Issue |
A&A
Volume 510, February 2010
|
|
---|---|---|
Article Number | A2 | |
Number of page(s) | 12 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/200810245 | |
Published online | 29 January 2010 |
A rotating molecular jet in Orion
L. A. Zapata - J. Schmid-Burgk - D. Muders - P. Schilke - K. Menten - R. Guesten
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Received 22 May 2008 / Accepted 23 October 2009
Abstract
We present CO(2-1), 13CO(2-1), CO(6-5),
CO(7-6), and SO(65-54) line
observations made with the IRAM 30 m and Atacama Pathfinder
Experiment (APEX) radiotelescopes and the Submillimeter
Array (SMA) toward the highly collimated (11)
and extended (
2')
southwest lobe of the bipolar outflow Ori-S6
located in the Orion South region. We report for all these lines, the
detection of velocity asymmetries about the flow axis with velocity
differences roughly on the order of 1 km s-1
over distances of about 5000 AU, 4 km s-1
over distances of about 2000 AU, and close to the source of
between 7 and 11 km s-1
over smaller scales of about 1000 AU. The redshifted gas
velocities are located to the southeast of the outflow's axis, the
blueshifted ones to the northwest. We interpret these velocity
differences as a signature of rotation, but also discuss some
alternatives which we recognize as unlikely in view of the asymmetries'
large downstream continuation. In particular, any
straightforward interpretation by an ambient velocity gradient does not
seem viable. This rotation across the Ori-S6 outflow
is observed out to (projected) distances beyond 2.5
104 AU from the flow's presumed origin.
Comparison of our large-scale (single dish) and small-scale (SMA)
observations suggests the rotational velocity to
decline not faster than 1/R with distance R
from the axis; in the innermost few arcsecs an increase of rotational
velocity with R is even indicated. The
magnetic field lines threading the inner rotating CO shell may
well be anchored in a disk of a radius of
50 AU; the field
lines further out need a more extended rotating base. Our high angular
resolution SMA observations also suggest this outflow to be energized
by the compact millimeter radio source 139-409,
a circumbinary flattened ring that is located in a small
cluster of very young stars associated with the extended and bright
source FIR4.
Key words: ISM: jets and outflows - techniques: high angular resolution - binaries: general - stars: pre-main sequence - ISM: molecules - radio lines: ISM
1 Introduction
Protostellar jets have the essential task of removing angular momentum from the cores of pre-/protostellar clouds in order for these to contract into new stellar objects. It is believed that T Tauri winds and protostellar jets are driven magnetocentrifugally from keplerian accretion disks close to the central stars (for a review see Shang et al. 2007; Shu et al. 2000; Königl & Pudritz 2000; Pudritz et al. 2007). In these models the magnetic fields that are anchored in the accretion disk-star system are responsible for accelerating the jets' material from the accretion disk. The material ejected from the disk therefore possesses angular momentum and, if not completely free to move away from the jet immediately, will thus show a toroidal velocity component, i.e. rotation about the jet axis.
In recent years a number of observations at optical wavelengths, using high spectral and angular resolution toward the launching zones of young jets from T Tauri stars, have attempted to identify velocity asymmetries that might be interpreted as signatures of jet rotation. These observations include Bacciotti et al. (2002), Coffey et al. (2004), Woitas et al. (2005), and Coffey et al. (2007) who detected systematic velocity shifts (of order 5 to 25 km s-1) across the jet's axis within the first 100-200 AU from the star, toward six T Tauri stars, e.g. DG tau, RW Aur, CW Tau, and Th 28. Furthermore, observations at infrared wavelengths have also revealed such velocity jumps in the launching zones of the Herbig-Haro objects HH 26 and HH 72 (Chrysostomou et al. 2008). All these authors have interpreted the velocity shifts as a signature of jet rotation produced by a magneto centrifugal wind.
The first tentative evidence of such an outflow rotation was presented by Davis et al. (2000) at infrared wavelengths. They reported velocity shifts of a few km s-1 across the HH212 jet at distances of about 104 AU from the ejecting object, using spectral line observations of the molecule H2.
At millimeter wavelengths there have likewise been attempts to find such signatures near the base of strong molecular outflows (HH 30: Pety et al. 2006; HH212: Codella et al. 2007; Lee et al. 2008; HH 211: Lee et al. 2007; CB 26: Launhardt et al. 2009). However, there seems to be evidence of rotation only in the CB 26, the HH212, and the HH 211 outflows.
Recent studies have proposed alternative explanations for observed velocity asymmetries in the jets. Soker (2005) remarked that the interaction of the jet with a twisted-tilted (warped) accretion disk can lead to the observed asymmetry in the jet's line-of-sight velocity profile, and thus the magneto centrifugal wind acceleration model is not required to explain such velocity jumps at the base of the optical and infrared jets. Cerqueira et al. (2006) proposed that a precessing jet whose ejection velocity changes periodically with a period equal to the precession period could also reproduce the line profiles of the jets.
Below, we address the issue of outflow rotation by combining new interferometer and single-dish observations of the Ori-S6 molecular outflow (Schmid-Burgk et al. 1990), which is located in the southernmost part of the very active high- and intermediate-mass star forming region Orion South. Single-dish observations suggested this outflow to originate near the millimeter sources CS3/FIR4 some 100'' to the south of the BN/KL object. Unfortunately the immediate vicinity of CS3/FIR4 contains at least two more molecular outflows (Zapata et al. 2005,2007), both almost perpendicular to Ori-S6, so that unique differentiation between the several emitting structures becomes only possible for single telescopes beyond some 20'' from the presumed region of origin. Interferometry however has permitted Zapata et al. (2004a,b) to propose the source of this outflow to be the elongated radio object 134-411 (possibly a compact thermal jet) that is located close to the position of CS3/FIR4, the position angle of its major axis being consistent with that of the outflow.
