Issue |
A&A
Volume 508, Number 2, December III 2009
|
|
---|---|---|
Page(s) | 773 - 778 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/200911809 | |
Published online | 27 October 2009 |
A&A 508, 773-778 (2009)
Herbig-Haro flows in 3D: the HH 83 jet![[*]](/icons/foot_motif.png)
T. A. Movsessian1 - T. Yu. Magakian1 - A. V. Moiseev2 - M. D. Smith3
1 - Byurakan Astrophysical Observatory, 378433 Aragatsotn reg., Armenia
2 -
Special Astrophysical Observatory, N.Arkhyz, Karachaevo-Cherkesia 369167, Russia
3 -
Centre for Astrophysics & Planetary Science, University of Kent, Canterbury CT2 7NH, UK
Received 8 February 2009 / Accepted 28 September 2009
Abstract
Aims. The kinematics of the HH 83 optical outflow, located in the L 1641 molecular cloud, are investigated.
Methods. Observations were carried out with the Fabry-Perot
scanning interferometer on the 6-m telescope of the Special
Astrophysical Observatory. The H
emission line was scanned with a spectral resolution of R = 8200.
Results. The radial velocity along the jet increases with
distance from the source, confirming previous results. It also shows
lower amplitude variations which are not correlated with intensity.
Both the spatial width of the jet as well as the FWHM of the H
emission line in the jet tend to decrease with distance from the
source. The velocity field across the jet demonstrates a decrease from
the center to the edges as well as some evidence for a transverse
velocity gradient. The blue-shifted bow shock is separated spatially
and spectrally into two distinct features, divided by about 2
and 250 km s-1, accordingly.
Conclusions. Evidence is provided that these split features
correspond to forward and reverse shocks caused by a rapid pressure
increase as the jet begins a new oblique impact on the surrounding
medium. Radial velocity variations lengthwise and transverse to the jet
axis are discussed. Linear extrapolation of the jet velocity up to the
location of the terminal shock region yields the radial velocity of the
reverse jet shock. The data are consistent with an abrupt outburst
about one thousand years ago which ejected material with total speeds
of up to 400 km s-1.
Key words: stars: formation - ISM: jets and outflows - ISM: clouds
1 Introduction
The Herbig-Haro object HH 83 was first noted as an unusual nebulous object with a cross-like knotty structure
on a deep UK Schmidt red plate (Reipurth 1985). Further investigation revealed that HH 83 consists
of a small reflection nebula associated with a well-collimated jet of 32
length (Reipurth 1989),
which at 450 pc distant (the usually adopted distance to the L 1641 cloud) is equal to 0.07 pc.
Infrared observations in the field revealed a source located to the south-east of HH 83, well aligned with
the jet axis (Moneti & Reipurth 1995; Reipurth 1989).
This optically invisible source coincides with IRAS 05311-0631 with a luminosity of 10.5
(Reipurth 1989).
It was detected in the radio at the VLA by Rodriguez & Reipurth (1998) and is also associated with a weakly
collimated molecular flow observed in CO (Bally et al. 1994).
An optical polarization map of the reflection nebula confirmed that it is illuminated by the infrared
source (Rolph et al. 1990).
At a separation of 105
from the base of the jet, a bow shock structure was found on H
images
but was not visible on [SII] images (Reipurth 1989). In addition, HH 83 possibly forms a parsec-scale outflow system with HH 84
which traces a projected distance of 1.46 pc
(Reipurth et al. 1997).
A spectral investigation of the jet revealed a pronounced velocity increase along its axis. In addition, the density and the excitation in the jet decrease with distance (Reipurth 1989).
It is interesting that the HH 83 jet showed no signs of H2 or [FeII] emission during HST/NICMOS near-infrared imaging although narrow band filters were not utilised. However, faint line emission was detected by Podio et al. (2006). The HST data revealed the complex reflection nebula in the vicinity of the bright source star which represents the edges of a conical cavity excavated by the flow. Its features are smoothly curved with very sharp edges (Reipurth et al. 2000).
