191 research outputs found
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
Laser Guide Star Adaptive Optics Integral Field Spectroscopy of a Tightly Collimated Bipolar Jet from the Herbig Ae star LkHa 233
We have used the integral field spectrograph OSIRIS and laser guide star
adaptive optics at Keck Observatory to obtain high angular resolution (0.06"),
moderate spectral resolution (R ~ 3800) images of the bipolar jet from the
Herbig Ae star LkHa 233, seen in near-IR [Fe II] emission at 1.600 & 1.644
microns. This jet is narrow and tightly collimated, with an opening angle of
only 9 degrees, and has an average radial velocity of ~ 100 km/s. The jet and
counterjet are asymmetric, with the red-shifted jet much clumpier than its
counterpart at the angular resolution of our observations. The observed
properties are in general similar to jets seen around T Tauri stars, though it
has a relatively large mass flux of (1.2e-7 +- 0.3e-7) M_sun/year, near the
high end of the observed mass flux range around T Tauri stars. We also
spatially resolve an inclined circumstellar disk around LkHa 233, which
obscures the star from direct view. By comparison with numerical radiative
transfer disk models, we estimate the disk midplane to be inclined i = 65 +- 5
degrees relative to the plane of the sky. Since the star is seen only in
scattered light at near-infrared wavelengths, we detect only a small fraction
of its intrinsic flux. Because previous estimates of its stellar properties did
not account for this, either LkHa 233 must be located closer than the
previously believed, or its true luminosity must be greater than previously
supposed, consistent with its being a ~4 M_sun star near the stellar birthline.Comment: Accepted for publication in the Ap
Physical properties of the jet from DG Tauri on sub-arcsecond scales with HST/STIS
We derive the physical properties at the base of the jet from DG Tau both
along and across the flow and as a function of velocity. We analysed seven
optical spectra of the DG Tau jet, taken with the Hubble Space Telescope
Imaging Spectrograph. The spectra were obtained by placing a long-slit parallel
to the jet axis and stepping it across the jet width. The resulting
position-velocity diagrams in optical forbidden emission lines allowed access
to plasma conditions via calculation of emission line ratios.
We find at the base of the jet high electron density, 10, and
very low ionisation, , which combine to give a total
density up to 3 10. This analysis confirms previous reports of
variations in plasma parameters along the jet, (i.e. decrease in density by
several orders of magnitude, increase of from 0.05 to a plateau at 0.7
downstream at 2 from the star). Furthermore, a spatial coincidence is
revealed between sharp gradients in the total density and supersonic velocity
jumps. This strongly suggests that the emission is caused by shock excitation.
The position-velocity diagrams indicate the presence of both fast accelerating
gas and slower, less collimated material. We derive the mass outflow rate,
, in the blue-shifted lobe in different velocity channels, that
contribute to a total of 8 4 10 M
yr. We estimate that a symmetric bipolar jet would transport at the low
and intermediate velocities probed by rotation measurements, an angular
momentum flux of 2.9 1.5 10 M yr
AU km s.
The derived properties of the DG Tau jet are demonstrated to be consistent
with magneto-centrifugal theory. However, non-stationary modelling is required
in order to explain all of the features revealed at high resolution.Comment: 16 pages, 18 figure
Magneto-Centrifugal Launching of Jets from Accretion Disks. II: Inner Disk-Driven Winds
We follow numerically the time evolution of axisymmetric outflows driven
magneto-centrifugally from the inner portion of accretion disks, from their
launching surface to large, observable distances. Special attention is paid to
the collimation of part of the outflow into a dense, narrow jet around the
rotation axis, after a steady state has been reached. For parameters typical of
T Tauri stars, we define a fiducial ``jet'' as outlined by the contour of
constant density at 10^4 cm^{-3}. We find that the jet, so defined, appears
nearly cylindrical well above the disk, in agreement with previous asymptotic
analyses. Closer to the equatorial plane, the density contour can either bulge
outwards or pinch inwards, depending on the conditions at the launching
surface, particularly the mass flux distribution. We find that even though a
dense, jet-like feature is always formed around the axis, there is no guarantee
that the high-density axial jet would dominate the more tenuous, wide-angle
part of the wind. Specifically, on the 100 AU scale, resolvable by HST and
ground-based adaptive optics for nearby T Tauri winds, the fraction of the wind
mass flux enclosed by the fiducial jet can vary substantially, again depending
on the launching conditions. We show two examples in which the fraction is ~20%
and ~45%. These dependences may provide a way to constrain the conditions at
the launching surface, which are poorly known at present.Comment: 11 pages, 6 figures. Accepted for publication in ApJ, scheduled for
vol. 595, October 1, 200
An HST Imaging Survey of Low-Mass Stars in the Chamaeleon I Star Forming region
We present new HST/WFPC2 observations of 20 fields centered around T Tauri
stars in the Chamaeleon I star forming region. Images have been obtained in the
F631N ([OI]6300A), F656N (Ha) and F673N ([SII]6716A+6731A) narrow-band filters,
plus the Johnson V-band equivalent F547M filter. We detect 31 T Tauri stars
falling within our fields. We discuss the optical morphology of 10 sources
showing evidence of either binarity, circumstellar material, or mass loss. We
supplement our photometry with a compilation of optical, infrared and
sub-millimeter data from the literature, together with new sub-mm data for
three objects, to build the Spectral Energy Distributions (SED) of 19 single
sources. Using an SED model fitting tool, we self-consistently estimate a
number of stellar and disk parameters, while mass accretion rates are directly
derived from our Ha photometry. We find that bolometric luminosities derived
from dereddened optical data tend to be underestimated in systems with high
alpha(2-24} IR spectral index, suggesting that disks seen nearly edge-on may
occasionally be interpreted as low luminosity (and therefore more evolved)
sources. On the other hand, the same alpha(2-24) spectral index, a tracer of
the amount of dust in the warmer layers of the circumstellar disks, and the
mass accretion rate appear to decay with the isocronal stellar age, suggesting
that the observed age spread (~0.5-5 Myr) within the cluster is real. Our
sample contains a few outliers that may have dissipated their circumstellar
disks on shorter time-scale.Comment: to appear on Astronomical Journal, accepted April 16, 2012 (AJ-10740
Organic molecules in the protoplanetary disk of DG Tau revealed by ALMA
Planets form in protoplanetary disks and inherit their chemical compositions.