The redshifted lobe of Ori-S6 is highly collimated (11), of
relatively low radial velocities (up to 15 to
20 km s-1 relative to ambient)
and extended over
2'
in a direction of PA around 210
.
That lobe is much better defined and stronger than its blueshifted
counterpart because it is interacting directly with the molecular
cloud, while the latter points in direction of the HII region
and is likely destroyed by it. Below, we will deal with the
redshifted lobe only.
Zapata et al. (2006) suggested that this outflow could be associated with a collimated SiO jet that they had detected in their SMA observations and that is emanating from the CS3/FIR4 region with almost the same orientation. On larger scales and at optical wavelengths, Henney et al. (2007); O'Dell et al. (2008) discussed the possibility that the redshifted lobe of the Ori-S6 outflow is associated with a strong shock (``Southwest Shock''), some 400'' downstream, that is imaged in the Fig. 6 of Henney et al. (2007).
CO position-velocity diagrams along the red lobe reveal a two-step structure over a distance of about 120'' from the source region. Over the inner 60'' there is a linear buildup of outflow velocities, from ambient (around 7 to 8 km s-1) to maximum values near 25 km s-1, which then terminates abruptly. At the same point the ambient velocities as seen in CO, which closer to CS3/FIR4 cover a rather wide range of between 7 and 9.5 km s-1, acquire an additional component, of around 10 km s-1, which further downstream becomes the dominant velocity. At this point a second, even faster linear buildup of radial velocity, again starting from ambient values, sets in which follows exactly the same direction as the first one. Quite likely the break in outflow velocity around the 60'' point is related to the change in ambient conditions.
In the present study we will concentrate on the inner 60'' section. Already early on Muders & Schmid-Burgk (1992) had noted this section, as seen in C18O(2-1), to be enveloped by a somewhat symmetrical cylindrical structure that seemed to undergo rotation about the outflow axis.
Throughout the paper, we have divided the outflow into three
main components: (1) on small scales (1000 AU), the
jet (concentrated in bullets); (2) on large scales (
5000 AU),
the shell, both observed with the SMA;
and (3) on very large scales (
104 AU),
the envelope, observed with single dish telescopes.
We present CO(2-1), 13CO(2-1), CO(6-5), CO(7-6), and SO(65-54) line observations made with both the IRAM 30 m and APEX telescopes as well as the submillimeter array toward the redshifted lobe of Ori-S6. We report the detection at all three observatories of velocity jumps across the flow axis and interpret these as a signature of rotation. We discuss some alternative suggestions to explain these velocity asymmetries. Finally, the SMA observations suggest the source of the extended collimated outflow to be the millimeter continuum source 139-409.
2 Observations
2.1 Submillimeter Array
2.1.1 SO(65-54) high resolution
Observations were made with the Submillimeter Array (SMA) during
2004 September 3. The SMA was in its extended
configuration, which includes 21 independent baselines ranging
in projected length from 16 to 180 m. The
phase reference center of the observations was RA = 05
35
14
,
dec = -05
24'00'' (J2000.0).
The size of the primary beam response at this frequency
is 50''. The receivers were tuned to a frequency of
230.534 GHz in the upper sideband (USB), while the lower
sideband (LSB) was centered on 220.534 GHz.
The SO(65-54) transition was detected in the LSB at a frequency of 219.9 GHz. The full bandwidth of the SMA correlator is 4 GHz (2 GHz in each band). The SMA digital correlator was configured in 24 spectral windows (``chunk'') of 104 MHz each, with 32 channels distributed over each spectral window, providing a resolution of 3.25 MHz (4.29 km s-1) per channel.
The zenith opacity (
),
measured with the NRAO tipping radiometer located at the Caltech
Submillimeter Observatory, was
0.09, indicating very good
weather conditions during the experiment. Observations of Callisto
provided the absolute scale for the flux density calibration. Phase and
amplitude calibrators were the quasars 0423-013 and
3C 120, with measured flux densities of 2.30
0.06 and 0.50
0.03 Jy, respectively. The absolute flux density calibration
uncertainty is estimated to be 20%, based on
SMA monitoring of quasars. Further technical descriptions of
the SMA and its calibration schemes can be found in Ho et al. (2004).
The data were calibrated using the IDL superset MIR,
originally developed for the Owens Valley Radio Observatory (Scoville et al. 1993)
and adapted for the SMA.
The calibrated data were imaged and analyzed in the standard manner
using the MIRIAD and AIPS packages. We used the ROBUST parameter set
to 0 for an optimal compromise between sensitivity and angular
resolution. The line
image rms-noise was 20 mJy beam-1
for each channel at an angular resolution of
with a PA = -73
.
2.1.2 12CO(2-1) and SO(65-54) low resolution
Observations were made during December 2007 and February 2009.
The SMA was in its compact configuration, which includes
28 independent baselines ranging in projected length
from 8 to 57 m. The receivers were tuned to a
frequency of 230.534 GHz in the upper sideband (USB), while
the lower sideband (LSB) was centered at 220.534 GHz. The
observations were made in mosacing mode using the half-power point
spacing between field centers and thus covering a total area of about
2'
2' in Orion South that contains the outflow reported by Schmid-Burgk et al.
(1990).
The full bandwidth of the SMA correlator is 4 GHz. The SMA digital correlator was configured in 24 spectral windows of 104 MHz each, with 128 channels distributed over each spectral window, providing a resolution of 0.812 MHz (1.05 km s-1) per channel.