We present in this paper scanning Fabry-Perot (FP) interferometric observations of the HH 83 system
in H
emission. This is a powerful method for obtaining kinematic information
of extended emission nebulae such as HH objects and jets. The main goal
of this investigation is to obtain full three dimensional (two spatial
and one spectral) characteristics of the complete system. The
observational method is described in Sect. 2.
In Sect. 3, we analyse the radial velocity and spatial
distributions in the jet and bow shock. A full discussion of the
implications
is provided in Sect. 4.
![]() |
Figure 1:
Channel maps of the HH 83 region obtained by FP scanning of the H |
Open with DEXTER |
2 Observations and data reduction
Observations were carried out at the prime focus of the 6 m
telescope of the Special Astrophysical Observatory (SAO, Russia)
under good conditions (the seeing was 1
). We used a scanning Fabry-Perot interferometer (IFP) placed in
the parallel beam of the SCORPIO focal reducer (Afanasiev & Moiseev 2005). The SCORPIO capabilities for IFP observations were
also described by Moiseev (2002). The detector was a TK1024
pixel CCD array. Observations were
performed with
pixel binning to reduce the readout time, so 512
512 pixel images were
obtained for each spectral channel. The field of view was 4.8
4.8
with a scale of 0.56
per pixel.
An interference filter with
Å centered on the H
line was used for pre-monochromatization.
For our observations we used a Queensgate ET-50 interferometer operating in the 50th order of interference at
the H
wavelength, and providing a spectral resolution of
Å (or
40 km s-1)
for a range of
Å (or
590 km s-1), free from order overlapping. The number of
spectral channels was 36 and the size of a single channel was
Å (
16 km s-1).
We reduced our interferometric observations using software developed at the SAO (Moiseev & Egorov 2008). After the primary
data reduction, subtraction of night sky lines and wavelength calibration, the observational material represents
``data cubes'' where each point in the 512 512
pixel field contains a 36 channel spectrum. We removed the ghost images
that appear in the plates of the
interferometer using the algorithms
described by Moiseev & Egorov (2008). Optimal data filtering was
performed by applying Gaussian smoothing over the spectral coordinate
with
channels and spatial smoothing by a two-dimensional Gaussian with
pixels, by using
the ADHOC software package
.
3 Results
The field covered by our FP images includes the entire HH 83 system: the jet, reflection nebula (actually the light reflected from the cavity walls) as well as the terminal shock region.
In Fig. 1, velocity channel maps are presented (to increase the S/N ratio, each image was obtained by binning over two spectral channels). This illustrates that the object morphology depends strongly on the radial velocity (here and elsewhere in the paper the heliocentric velocities are used; to obtain the velocity with respect to the source one should subtract 21 km s-1).
The jet itself does not show prominent variations in its structure with the exception of the change of radial velocity along the axis, i.e. no new details appear or disappear according to the velocity.
The reflection nebula, as can also be seen from Fig. 1, is visible in all channels but
becomes slightly brighter at low velocities. This is probably a result of the presence of the reflected
H
emission from the invisible stellar source.
The most interesting is the terminal shock region which is split into two discrete structures - a narrow and more elongated bow outside of a clumpy and thick arc with a velocity separation between them of about 250 km s-1.
Below we discuss these parts of HH 83 separately.
3.1 Velocity variations along the HH 83 jet
The position-velocity diagram along the flow axis with the slit width of 5 pixels (corresponding to 2.8
)
is presented in Fig. 2.
It was created through the specific property of 3D data cubes which
allows the construction of pseudo-slits with any selected direction and
width. In the present case, the diagram was obtained by integrating the
H
intensity in each spectral channel across the slit width.
The slit was made wide and oriented as shown in Fig. 2 to include all knots.
