more compact component (density >
10^5 cm^-3) likely associated with the hottest gas. A further neutral component
is also evidenced, having temperature lass than 10000 K and density > 10^4
cm^-3. The gas fractional ionization was estimated solving the ionization
equilibrium equations of atoms detected in different ionization stages. We find
that neutral and fully ionized regions co-exist inside the shock. Also,
indications in favor of at least partially dissociative shock as the main
mechanism for molecular excitation are derived. Chemical abundances are
estimated for the majority of the detected species. On average, abundances of
non-refractory/refractory elements are lower than solar of about 0.15/0.5 dex.
This testifies the presence of dust inside the medium, with a depletion factor
of Iron of about 40%.Comment: Accepted by The Astrophysical Journa
Searching for jet rotation in Class 0/I sources observed with GEMINI/GNIRS.
Original article can be found at: http://www.aanda.org/
Copyright The European Southern ObservatoryContext: 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 HH 34, 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 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
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.Comment: Accepted by ApJ, 16 pages, 5 figure
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 \AA, [S
II] 6716,6731 \AA, and [N II] 6548, 6583 \AA that 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.Comment: 24 pages, 10 figures, accepted for publication in the Astronomy and
Astrophysic
Classical T Tauri-like Outflow Activity in the Brown Dwarf Mass Regime
Over the last number of years spectroscopic studies have strongly supported
the assertion that protostellar accretion and outflow activity persists to the
lowest masses. In this paper we present the results of our latest investigation
of brown dwarf (BD) outflow activity and report on the discovery of two new
outflows. Here ISO-Oph 32 is shown to drive a blue-shifted outflow with a
radial velocity of 10-20 km/s and spectro-astrometric analysis constrains the
position angle of this outflow to 240 +/- 7 degrees. The BD candidate ISO-Cha1
217 is found to have a bipolar outflow bright in several key forbidden lines
(radial velocity = -20 km/s, +40 km/s) and with a PA of 190-210 degrees. A
striking feature of the ISO-Cha1 217 outflow is the strong asymmetry between
the red and blue-shifted lobes. This asymmetry is revealed in the relative
brightness of the two lobes (red-shifted lobe is brighter), the factor of two
difference in radial velocity (the red-shifted lobe is faster) and the
difference in the electron density (again higher in the red lobe). Such
asymmetries are common in jets from low mass protostars and the observation of
a marked asymmetry at such a low mass supports the idea that BD outflow
activity is scaled down from low mass protostellar activity.
In addition to presenting these new results, a comprehensive comparison is
made between BD outflow activity and jets launched by CTTSs. In particular, the
application of current methods for investigating the excitation conditions and
mass loss rates in CTT jets to BD spectra is explored.Comment: Accepted by Astrophysical Journa
Magnetic Fields in Stellar Jets
Although several lines of evidence suggest that jets from young stars are
driven magnetically from accretion disks, existing observations of field
strengths in the bow shocks of these flows imply that magnetic fields play only
a minor role in the dynamics at these locations. To investigate this apparent
discrepancy we performed numerical simulations of expanding magnetized jets
with stochastically variable input velocities with the AstroBEAR MHD code.
Because the magnetic field B is proportional to the density n within
compression and rarefaction regions, the magnetic signal speed drops in
rarefactions and increases in the compressed areas of velocity-variable flows.
In contrast, B ~ n^0.5 for a steady-state conical flow with a toroidal field,
so the Alfven speed in that case is constant along the entire jet. The
simulations show that the combined effects of shocks, rarefactions, and
divergent flow cause magnetic fields to scale with density as an intermediate
power 1 > p > 0.5. Because p > 0.5, the Alfven speed in rarefactions decreases
on average as the jet propagates away from the star. This behavior is extremely
important to the flow dynamics because it means that a typical Alfven velocity
in the jet close to the star is significantly larger than it is in the
rarefactions ahead of bow shocks at larger distances, the one place where the
field is a measurable quantity. We find that the observed values of weak fields
at large distances are consistent with strong fields required to drive the
observed mass loss close to the star. For a typical stellar jet the crossover
point inside which velocity perturbations of 30 - 40 km/s no longer produce
shocks is ~ 300 AU from the source
Optical Emission in the Beam of Stellar Jets: A Possible Mechanism
We propose a mechanism for the optical emission observed in the beam section of stellar jets based on the stability properties of circularly polarized Alfven waves propagating in the partially ionized medium filling the jet's channel. We first derive the relevant magnetohydrodynamic equations, which include the Hall term for such a partially ionized medium, a term which give rise to dispersive effects. Quasi-equipartition of thermal and magnetic energies is assumed, and the model is developed in the one-dimensional approximation but keeping the three components of the vectorial fields. Mild compressions or very weak shocks occur in the flow when instabilities develop, depending on the relative sign of the angular momentum of the underlying accretion disk and the external magnetic field. Simulations performed under quite general conditions show that the temperature/density perturbations associated with the instability are consistent with the average luminosity contrast observed in the jet's beam between bright knots and interknot regions
A highly-collimated SiO jet in the HH212 protostellar outflow
We mapped the HH212 Class 0 outflow in SiO(2--1, 5--4) and continuum using
the PdBI in its extended configurations. The unprecedented angular resolution
(down to 0.34") allows accurate comparison with a new, deep H2 image obtained
at the VLT. The SiO emission is confined to a highly-collimated bipolar jet
(width 0.35") along the outflow axis. The jet can be traced down to within 500
AU of the protostar, in a region that is heavily obscured in H2 images. Where
both species are detected, SiO shows the same overall kinematics and structure
as H2, indicating that both molecules are tracing the same material. We find
that the high-velocity SiO gas near the protostar is not tracing a wide-angle
wind but is already confined to a flow inside a narrow cone of half-opening
angle < 6 deg.Comment: Astronomy and Astrophysics Letter, in pres
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