The SO(65-54) and 12CO(2-1) transitions were detected in the LSB and USB at a frequency of about 219.9 GHz and 230.5 GHz, respectively.
The zenith opacity (
),
was
0.1-0.3,
indicating reasonable weather conditions. Observations of Uranus
provided the absolute scale for the flux density calibration. Phase and
amplitude calibrators were the quasars 0530+135
and 0541-056.
The line image rms-noise was 150 mJy beam-1
for each channel at an angular resolution of
with a PA = -4.9
.
For the SO(65-54) we
tapered our angular resolution at
with a PA = -7.7
.
2.2 Single-dish observations
2.2.1 IRAM 30 m
Initial CO and 13CO(2-1) observations were
performed in February 1991 at the IRAM 30 m
telescope, covering a 70''
80'' area centered 10'' north of the presumed source
of the outflow. The beam size at 220 GHz was 13'',
the main beam efficiency 0.46. We used position switching with
the reference fixed at 30' west of the grid center and chose a
velocity resolution of 0.14 km s-1.
Observed points were 6'' apart, and the observation time per
point was 80 s.
These measurements were extended and complemented by 13CO(2-1)
OTF observations on February 2 and 4, 2008,
with a total observing time per night of 45 min. Now the center of the
100''
72'' grid was placed 36'' downstream from the
presumed source and the grid rotated to align with the outflow. We
co-added the OTF dumps every 6'' along scans
6'' apart. Velocity resolution was 0.05 km s-1.
Both frequency switching (throw 15 MHz) and position
switching were used.
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Figure 1: IRAM CO(2-1) integrated intensity map of the Ori-S6 collimated molecular outflow ( color image) overlaid with the SMA integrated intensity of the CO(2-1) ( black contours) and SO(65-54) ( red contours) images. For CO the contours are 4, 6, 8, 10, 12, 14, 16, 18, and 20 times 1.1 Jy beam-1 km s-1, the rms-noise of the image, while for SO the contours are 4-20 times 0.7 Jy beam-1 km s-1, the rms-noise of the image. The molecular emission in the SO map is integrated over velocities between +8 and +25 km s-1, for the SMA CO(2-1) map from +11 to +25 km s-1 and for the IRAM 30 m CO(2-1) map from +12 to +23 km s-1. The yellow diamond and triangle denote the position of the source FIR 4 (Mezger et al. 1990) and the millimeter source CS 3 (Mundy et al. 1986), respectively. The white stars mark the position of the compact millimeter sources reported by Zapata et al. (2005). Note the SO(65-54) maps did not cover the same large areas as the CO(2-1) maps from IRAM 30 m and SMA. The dashed white lines mark approximately the axis of the outflow before and after deflection. The white dot marks the point O. The pink line marks the position where the intensity cuts (shown in Fig. 8) were made. |
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Figure 2:
SMA SO(65-54) integrated
intensity map of the redshifted component of the Ori-S6 outflow.
The contours are -3, 3-20 times 0.7 Jy beam-1 km s-1,
the rms-noise of the image. The scale bar indicates the integrated
molecular emission in units of 1 |
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2.2.2 APEX
Some preliminary CO(6-5) and (7-6) measurements were performed in
October 2007 with the 12 m Atacama Pathfinder
Experiment (APEX) telescope located on Chajnantor in Chile. The same
lines were
simultaneously reobserved in depth on September 21
and 22, 2008 with the 2
7-element CHAMP+ array, covering
a 120''
120'' field around the origin of Ori-S6. Zenith opacity at
5100 m altitude was
0.35 at 690 GHz
and 0.42 at 806 GHz. At 690 GHz the
APEX beam is 9'', at 806 GHz 7.5''.
Spectral resolution was chosen to be 0.73 MHz, corresponding
to 0.32 resp. 0.27 km s-1 per
channel, in a way that the 2048 channels per band resulted in
a total velocity coverage of 650 resp.
550 km s-1 per band,
sufficient to detect the highest-velocity bullets possibly present in
this very active star-forming region Orion South.
The single-dish measurements were reduced with the standard
CLASS software of the Gildas package,
with only first-order baselines subtracted from the data.
3 Results
3.1 The jet
In Figs. 1, 2 and 12, we show the SO(65-54) redshifted emission of the innermost part of the Ori-S6 outflow as mapped with the SMA. The emission is here integrated over velocitiesm of between +8 and +25 km s-1. Our SO observations detected only the innermost part of the outflow because of the small primary beam size response of the SMA and because the beam was centered to the north of the millimeter source 139-409. In Fig. 2, we also mark the primary beam of the SMA. The molecular emission is well resolved and shows a collimated jet with clumpy morphology that is being ejected from that source. The resolved spatial size of each molecular gas bullet or clump is about 1000 to 2000 AU.
Position-velocity diagrams along directions perpendicular to
the SO jet (i.e. along PA 135)
are presented in Fig. 3
for four molecular gas bullets (A-D, see Fig. 2). The bullet
named E, seems not to be real because it falls far outside the
primary beam response of the SMA. Three of these bullets (B-D)
again show velocity jumps across the symmetry axis, in the same way as
those observed on large scales (IRAM 30 m, APEX, and
SMA) but with larger velocity excursions (7 to
11 km s-1) over scales of
1000 AU,
see Table 1,
where we give the parameters of the Gaussian least square fits to the
profiles. The kinematics of the molecular gas in the bullets seems
consistent with a rigid body law where the velocity is proportional to
the distance from the rotation axis. This feature seems least evident
in the innermost bullets, perhaps because of our poor spectral
resolution (4 km s-1) as well
as their very compact size of around one SMA beam (
1''). Note
that the molecular bullets appear to increase their radial velocities
with the distance from the ejecting object (Table 1). A similar
increase had already been discovered on the much larger 30 m
scale out to 70'' from the source in the CO flow
(Schmid-Burgk
et al. 1990).