To measure the distances of the knots from the source we used precise coordinates from Moneti & Reipurth (1995) and Rodriguez & Reipurth (1998); previous source positions given by Reipurth (1989) and Mundt et al. (1991) contain errors.
![]() |
Figure 2:
Left panel: the position-velocity diagram for the HH 83 system. Right panel: the monochromatic image of the
system in H |
Open with DEXTER |
Our data provide detailed information about the velocity field in the entire HH 83 outflow system. In Fig. 3, the dependence of the heliocentric radial velocity along the HH 83 jet on the distance from the infrared source HH 83 IRS is presented. For better identification of knots, the intensity distribution along the jet is superimposed and knots D, F and G are labelled.
Both Figs. 2 and 3 display the obvious velocity increase along the HH 83 jet, first found and discussed by Reipurth (1989), and clearly show the split of the terminal shock region into two separate velocity structures. The velocity field in the jet part of the outflow is in agreement with the long-slit data of Reipurth (1989).
In Fig. 3, the straight line connects the velocity trend in the jet with the high velocity filament in the terminal shock region. One can note that such a linear extrapolation of the velocity up to the terminal shock region indeed is the good description of the velocity field in this structure. We will discuss this region in more detail in Sect. 3.3.
Besides the general increase of the radial velocity with distance from the source, we note the smaller amplitude variations along the jet axis. As can be seen from Fig. 3, there is no significant correlation between the velocity and the emission intensity in the jet. Since the line emission depends on the shock properties and not on the velocity of the gas in the shock, this is not surprising.
On the other hand, these velocity variations in the jet are very
important since they are related to the inner shocks in the jet. To
estimate the shock velocity, we followed the approach of Hartigan et al. (2001). Correcting for the inclination angle of 45
(Reipurth 1989),
we obtain the true space velocities of the emitting material in the jet
(assuming, of course, that the gas really is moving along the jet
axis). Taking into account also the general trend of the velocity
increase, we get the entire range of the space velocity variations in
the
jet as
115 km s-1.
We note that this is a somewhat higher value than 100 km s-1, obtained for HH 111 by Hartigan et al. (2001). This conforms with the higher level of ionization of HH 83 (Podio et al. 2006). In addition, from Fig. 3
it is evident that the largest amplitude velocity variations are
observed in the brightest part of the jet, i.e. near knots C-D-F-G, and
become lower further out. This again can be compared with
the prominent decrease of
and
between knots C and F, as shown by Podio et al. (2006).
![]() |
Figure 3:
The heliocentric radial velocity distribution (filled circles) along
the HH 83 system versus the angular distance from the source. The
working surface is separated into Mach
disk and bow-shock structures. The positions of knots D, F, and G are
identified by arrows on the superposed plot of the intensity of H |
Open with DEXTER |
![]() |
Figure 4: The spatial FWHM of the HH 83 outflow (without deconvolution) plotted against the distance from the source. The general trend is shown by the dashed line. |
Open with DEXTER |
![]() |
Figure 5:
The positions of pseudo-slits traced across knots A-B, D, F and G of the HH 83 jet ( left panel). The size of each one of pseudo-slits is
|
Open with DEXTER |
3.2 The HH 83 jet in the transverse direction
Our data show that the jet is broader than the seeing FWHM.
This means that it is spatially resolved. The good spatial sampling
allows us to accurately fit the spatial profiles along the restored
image of the jet in H,
measuring in this way the HH 83 jet width. The results, displayed in Fig. 4, can be compared with the measurements of Mundt et al. (1991) which were done
for the [SII] line. Although one can come to the conclusion that in H
the jet is slightly wider than in [SII], it is of little significance
because our data were not deconvolved. However, the definite narrowing
of the jet with distance from the source can be noted, which is not the
case for the [SII] emission.