It is thus crucial to map the distribution and investigate the formation of
simple organics, such as formaldehyde and methanol, in protoplanetary disks. We
analyze ALMA observations of the nearby disk-jet system around the T Tauri star
DG Tau in the o-HCO and CHOH E,
A transitions at an unprecedented resolution of ,
i.e., au at a distance of 121 pc. The HCO emission originates from
a rotating ring extending from au with a peak at au, i.e., at
the edge of the 1.3mm dust continuum. CHOH emission is not detected down to
an r.m.s. of 3 mJy/beam in the 0.162 km/s channel. Assuming an ortho-to-para
ratio of 1.8-2.8 the ring- and disk-height-averaged HCO column density is
cm, while that of CHOH is
cm. In the inner au no o-HCO emission
is detected with an upper limit on its beam-averaged column density of
cm. The HCO ring in the disk of DG Tau is
located beyond the CO iceline (R au). This suggests that the
HCO abundance is enhanced in the outer disk due to formation on grain
surfaces by the hydrogenation of CO ice. The emission peak at the edge of the
mm dust continuum may be due to enhanced desorption of HCO in the gas phase
caused by increased UV penetration and/or temperature inversion. The
CHOH/HCO abundance ratio is , in agreement with disk chemistry
models. The inner edge of the HCO ring coincides with the radius where the
polarization of the dust continuum changes orientation, hinting at a tight link
between the HCO chemistry and the dust properties in the outer disk and at
the possible presence of substructures in the dust distribution.Comment: 8 pages, 6 figures, accepted for publication on A&A Letter
POISSON project - I - Emission lines as accretion tracers in young stellar objects: results from observations of Chamaeleon I and II sources
We present the results of the analysis of LR optical-NIR spectra (0.6-2.4 um)
of a sample 47 YSOs in the ChaI and II star-forming clouds. These data are part
of the POISSON project (Protostellar Optical-Infrared Spectral Survey on NTT).
The aim is to determine the accretion luminosity (Lacc) and mass accretion rate
(Macc) of the sources through the analysis of the detected emission features.
We also aim at verifying the reliability and consistency of the existing
empirical relationships connecting emission line luminosity and Lacc. We employ
five tracers (OI-6300A, Ha, CaII-8542A, Pab, and Brg) to derive the accretion
luminosity. The tracers provide Lacc values showing different scatters when
plotted as a function of L*. The Brg seems to be the most reliable, because it
gives the minimum Lacc dispersion over the entire range of L*, whereas the
other tracers provide much more scattered Lacc values, which are not expected
for our homogeneous sample. The comparison between Lacc(Brg) and Lacc obtained
from the other tracers also shows systematic differences among the empirical
relationships. These may probably be ascribed to different excitation
mechanisms contributing to the line emission, which may vary between our sample
and those where the relationships were calibrated. Adopting the Lacc derived
from Brg, we find Lacc=0.1L*-1L* for all sources, and Macc of the order of
10^-7-10^-9 Msun/yr. The Macc derived in ChaI are proportional to M*^2, as
found in other low-mass star-forming regions. The discrepancies observed in the
case of Lacc(Brg) and Lacc(Pab) can be related to different intrinsic Pab/Brg,
ratios. The derived ratios show the existence of two different emission
modalities, one that agrees with predictions of both wind and accretion models,
the other suggesting optically thick emission from relatively small regions
(10^21-10^22 cm^-3) with gas at low temperatures (<4000K).Comment: 22 pages, 8 figures, accepted for publication in A&A; institute
affiliations and typos correcte
ALMA polarimetric studies of rotating jet/disk systems
We have recently obtained polarimetric data at mm wavelengths with ALMA for
the young systems DG Tau and CW Tau, for which the rotation properties of jet
and disk have been investigated in previous high angular resolution studies.
The motivation was to test the models of magneto-centrifugal launch of jets via
the determination of the magnetic configuration at the disk surface. The
analysis of these data, however, reveals that self-scattering of dust thermal
radiation dominates the polarization pattern. It is shown that even if no
information on the magnetic field can be derived in this case, the polarization
data are a powerful tool for the diagnostics of the properties and the
evolution of dust in protoplanetary disks.Comment: 9 pages, 3 figures, to appear in "Jet Simulations, Experiments and
Theory. Ten years after JETSET, what is next ?", C. Sauty ed., Springer
Natur
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