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Figure 4:
SMA SO(65-54) low
resolution integrated intensity map of the Ori-S6 molecular
outflow ( red thin contours) overlaid with the SMA
high resolution integrated intensity of the CO(2-1) ( grey
scale) and SO(65-54)
( red thick contours) images. Note that the SO(65-54) map
represented in red thick contours has a better angular resolution and
sensitivity than the red thin contour map, and it did not
cover the entire outflow. The red thin contours start from 15%
to 90% with steps of 7% the flux peak of the image,
while the red thick contours start from 30% to 90% with steps
of 10% the flux peak of the image. The molecular emission in
the SO maps is integrated over velocities between +8 and
+25 km s-1, for the SMA
CO(2-1) map from +11 to +25 km s-1.
We tapered the SMA SO low resolution data (red thin contours)
to obtain a better sensitivity. The synthesized beam of the red thin
contour SO map is shown in the bottom left corner of the image
and has a size of |
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Figure 5:
SMA integrated intensity CO(2-1) contour map of the S6-Ori
outflow showing the rotating shell (blue and red contours) and the
inner high velocity jet (grey scale). The contours are from 15% to 90%
with steps of 10% of the peak of the line emission, the peak
being 11 Jy beam-1. The
redshifted emission ( red contours) is integrated
from 14 to 16 km s-1,
the blueshifted one ( blue contours)
from 18 to 20 km s-1,
and the grey scale from 20 to 30 km s-1.
Note that the blue- and red-shifted velocities refer to the outflow's
mean velocity at a given distance from the source. The dashed line
marks approximately the outflow's symmetry axis, and the continuous
black line traces the positions where the position-velocity diagram in
Fig. 6
was made. The synthesized beam is shown in the bottom left corner of
the image and has a size of |
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Figure 6:
Position-velocity diagram of the transversal cut (along PA 125 |
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Zapata et al.
(2007) reported the millimeter source 139-409 to be
a circumbinary molecular ring of a size of a few hundred astronomical
units (
18 AU) that is produced by two intermediate-mass stars with
very compact circumstellar disks of sizes and separations of less
than 50 AU. The circumbinary disk is seen almost
edge-on with
.
The redshifted molecular gas is located toward the west, the
blueshifted one toward the east. Note that the sense of rotation of the
molecular material in this circumbinary disk is opposite to that found
in the jet.
3.2 The shell
Figure 1
shows a map of the high velocity CO(2-1) emission from the
Ori-S6 redshifted lobe intensity integrated from +12 to
+23 km s-1,
as obtained at the IRAM 30 m telescope (Schmid-Burgk et al.
1990), overlaid with both our SMA total integrated
intensity SO(65-54) and
CO(2-1) maps. In Fig. 13 a channel map of
the SMA CO(2-1) emission is also shown. In Fig. 1. one can see the
SO emission to trace the inner, very collimated jet ejected
along a PA of 45
from the source 139-409 that is located in a small cluster of
young stars associated with the extended and bright
source FIR4 (Zapata et al. 2006,2007).
The CO emission on the other hand delineates the more extended
parts of the outflow, with its major axis bending towards a PA
of some 30
within a small distance from the source. The small difference between
both position angles can be explained by the outflow undergoing a
deflection, possibly due to the high density (
106 cm3)
molecular cloud located behind the Orion Nebula as had already been
suggested for other outflows populating this region (Henney
et al. 2007; Zapata et al. 2006).
A process for the deflection of an outflow has been proposed
and modeled by Cantó & Raga (1996). In Fig. 4, we show the
continuation of the SO outflow through the the same positions
of the CO(2-1) shell, confirming that both sections are part
of the same molecular outflow.
Figure 5
presents an overlay of two CO(2-1) emission intervals of the
south-western lobe of the Ori-S6 outflow made with the SMA,
one integrated over the velocities from 14 to
16 km s-1 (blue), the other
from 18 to 20 km s-1
(red). It clearly shows a velocity ``jump'' across the
outflow, with the redshifted gas velocities located toward the
south-east, the blueshifted ones toward the north-west. Note that the
blue- and red-shifted velocities here are given with respect to the
outflow's mean velocity at a given distance from the center. With a
distance of 415 pc to the Orion Nebula (Menten et al. 2007),
the separation between these two components is about 2 103 AU.
Table 1: Parameters of the SO(65-54) line from the molecular gas bullets.
The position-velocity diagram of Fig. 6, taken along the
black line across the outflow in Fig. 5, displays the
decrease of radial velocity in the rotating shell with distance from
the flow axis. The largest radial velocity (22 km s-1)
at this distance from the source is found right on the outflow axis,
from which the velocity decreases outwards at different rates on either
side. At distances of 3'' it has diminished by almost
3 km s-1 (i.e. the
shell sector rotating away from the observer) resp.
7 km s-1 (rotating towards the
observer). We interpret this difference as evidence for a superposition
of rotation about the axis onto the outflow's general radial velocity
field. The large optical depths of the tangential line of sight through
the narrow shell plus the narrow width of the rotation shell itself
cause the interferometer to only see the tangential shell zones while
largely overlooking the weaker, extended material closer to the axis.