To measure the radial velocity changes across the
HH 83 jet, we traced several pseudo-slits, crossing the jet
in knots C, D, F and G. Their width was set to five pixels (or 2.8
)
to increase the signal-to-noise ratio. Radial velocities were measured
by Gaussian fitting of each profile along the pseudo-slit. As expected,
the velocity fields obtained show a peak of the negative radial
velocity near the jet axis and a decline moving to both sides (see,
e.g., Movsessian et al. (2007) for another observational example and Beck et al. (2007) for a theoretical discussion).
As can be seen from Fig. 5,
where we show the positions of pseudo-slits and the velocity
diagrams, in three cases the minimum of the negative velocity is
shifted in the north-east direction from the jet axis (actually, since
the axis of the not-straight jet is difficult to define, we used for
each knot the local maximum of intensity as the zero-point): in knot D
by 0.44
,
F by 0.34
and G by 0.52
.
In knot C the situation is reversed: the shift is 0.42
in the opposite direction.
Somewhat similar shifts have been found in the work of Coffey et al. (2007). However, we are not sure about the rotational interpretation of the shift in the case of HH 83. We will discuss this velocity trend in more detail in Sect. 4.
3.3 The terminal shock region
![]() |
Figure 6: The working surface of
the HH 83 outflow, split into high-velocity
(grey scale) and low-velocity (contoured) structures. These features
are obtained by integrating the intensities in Gaussian-fitted
components in the range of |
Open with DEXTER |
As mentioned above, our data show that the terminal shock region in the HH 83 system is divided into two distinct components which are well separated spectrally as well as spatially.
Spectral Gaussian fitting of the two components allows us to obtain the velocity field in each structure separately and to
restore images in both components. It is clear
from Fig. 6 that these two structures are shifted spatially by about 2
(
1000 AU)
and the narrow bow with low radial velocity is located in front of the
knotty and more compact high velocity structure. One should also note
that in the case of HH 83, the bow shock region is unusually
asymmetric about the jet axis: its northern wing is much brighter than
the southern one. However, this asymmetry can be caused either by
various degrees of absorption in the dark cloud or by an anisotropic
distribution of the ambient medium.
Besides the evident division of both components in velocity, it should be noted that the high velocity component does not show significant variation of radial velocity in the transversal direction, while in the narrow low velocity structure the radial velocity decreases towards its wing from -70 km s-1 near the apex to -40 km s-1 near the edge of the bow-shaped structure.
3.4 Line profiles
The H
emission in the HH 83 jet mainly shows simple single-peaked
profiles. The exception is knot C where this emission is split into two
components with radial velocities of -160 and -70 km s-1. Some traces
of the high-velocity component are also detected in knots A and B. It is interesting that just near knot C a filament of the
reflection nebula crosses the jet.
Inspection of the line profiles in other parts of the reflection nebula shows that their spectra are not purely
continuum, and weak (compared to the jet) H
emission also exists in this region. However, contrary to the jet
emission, the radial velocity of H
in scattered light is about
+40 km s-1 and its FWHM is 400 km s-1. This fully confirms the reflection origin of the H
emission in the nebula.
![]() |
Figure 7:
The H |
Open with DEXTER |
The FWHM variations of the H
emission along the jet axis are presented in Fig. 7.
As mentioned above, at the base of the jet the existence of the second,
high-velocity component is evident. This component was fitted
separately and excluded from the measurements of the
general FWHM of H
in the jet. The significant variations of the FWHM can be seen at the beginning of the jet although the main trend, distinct in Fig. 7, is a general decrease of the FWHM with distance from the source.
This is consistent with a gradual decrease in the strength of shocks, as described in Sect. 3.1.
4 Discussion and conclusion
The increase of the jet velocity with distance from the source was discussed in detail by Reipurth (1989). His data led to the conclusion that this acceleration is not linear but decreases near the jet end. Our observations (see Fig. 3) are in agreement with Reipurth's data although our detection of a high-velocity component in the terminal shock region allows us to describe the general trend of radial velocity in the HH 83 flow as nearly linear, with superimposed variations of total amplitude of about 80 km s-1.
