A&A 402, L47-L54 (2003)
DOI: 10.1051/0004-6361:20030339
A. O. H. Olofsson1 -
G. Olofsson2 -
Å. Hjalmarson1 -
P. Bergman1 -
J. H. Black1 -
R. S. Booth1 -
V. Buat3 -
C. L. Curry4 -
P. J. Encrenaz5 -
E. Falgarone6 -
P. Feldman7 -
M. Fich4 -
H. G. Florén2 -
U. Frisk8 -
M. Gerin6 -
E. M. Gregersen9 -
J. Harju10 -
T. Hasegawa11 -
L. E. B. Johansson1 -
S. Kwok11 -
B. Larsson2 -
A. Lecacheux12 -
T. Liljeström13 -
R. Liseau2 -
K. Mattila10 -
G. F. Mitchell14 -
H. L. Nordh15 -
M. Olberg1 -
H. Olofsson2 -
L. Pagani5 -
R. Plume11 -
I. Ristorcelli16 -
G. Rydbeck1 -
Aa. Sandqvist2 -
F. von Schéele8 -
G. Serra16,
-
N. F. Tothill14 -
K. Volk11 -
C. D. Wilson9
1 - Onsala Space Observatory (OSO),
439 92 Onsala, Sweden
2 -
Stockholm Observatory, SCFAB, Roslagstullsbacken 21, 106 91 Stockholm, Sweden
3 -
Laboratoire d'Astronomie Spatiale, BP 8, 13376 Marseille Cedex 12, France
4 -
Department of Physics, University of Waterloo, Waterloo, ON N2L 3G1, Canada
5 -
LERMA & FRE 2460 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75140 Paris, France
6 -
LERMA & FRE 2460 du CNRS, École Normale Supérieure, 24 rue Lhomond, 75005 Paris, France
7 -
NRC-HIA, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
8 -
Swedish Space Corporation, PO Box 4207, 171 04 Solna, Sweden
9 -
Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1, Canada
10 -
Observatory, PO Box 14, University of Helsinki, 00014 Helsinki, Finland
11 -
Department of Physics and Astronomy. University of Calgary, Calgary, ABT 2N 1N4, Canada
12 -
LESIA, Observatoire de Paris, Section de Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France
13 -
Metsähovi Radio Observatory, Helsinki University of Technology, Otakaari 5A, 02150 Espoo, Finland
14 -
Department of Astronomy and Physics, Saint Mary's University, Halifax, NS B3H 3C3, Canada
15 -
Swedish National Space Board, Box 4006, 171 04 Solna, Sweden
16 -
CESR, 9 avenue du Colonel Roche, BP 4346, 31029 Toulouse, France
Received 3 December 2002 / Accepted 14 February 2003
Abstract
New results from water mapping observations of the Orion KL region using the submm/mm wave satellite Odin (2.1
beam size at 557 GHz), are presented. The ortho-H2O
ground state transition was observed in a
rectangular grid with a spacing of 1
,
while the same line of H218O was measured in two positions, Orion KL itself and 2
south of Orion KL. In the main water species, the KL molecular outflow is largely resolved from the ambient cloud and it is found to have an extension of 60
-110
.
The H2O outflow profile exhibits a rather striking absorption-like asymmetry at the line centre. Self-absorption in the near (or "blue'') part of the outflow (and possibly in foreground quiescent halo gas) is tentatively suggested to play a role here. We argue that the dominant part of the KL H218O outflow emission emanates from the compact (size
15
)
low-velocity flow and here estimate an H2O abundance of circa 10-5 compared to all H2 in the flow - an order of magnitude below earlier estimates of the H2O abundance in the shocked gas of the high-velocity flow. The narrow ambient cloud lines show weak velocity trends, both in the N-S and E-W directions. H218O is detected for the first time in the southern position at a level of
0.15 K and we here estimate an H2O abundance of (1-8)
.