What is visible of this material, i.e. the bridge between the
22 km s-1 on-axis component
and the 19 km s-1 shell signal
(the one rotating away) has no correspondence on the other
side of the axis because the emission is spread out there over the much
larger velocity interval of 7 km s-1
instead of 3 km s-1. Note that
the shell signals both seem to have a long structure; however, their
major axes do not show the same magnitude of inclination towards the
outflow axis (the short white lines mark angles of
60
to the vertical of the diagram). In fact the redshifted
component appears to drop less steeply than its blueshifted
counterpart. If the shell's rotation were radius-independent,
this magnitude should be the same for both. If on the other
hand the rotation speed increased outward, the ``blueshifted'' outer
edge would have a still lower radial velocity, the
``redshifted''one a higher one than in the radius-independent case,
thus explaining this apparent asymmetry. With the aid of such
asymmetries one might in principle be able to estimate the forces
(magnetic?) that determine the rotation dynamics of the
outflow shell.
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Figure 7:
CO channel maps (channel width equal to 1.5 km s-1)
centered on velocities 12.0 km s-1
( top, CO(7-6)) resp. 4.25 km s-1
( bottom, CO(6-5)). The higher-velocity components
of the outflow (here: integrated from 14.5 to
15.1 km s-1) are indicated by
the white lines. Note that in the top panel a
contribution from the outflow proper has been cursorily removed
(see text). We estimated this outflow contribution by
extrapolating from higher velocities down to the relevant velocity
value.
This can be done because the outflow's Gaussian cross section and
amplitude at any position from the origin vary only slowly with
velocity. The coordinate system here is rotated about the
point O by -28 |
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Figure 8:
Variation of line intensities (IRAM 30 m) across the
outflow at a downstream distance of 45'' from point O
(see Fig. 1):
the black line is 13CO(2-1) at velocities
around 7.7 km s-1
(i.e. ambient), shifted upwards by 8 K. The
dotted black line is also 13CO(2-1) at
velocities around 15 km s-1.
The blue and red lines show the spatial separation between redshifted
(11.6 km s-1) and blueshifted
(4.6 km s-1) components of the
Ori-S6 outflow from CO(2-1) (strong lines) and 13CO(2-1) |
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3.3 The envelope
In Fig. 7 we show the large envelope surrounding the jet as mapped by the APEX telescope using the CO(7-6) and CO(6-5) lines.
The single-dish observations of CO(2-1), (6-5), and
(7-6) complement the SMA picture. Of course
at the velocities of the ambient gas these lines are optically very
thick; it therefore takes the rare isotopomeres to discern any
near-ambient-velocity structure close to the outflow axis. 13CO(2-1)
shows a clear and simple picture: over a distance of at least
some 50'' along the outflow two ranges of increased intensity
run parallel to the outflow axis, one on either side, of constant and
equal distance (15'')
from the axis, and of equal intensity if observed around
=
7.7 km s-1. At lower
velocities the ridge to the north-west begins to dominate, at higher
values the south-eastern one. We thus suspect the ambient
LSR velocity in the immediate vicinity of the outflow to be
around 7.7 km s-1, with the
two ridges marking the edges of a tubular wall that surrounds the flow
axis. The depression between the ridges seems relatively stronger for
the rarer isotopomere C18O, maybe
indicating preferential destruction due to less self-shielding against
the UV generated in the jet's shock. A typical 13CO intensity
cross section through the outflow is shown in Fig. 8 for
=
7.7 km s-1, taken at a
downstream distance of 45'' from the bending point (marked by
the white dot in Fig. 1).
This point serve as the zero-point is from now on for all single-dish
distance determinations along the outflow, because the outflow follows
a straight line from here. Its coordinates are RA = 05
35
13.04
,
dec = -05
24'20.0'' (J2000.0).
At velocities a few km s-1 away from
near-ambient values a structural asymmetry appears in the emission of
both CO(2-1) and 13CO(2-1), see Fig. 8. Some 3 to
4 km s-1 below
the 7.7 km s-1 value the
intensity of both species is strongest along a strip situated between
the ambient tubular wall and the outflow axis (as defined by
the high-velocity cross-section profile) on the north-western
side of the axis, at
some 3 to 4 km s-1 above
this value the emission peaks along a corresponding strip on the south-eastern
side. These two zones are spatially separated by
10'' to 15'' and by
7 km s-1
in radial velocity, and this asymmetry extends downstream to projected
distances of at least 2.5
104 AU from the source. The sense of
the asymmetry corresponds to our SMA results, but the
velocities in question have a different context: the
SMA variations are a superposition of an asymmetry of a few
km s-1 onto the high-velocity
components of the outflow, the single-dish data concern motions outside
the core flow zone.
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Figure 9:
Position-velocity diagrams (CO(6-5) and CO(7-6)) of the perpendicular
crosscut through the outflow at a downstream distance 37''
from point O (see Fig. 8). The outflow axis
is marked by the horizontal white line, the regions discussed in the
text for velocity asymmetry is indicated by the circles. Colour scale
is
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Figure 10: Sections of the position-velocity diagrams (CO(7-6)) of perpendicular crosscuts through the outflow at different distances (given by the numbers on the panels, arcsec) from point O. The outflow axis is marked by the horizontal grey line. Top: blueshifted velocities between 4 and 5 km s-1, bottom: redshifted ones from 11.5 to 12.5 km s-1. Note that the structures near the outflow axis persist for at least six beams. |
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![]() |
Figure 11:
Red-blue asymmetry of CO(6-5) emission across the outflow at increasing
distances from point O. Grey: Cross section at
15 km s-1 (core of the
outflow), red:
at 12.7 km s-1, blue:
at 4.5 km s-1, each
integrated over a velocity interval of a width of
0.5 km s-1. The |
Open with DEXTER |
Typical p-v diagrams taken
perpendicularly across the outflow at a distance of 37'' from
point O, as obtained with the somewhat higher
resolutions of the APEX measurements compared with those of
IRAM CO(2-1), are shown in Fig. 9.