Several of the early scenarios raised and discussed by Reipurth can now
be discounted, also taking into account the general progress in the
understanding of stellar outflows. The practically linear law of the
HH 83 outflow velocity
field, which was extended by our data to a distance of 105
,
is consistent with the idea of a
time dependent outflow. Moreover, it seems that such a linear dependence over about a thousand years points
to an abrupt outburst with a subsequent decrease of ejection - a suggestion
made by Reipurth (1989) as another possible scenario for HH 83 and supported by the recent discussions
that connect HH ejections to FU Ori type stellar activity (Reipurth & Bally 2001).
Such an approach combines the nearly instantaneous release of energy,
typical of the FU Ori phenomenon, with a gradual fading of the
outflow on timescales of the order of 103 years. The total jet speed probably
reaches 400 km s-1, assuming an angle of 45
to the line of sight.
A scenario based on a steady jet flow over timescales exceeding the dynamical time cannot be excluded. However, this requires a gradual acceleration mechanism for which there is no obvious candidate. Alternatively, a high-speed undetected jet could be present. In this case, a neutral core jet shocks and accelerates surrounding material to produce the visible jet and itself becomes visible only when directly impacting at the bow shock.
Less pronounced but still complex variations of the velocity and FWHM in the jet knots are very intriguing. They, perhaps, reflect the history of the smaller scale ejections from the central star, which created the separate knots in the jet. However, as was also mentioned above, the jet knots could be recently formed shock structures, as smooth velocity waves steepen within the jet. In this case, high knot pressure can produce a rapid transverse expansion. Note that, close to the source, radial velocities in the bright inner jet suggest total inner jet speeds of only 150 km s-1, which is not much in excess of the FWHM for the same jet section. Hence, the jet may widen considerably and the present set of knots will fade rapidly rather than be seen to propagate down the jet, unless the motions giving rise to the velocity width are confined.
Velocity variations detected transverse to the jet axis can, in principle, be considered to be the result of two combined phenomena. First, the velocity drop near both edges of the jet may be due to a turbulent interaction with the ambient medium or with an enveloping wind. Secondly, the shift between the velocity and intensity peaks can be interpreted as due to an additional azimuthal velocity component. Such azimuthal components are often taken to be a signature of jet rotation. Evidence for rotation has been found for several jets (see, e.g., Woitas et al. 2005; Coffey et al. 2007, and references there). In our case, however, one should keep in mind that the shifts shown in Fig. 5 are detected at rather large distances from the source compared with observations of Coffey et al. (2007), and are not consistent with a rotation hypothesis, being directed in opposite directions. Thus, we do not suggest evidence for jet rotation, especially because alternate interesting explanations exist, e.g. see Soker (2005,2007); besides, for the large distances from the exciting source it is more likely that any evidences of rotation will be lost in the turbulent structures inherent to the jet (Beck et al. 2007). Moreover, the transverse velocity gradient across the jet may be the result of a non-circular jet in which one side is aligned closer to the line of sight than the other. Of course, observational tests of such ideas require much higher spatial resolution than achieved so far.
The split of the emission line in knot C as well as the general decrease of FWHM with distance from the source can be the result of higher turbulent motion near the source where the jet passes the cavity; then it becomes more laminar when propagating out of the cloud.
The bow shock structure is represented by two divided structures, separated spatially as well as kinematically. The extrapolation of the jet velocity gradient up to the terminal shock region, where the jet rams into the ambient medium, strongly supports the classical working surface concept where an advancing bow shock is followed by a Mach disk through which the jet flow is halted by a reverse shock.
However, we suggest here that the shock configuration is non-steady. The forward shock appears to be just entering
a thick arc of cold stationary molecular gas identified in CO (Bally et al. 1994).
