91 research outputs found
Spatially Resolved Observations of the Bipolar Optical Outflow from the Brown Dwarf 2MASSJ12073347-3932540
Studies of brown dwarf (BD) outflows provide information pertinent to
questions on BD formation, as well as allowing outflow mechanisms to be
investigated at the lowest masses. Here new observations of the bipolar outflow
from the 24 M BD, 2MASSJ12073347-3932540 are presented. The outflow was
originally identified through the spectro-astrometric analysis of the
[OI]6300 emission line. Follow-up observations consisting of spectra
and [SII], R-band and I-band images were obtained. The new spectra confirm the
original results and are used to constrain the outflow PA at
65. The [OI]6300 emission line region is spatially resolved
and the outflow is detected in the [SII] images. The detection is firstly in
the form of an elongation of the point spread function along the direction of
the outflow PA. Four faint knot-like features (labelled {\it A-D}) are also
observed to the south-west of 2MASSJ12073347-3932540 along the same PA
suggested by the spectra and the elongation in the PSF. Interestingly, {\it D},
the feature furthest from the source is bow-shaped with the apex pointing away
from 2MASSJ12073347-3932540. A color-color analysis allows us to conclude that
at least feature {\it D} is part of the outflow under investigation while {\it
A} is likely a star or galaxy. Follow-up observations are needed to confirm the
origin of {\it B} and {\it C}. This is a first for a BD, as BD optical outflows
have to date only been detected using spectro-astrometry. This result also
demonstrates for the first time that BD outflows can be collimated and
episodic.Comment: Accepted by ApJ, ref ApJ89096R
A Search for Consistent Jet and Disk Rotation Signatures in RY Tau
We present a radial velocity study of the RY Tau jet-disk system, designed to
determine whether a transfer of angular momentum from disk to jet can be
observed. Many recent studies report on the rotation of T Tauri disks, and on
what may be a signature of T Tauri jet rotation. However, due to observational
difficulties, few studies report on both disk and jet within the same system to
establish if the senses of rotation match and hence can be interpreted as a
transfer of angular momentum. We report a clear signature of Keplerian rotation
in the RY Tau disk, based on Plateau de Bure observations. We also report on
the transverse radial velocity profile of the RY Tau jet close to the star. We
identify two distinct profile shapes: a v-shape which appears near jet shock
positions, and a flat profile which appears between shocks. We do not detect a
rotation signature above 3 sigma uncertainty in any of our transverse cuts of
the jet. Nevertheless, if the jet is currently in steady-state, the errors
themselves provide a valuable upper limit on the jet toroidal velocity of 10
km/s, implying a launch radius of < 0.45 AU. However, possible contamination of
jet kinematics, via shocks or precession, prevents any firm constraint on the
jet launch point, since most of its angular momentum could be stored in
magnetic form rather than in rotation of matter.Comment: 10 pages, 7 figures, accepted by The Astrophysical Journa
Numerical simulations of stellar jets and comparison between synthetic and observed maps: clues to the launch mechanism
High angular resolution spectra obtained with the Hubble Space Telescope
Imaging Spectrograph (HST/STIS) provide rich morphological and kinematical
information about the stellar jet phenomenon, which allows us to test
theoretical models efficiently. In this work, numerical simulations of stellar
jets in the propagation region are executed with the PLUTO code, by adopting
inflow conditions that arise from former numerical simulations of magnetized
outflows, accelerated by the disk-wind mechanism in the launching region. By
matching the two regions, information about the magneto-centrifugal
accelerating mechanism underlying a given astrophysical object can be
extrapolated by comparing synthetic and observed position-velocity diagrams
(PVDs). We show that quite different jets, like those from the young T Tauri
stars DG-Tau and RW-Aur, may originate from the same disk-wind model for
different configurations of the magnetic field at the disk surface. This result
supports the idea that all the observed jets may be generated by the same
mechanism.Comment: 15 pages, 18 figures, accepted for publication by A&
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.Comment: 33 pages, 16 figures, accepted by Ap
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.Comment: Proceedings of IAU meeting 275, "Jets at All Scales
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.Comment: 18 pages, 26 figures, Accepted by Ap
Jet rotation investigated in the near-ultraviolet with HST/STIS
We present results of the second phase of our near-ultraviolet investigation
into protostellar jet rotation using HST/STIS. We obtain long-slit spectra at
the base of five T Tauri jets to determine if there is a difference in radial
velocity between the jet borders which may be interpreted as a rotation
signature. These observations are extremely challenging and push the limits of
current instrumentation, but have the potential to provide long-awaited
observational support for the magneto-centrifugal mechanism of jet launching in
which jets remove angular momentum from protostellar systems. We successfully
detect all five jet targets (from RW Aur, HN Tau, DP Tau and CW Tau) in several
near-ultraviolet emission lines, including the strong Mg II doublet. However,
only RW Aur's bipolar jet presents sufficient signal-to-noise for analysis. The
approaching jet lobe shows a difference of 10 km/s in a direction which agrees
with the disk rotation sense, but is opposite to previously published optical
measurements for the receding jet. The near-ultraviolet difference is not found
six months later, nor is it found in the fainter receding jet. Overall, in the
case of RW Aur, differences are not consistent with a simple jet rotation
interpretation. Indeed, given the renowned complexity and variability of this
system, it now seems likely that any rotation signature is confused by other
influences, with the inevitable conclusion that RW Aur is not suited to a jet
rotation study.Comment: 13 pages, 21 figures, Accepted by The Astrophysical Journa
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
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 LkH321, 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
LkH321. 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).Comment: 17 pages, 10 figures, Accepted by Ap
Protostellar Outflows at the EarliesT Stages (POETS). III. H2O masers tracing disk-winds and jets near luminous YSOs
The goal of the Protostellar Outflows at the EarliesT Stages (POETS) survey
is to image the disk-outflow interface on scales of 10-100 au in a
statistically significant sample (36) of luminous young stellar objects (YSO),
targeting both the molecular and ionized components of the outflows. The
outflow kinematics is studied at milliarcsecond scales through VLBI
observations of the 22 GHz water masers. We employed the JVLA at 6, 13, and 22
GHz in the A- and B-Array configurations to determine the spatial structure and
the spectral index of the radio continuum emission. In about half of the
targets, the water masers observed at separation <= 1000 au from the YSOs trace
either or both of these kinematic structures: 1) a spatially elongated
distribution oriented at close angle with the direction of collimation of the
maser proper motions (PM), and 2) a linear LSR velocity (Vlsr) gradient across
the YSO position. The kinematic structure (1) is readily interpreted in terms
of a protostellar jet, as confirmed in some targets via the comparison with
independent observations of the YSO jets, in thermal (continuum and line)
emissions, reported in the literature. The kinematic structure (2) is
interpreted in terms of a disk-wind (DW) seen almost edge-on on the basis of
several pieces of evidence: first, it is invariably directed perpendicular to
the YSO jet; second, it agrees in orientation and polarity with the Vlsr
gradient in thermal emissions (when reported in the literature) identifying the
YSO disk at scales of <= 1000~au; third, the PMs of the masers delineating the
Vlsr gradients hint at flow motions at a speed of 10-20 km/s directed at large
angles with the disk midplane. In the remaining targets, the maser PMs are not
collimated but rather tend to align along two almost perpendicular directions,
and could originate in DW-jet systems slightly inclined (<= 30 deg) with
respect to edge-on.Comment: 61 pages (Main: 13 pages, Appendix: 48 pages), 18 figures (5 + 13),
28 tables (1 + 27), accepted for publication in section 6. Interstellar and
circumstellar matter of A&
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