No important long-range gradient of the ambient velocities is
evident, and there appears in particular no obvious tilt across the
outflow axis (the horizontal white line). However, in both
diagrams one notes conspicuous excursions of the iso-intensity lines,
somewhat anti-symmetrically offset from the outflow's axis,
at velocities slightly beyond the optically thick values,
i.e. below 5 resp. above 11 km s-1
(big white circles). These might seem spurious; but when
observing at different downstream
positions one notes their appearance in each p-v crosscut
between 20'' and 60'' from point O
(see Fig. 10),
contrary to some other local excursions also visible in Fig. 9, which do not
persist across this range. Figure 10 does show
noticeable differences between the high- and the low-velocity diagrams.
But complete antisymmetry between the encircled regions of
Fig. 9
cannot be expected because the outflow proper will have different
effects on the two velocity windows, contaminating the redshifted
rotation component. This component seems to extend further from the
axis, and it may consist of two somewhat distinct parts, one
at the axial distance of the blueshifted component
(about 5''), the other one father away
(see panel 37 of Fig. 10). Higher
resolution studies and better knowledge of the central outflow's
contribution are needed to decide this issue.
In the two transition regions between the typical excursion
and the ambient velocities, i.e. at around 6 resp.
10.5 km s-1, there are hints
in Fig. 11
(as well as at other downstream distances) of emission deficits
at about the same offsets from the axis as those where the excursion
emission peaks (Iso-intensity lines change from convex to concave along
the
axis). This seems to indicate in situ acceleration in opposite
directions on either side of the outflow, rather than a mere ambient
velocity gradient to be at the root of the observed velocity
antisymmetry.
![]() |
Figure 12: SMA channel maps of the CO(2-1) line emission from the outflow Ori-S6. The emission is summed in velocity bins of 1 km s-1. The central velocity is indicated in the upper right-hand corner of each panel (the systemic velocity of the ambient molecular cloud is about 6-7 km s-1). |
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![]() |
Figure 13: SMA channel maps of the SO(65-54) line emission from the outflow Ori-S6. The emission is summed in velocity bins of 4 km s-1. The central velocity is indicated in the upper right-hand corner of each panel (the systemic velocity of the ambient molecular cloud is about 6-7 km s-1). |
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Spatial intensity profiles across the flow (Fig. 11), taken for the
two velocity excursions in question at various downstream distances
from point O, clearly show a persistent ``red-blue'' asymmetry
about the axis that is defined by the high-velocity crosscut here
depicted in grey. Mapping the two ``excursion channels''
of 3.5 to 5.0 km s-1
resp. 11.25 to 12.75 km s-1
results in the spatial distributions of Fig. 7 which represent the
considerable velocity difference of some 6 to
8 km s-1 over a distance of
around 10-15'' or 4-6
103 AU. Of course this
variation does not signal an actual velocity difference
of 6 km s-1 or so
between the two sides, the bulk of the line profiles being hidden by a
large optical depth in a way that only their extreme wings can be seen.
The fact that either side predominantly shows just one of the two
wings, the ``red'' or the ``blue'' one, indicates however that the true
radial velocity shift across the flow must be roughly on the order of
at least 0.5 km s-1,
corresponding to 20 km s-1 per pc;
were it much smaller, then one would measure about the same wing
emission on either side. This in turn indicates that the rotation
velocity between the inner SMA region and the annular zone of
axial distance R = 10''
to 15'' does not decrease
much faster than 1/R.
Note that in order to somewhat correct Fig. 7 for the contribution from the outflow proper to the redshifted intensities we have rather cursorily subtracted part of the outflow's higher velocities (between 14 and 18 km s-1) from the data.
4 Discussion
Three independent observations all show velocity asymmetries about the outflow axis which suggest rotation on different length scales. Seen along the redshifted lobe down from the origin this rotation would be clockwise for the SO clumps, the CO jet shell, and the ambient envelope alike.
Although this congruence of velocity shifts on three different
length scales lends some credence to a rotation model for Ori-S6, we
have to discuss alternative explanations for the observed red-blue
asymmetries. The most obvious alternative would be a general
large-scale velocity gradient in the region. Indeed, as the p-v diagram
of Fig. 9
shows, there is some overall velocity
change in the brightest component, from higher to lower velocities
along a direction roughly SE to NW. Between the edges of this
diagram, i.e. over a distance of 80'', the
peak
is seen to change by about 1 km s-1.
Were this a smooth gradient, it would amount to
5 km s-1 per pc,
a value high but not uncommon for molecular cloud clumps. Over
the 5'' resp. 10'' distances between the two sides of
the CO jet shell (Fig. 5) resp. the
``ambient'' CO tube (Fig. 11) this would
however amount to velocity asymmetries below 0.1 km s-1,
much less than what is observed. In fact, over these distances
the brightest component does not seem to vary at all in velocity;
only beyond, and then only in the SE part of the
diagram over the very limited extent of some 10'',
a much larger gradient appears, on the order of
60 km s-1 per parsec.
This huge gradient connects suspiciously to the velocity dominant still
further away from the flow. One could postulate such a large,
spatially very limited velocity gradient that by chance coincides with
the outflow positions. But that gradient would have to be closely
matched with the flow for these asymmetries to be evident over a length
of scales on the order of 60''. This seems unlikely. Also, the
convex-concave transitions mentioned above appear to favor
in situ acceleration over any effects of an ambient velocity
gradient. We thus discard the notion of the asymmetries being caused by
such a gradient in the ambient gas.