This arc is roughly 110
from HH 83-IRS
and runs orthogonal to the jet. Here, the reverse shock is oblique and, hence, the
common term Mach disk such as used by Morse et al. (1992) is not an apt description.
Furthermore, the separation of the forward and reverse shocks is not predicted from hydrodynamic numerical simulations
(Blondin et al. 1989).
To explain the separation, we propose that
the shocks have recently strengthened due to the obstructing arc,
resulting in a pressure increase within the shocked layer. The
adjustment of the two shocks to this increase in pressure then leads to
an expanding layer, and hence a detectable spatial separation. The lack
of detectable [SII] emission may then also be attributed to the
truncated nature of the post-shock cooling layers in which incomplete
cooling occurs. For this to occur over an angular distance of two
arcseconds at a distance of 420 pc requires a cooling length in
excess of
cm.
The atomic cooling time scale at 10 000 K is of the order of 1011/n s where n is the atomic density in units of cm-3, e.g. Smith & Rosen (2003).
Therefore, taking a speed of 300 km s-1 (the tangential component of the
gas speed approaching the shock surface), a truncated shock is expected for densities below 250 cm-3.
In comparison, the bow shock and putative Mach disk in HH34S are separated by
cm,
where magnetic field pressure or cool neutral atomic gas were suggested to
occupy the gap (Morse et al. 1992).
The highest jump in (projected) velocity of 250 km s-1 occurs directly at the projected interface of the two shock waves. This also suggests that the interaction is via two deflecting oblique shocks and that the post-shock ambient gas is deflected in the opposite direction transverse to the jet gas.
A problem remains of how the thick arc has come to obstruct the jet. The bipolar outflow and star are probably quite old with signs of having blown out through the cloud. Hence, for the jet to now still be obstructed would suggest that the jet orientation has changed, gradually eroding the cloud in new directions.
We detected the H
emission in reflected light from the cavity walls, which indicates that
the illuminating source is an emission line star deeply embedded in the
dark cloud. It is very probably a classical T Tauri star and not a
Herbig AeBe star, with its high IRAS luminosity at this early
evolutional state largely derived from accretion.
The authors are grateful to Bo Reipurth for the discussion of the manuscript and many helpful comments. They also thank the referee whose valuable suggestions led to a significantly improved paper. This project was mainly supported by INTAS grant 00-00287 and partially supported by ANSEF grant No. PS103-01.
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Footnotes
- ... jet
- Based on observations collected with the 6m telescope of the Special Astrophysical Observatory (SAO) of the Russian Academy of Sciences (RAS), operated with the financial support of the Science Department of Russia (registration number 01-43.).
- ...
- The ADHOC software package was developed by J. Boulesteix (Marseille Observatory) and is publicly available on the Internet.
All Figures
![]() |
Figure 1:
Channel maps of the HH 83 region obtained by FP scanning of the H |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Left panel: the position-velocity diagram for the HH 83 system. Right panel: the monochromatic image of the
system in H |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
The heliocentric radial velocity distribution (filled circles) along
the HH 83 system versus the angular distance from the source. The
working surface is separated into Mach
disk and bow-shock structures. The positions of knots D, F, and G are
identified by arrows on the superposed plot of the intensity of H |
Open with DEXTER | |
In the text |
![]() |
Figure 4: The spatial FWHM of the HH 83 outflow (without deconvolution) plotted against the distance from the source. The general trend is shown by the dashed line. |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
The positions of pseudo-slits traced across knots A-B, D, F and G of the HH 83 jet ( left panel). The size of each one of pseudo-slits is
|
Open with DEXTER | |
In the text |
![]() |
Figure 6: The working surface of
the HH 83 outflow, split into high-velocity
(grey scale) and low-velocity (contoured) structures. These features
are obtained by integrating the intensities in Gaussian-fitted
components in the range of |
Open with DEXTER | |
In the text |
![]() |
Figure 7:
The H |
Open with DEXTER | |
In the text |
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Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.