Since the outflow originates in a binary system (Zapata et al. 2007) a recent calculation (Murphy et al. 2008) of two nearly parallel jets of unequal speed stemming from such a system may be of relevance for Ori-S6. In that model the two jets eventually merge, as witnessed by a persistent kink in the final structure. Furthermore, due to binarity precession begins to show after some time in the form of a bending jet trajectory. Although neither of these effects can as yet be definitely excluded for our inner jet, SMA data of the large-distance CO structure clearly speaks against any sizeable precession or kink. Nor would it seem likely that the two flow velocities should not over gradually adjust to each other their large common path and thus wipe out any initial differences in speed. We therefore look for other explanations for the observed velocity jumps across the flow axis.
Soker (2005) alternative proposal that left-right asymmetries in jets could result from (non-magnetic) interaction at the base between jet and a warped disk, would on the other hand at the very least in our case require a warp spatially static over the long times that it takes to build up the outflow out to 100'' from the source: 0.2 pc/15 km s-1 is of order 13 000 years, so even accounting for possibly reducing projection factors a very large period of standstill would be demanded of the close binary system at the source of Ori-S6. Hence we also discard Soker's mechanism and relate the observed jumps to rotation instead.
Where would this rotation originate? The CO jet shown in
Fig. 5
is considered to be ejected from a rotating protostellar disk and then
accelerated and collimated by MHD forces. Anderson et al. (2003)
provide a formula that allows the relation of jet properties measured
at large distances from the disk to the position (the ``footpoint'') on
the disk from where the observed jet section first emerges:
Here





















A similar estimate for the CO envelope of nearly ambient velocity,
shown in cross-section in Fig. 7, must await more
detailed investigations since its apparent value
is (very) small and not determined; likewise
can be estimated only very roughly, as explained
above - the asymmetric wings say little quantitatively about
the bulk toroidal motions of the envelope. A trial with
5'',
0.4 km s-1, and
1 km s-1 would result in
footpoint radii about a factor of ten larger than the CO jets
shell's, suggesting an origin quite different from that of the
jet's disk.
Anderson
et al. (2003) also provide an expression for the
ratio between toroidal and poloidal components of the magnetic field
strength at the observed jet positions (see their
Eq. (2)). Using the same parameter values as employed for
estimating the jet shell's footpoint radius and setting
to 30
we arrive at a field strength ratio (toroidal over poloidal) of around
six. The magnetic field in the shell thus seems to be tightly
wound up, thereby keeping the shell material well collimated
by its hoop stresses.
Where in the system 139-409 the jet actually originates has
yet to be determined. The sense of rotation of the circumbinary ring is
nearly opposite to that of the jet and the outflow, and the jet leaves
the system under an angle of 45
with the ring plane. One should therefore
expect the origin of the flow to be at the circumstellar disk of one of
the binary components. The two disks need not align with the ring.
Further deep observations of highest resolution will be required to
clarify the jet-disk connection.
5 Summary
The Ori-S6 outflow presents a promising laboratory for future studies of magneto-centrifugal models of jet acceleration. We have observed the compact jet and its larger-scale molecular envelope on three different spatial scales and found the following:
- the SO bullets in the jet, its CO shell and also the more distant CO envelope all show rotation about the outflow axis; the sense of rotation is the same for all;
- the inner jet is composed of individual bullets that appear
to follow a solid body rotation law with peak
values around 5 km s-1. The CO shell, at a distance of about
1000 AU from the axis, rotates at
2 km s-1; the CO envelope, at a distance of
2000 AU from the axis, i.e. several times the bullets' radii, rotates at
0.5 km s-1;
- the rotation is observed out to at least 25 000 AU downstream from the source;
- the magnetic field lines embedded in the jet's
CO shell can well thread a protostellar disk of radius
50 AU. For the wider CO envelope the footpoint in the equatorial plane of the disk must lie considerably further out;
- the exact identification of the outflow source remains open since at the obvious position there is a circumbinary ring containing two intermediate mass stars. Orientation and sense of rotation of the ring do not coincide with those of the outflow.
We would like to thank Dr. Bernd Klein for having taken the recent CO data on the 30 m telescope, and Dr. Rainer Mauersberger for reactivating the IRAM data of 1989. Facilities: IRAM 30 m APEX and SMA.
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Footnotes
- ... (SMA)
- The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics, and is funded by the Smithsonian Institution and the Academia Sinica.
- ... SMA
- The MIR cookbook by C. Qi can be found at http://cfa-www.harvard.edu/ cqi/mircook.html
- ... package
- http://www.iram.fr/IRAMFR/GILDAS
All Tables
Table 1: Parameters of the SO(65-54) line from the molecular gas bullets.
All Figures
![]() |
Figure 1: IRAM CO(2-1) integrated intensity map of the Ori-S6 collimated molecular outflow ( color image) overlaid with the SMA integrated intensity of the CO(2-1) ( black contours) and SO(65-54) ( red contours) images. For CO the contours are 4, 6, 8, 10, 12, 14, 16, 18, and 20 times 1.1 Jy beam-1 km s-1, the rms-noise of the image, while for SO the contours are 4-20 times 0.7 Jy beam-1 km s-1, the rms-noise of the image. The molecular emission in the SO map is integrated over velocities between +8 and +25 km s-1, for the SMA CO(2-1) map from +11 to +25 km s-1 and for the IRAM 30 m CO(2-1) map from +12 to +23 km s-1. The yellow diamond and triangle denote the position of the source FIR 4 (Mezger et al. 1990) and the millimeter source CS 3 (Mundy et al. 1986), respectively. The white stars mark the position of the compact millimeter sources reported by Zapata et al. (2005). Note the SO(65-54) maps did not cover the same large areas as the CO(2-1) maps from IRAM 30 m and SMA. The dashed white lines mark approximately the axis of the outflow before and after deflection. The white dot marks the point O. The pink line marks the position where the intensity cuts (shown in Fig. 8) were made. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
SMA SO(65-54) integrated
intensity map of the redshifted component of the Ori-S6 outflow.
The contours are -3, 3-20 times 0.7 Jy beam-1 km s-1,
the rms-noise of the image. The scale bar indicates the integrated
molecular emission in units of 1 |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
SMA SO(65-54) low
resolution integrated intensity map of the Ori-S6 molecular
outflow ( red thin contours) overlaid with the SMA
high resolution integrated intensity of the CO(2-1) ( grey
scale) and SO(65-54)
( red thick contours) images. Note that the SO(65-54) map
represented in red thick contours has a better angular resolution and
sensitivity than the red thin contour map, and it did not
cover the entire outflow. The red thin contours start from 15%
to 90% with steps of 7% the flux peak of the image,
while the red thick contours start from 30% to 90% with steps
of 10% the flux peak of the image. The molecular emission in
the SO maps is integrated over velocities between +8 and
+25 km s-1, for the SMA
CO(2-1) map from +11 to +25 km s-1.
We tapered the SMA SO low resolution data (red thin contours)
to obtain a better sensitivity. The synthesized beam of the red thin
contour SO map is shown in the bottom left corner of the image
and has a size of |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
SMA integrated intensity CO(2-1) contour map of the S6-Ori
outflow showing the rotating shell (blue and red contours) and the
inner high velocity jet (grey scale). The contours are from 15% to 90%
with steps of 10% of the peak of the line emission, the peak
being 11 Jy beam-1. The
redshifted emission ( red contours) is integrated
from 14 to 16 km s-1,
the blueshifted one ( blue contours)
from 18 to 20 km s-1,
and the grey scale from 20 to 30 km s-1.
Note that the blue- and red-shifted velocities refer to the outflow's
mean velocity at a given distance from the source. The dashed line
marks approximately the outflow's symmetry axis, and the continuous
black line traces the positions where the position-velocity diagram in
Fig. 6
was made. The synthesized beam is shown in the bottom left corner of
the image and has a size of |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Position-velocity diagram of the transversal cut (along PA 125 |
Open with DEXTER | |
In the text |
![]() |
Figure 7:
CO channel maps (channel width equal to 1.5 km s-1)
centered on velocities 12.0 km s-1
( top, CO(7-6)) resp. 4.25 km s-1
( bottom, CO(6-5)). The higher-velocity components
of the outflow (here: integrated from 14.5 to
15.1 km s-1) are indicated by
the white lines. Note that in the top panel a
contribution from the outflow proper has been cursorily removed
(see text). We estimated this outflow contribution by
extrapolating from higher velocities down to the relevant velocity
value.
This can be done because the outflow's Gaussian cross section and
amplitude at any position from the origin vary only slowly with
velocity. The coordinate system here is rotated about the
point O by -28 |
Open with DEXTER | |
In the text |
![]() |
Figure 8:
Variation of line intensities (IRAM 30 m) across the
outflow at a downstream distance of 45'' from point O
(see Fig. 1):
the black line is 13CO(2-1) at velocities
around 7.7 km s-1
(i.e. ambient), shifted upwards by 8 K. The
dotted black line is also 13CO(2-1) at
velocities around 15 km s-1.
The blue and red lines show the spatial separation between redshifted
(11.6 km s-1) and blueshifted
(4.6 km s-1) components of the
Ori-S6 outflow from CO(2-1) (strong lines) and 13CO(2-1) |
Open with DEXTER | |
In the text |
![]() |
Figure 9:
Position-velocity diagrams (CO(6-5) and CO(7-6)) of the perpendicular
crosscut through the outflow at a downstream distance 37''
from point O (see Fig. 8). The outflow axis
is marked by the horizontal white line, the regions discussed in the
text for velocity asymmetry is indicated by the circles. Colour scale
is
|
Open with DEXTER | |
In the text |
![]() |
Figure 10: Sections of the position-velocity diagrams (CO(7-6)) of perpendicular crosscuts through the outflow at different distances (given by the numbers on the panels, arcsec) from point O. The outflow axis is marked by the horizontal grey line. Top: blueshifted velocities between 4 and 5 km s-1, bottom: redshifted ones from 11.5 to 12.5 km s-1. Note that the structures near the outflow axis persist for at least six beams. |
Open with DEXTER | |
In the text |
![]() |
Figure 11:
Red-blue asymmetry of CO(6-5) emission across the outflow at increasing
distances from point O. Grey: Cross section at
15 km s-1 (core of the
outflow), red:
at 12.7 km s-1, blue:
at 4.5 km s-1, each
integrated over a velocity interval of a width of
0.5 km s-1. The |
Open with DEXTER | |
In the text |
![]() |
Figure 12: SMA channel maps of the CO(2-1) line emission from the outflow Ori-S6. The emission is summed in velocity bins of 1 km s-1. The central velocity is indicated in the upper right-hand corner of each panel (the systemic velocity of the ambient molecular cloud is about 6-7 km s-1). |
Open with DEXTER | |
In the text |
![]() |
Figure 13: SMA channel maps of the SO(65-54) line emission from the outflow Ori-S6. The emission is summed in velocity bins of 4 km s-1. The central velocity is indicated in the upper right-hand corner of each panel (the systemic velocity of the ambient molecular cloud is about 6-7 km s-1). |
Open with DEXTER | |
In the text |
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