Key words: ISM: abundances - ISM: individual objects: Orion KL - ISM: molecules - submillimeter
The Orion KL molecular outflow region, at a distance of only 500 pc,
has been studied in great detail with a wide variety of instruments
(cf. Melnick et al. 2000b; Wilson et al. 2001).
There are strong indications that the compact H II source called I
(Menten & Reid 1995) is the centre of the dynamical activity,
being the central position for a dense group of H2O and SiO masers
(Gaume et al. 1998; Wright et al. 1995).
Still, the compact luminous "Hot Core'', as revealed by interferometric observations, is centred at least 500 AU away from source I
(Wright et al. 1996; Vicente & Martín-Pintado 2002),
leaving some doubts that source I is the power source for the Hot Core.
Other compact molecular cores are found in the close vicinity of source I,
like the "Compact Ridge'' and the "Northern Cloud''
(Wright et al. 1996).
Mid-IR mapping reveals a number of bright, more or less pointlike
sources (Gezari et al. 1998) in addition to the IR
nebula named after its discoverers (Kleinmann & Low 1967).
It is generally assumed that the mid-IR nebulosity is due to dust emission
as the scattering efficiency of small grains is low at these
wavelengths. On the other hand, the next brightest mid-IR
source, Irc2 (Rieke et al. 1973), appears to be a reflection nebula,
at least in view of the observed polarisation at 3.6 m (Dougados et al. 1993).
This source, traditionally identified as the
dynamical centre, may well be powered by near- or mid-IR emission
from source I. If so, source I must be heavily obscured in our
line of sight (presumably by the Compact Ridge) and much less
so in the direction of Irc2. The spatial distribution of the
SiO
,
emission and the
SiO
,
masers, together with the observed velocity patterns,
suggest that source I is surrounded by a flared disc oriented in the
SW-NE direction (Wright et al. 1995) allowing radiation to
reach the two dense compact cores, Irc2 and the "hot core''.
However, such a protostar with a disc seen more or less edge-on, should
give rise to a well-behaved bipolar outflow in the SE-NW directions.
Even though weak bipolar tendencies have been reported
(H. Olofsson et al. 1982; Masson et al. 1987;
Wilson et al. 2001), it is
generally agreed that the outflow is viewed almost face-on and has a
very wide opening angle. Seen at high spatial
resolution, it is clear that much of the shock-excited high-velocity 2 m H2 emission is coming from numerous condensations (Sugai et al.
1995; Salas et al. 1999). Whether these condensations are
accelerated by a strong wind (not yet observed), or are originating from the
close environment of the protostar, is not clear. Recent proper
motion observations of HH objects at large projected distances from
their dynamical centres suggest an explosive event 1000 years ago
(Doi et al. 2002). This might have been a powerful version
of the FU Ori phenomenon.
In addition to exploring the dynamical properties of the Orion/KL
outflow, much observational effort has been spent on the chemical
aspects, due to the brightness of the source and the richness of its
microwave spectrum (cf. Sutton et al. 1995).
Most of the molecular emission comes from the
molecular cores near the dynamical centre, and from the inner
(lower velocity) expanding
region called the "Plateau'' source (named after the shapes of the
emission lines, cf. Genzel & Stutzki 1989). The diameter
of this expanding region is 15
-30
(Masson et al. 1987; Pardo et al. 2001), while the
extent of the shock-excited 2
m H2 emission and (post-shock) HCO+ flow is larger than 1
(Sugai et al. 1995; Salas et al. 1999;
Olofsson et al. 1982; Vogel et al. 1984).
On a larger scale, the KL region is one of the centres of active star formation along the elongated "Extended Ridge'' that makes up the OMC-1 GMC, which in turn is a prominent component of the Orion A molecular cloud. Comprehensive studies on the physical and chemical molecular structure of this region can be found in e.g. Castets et al. (1990), Dutrey et al. (1993), and Ungerechts et al. (1997). The H II region M42, powered by the Trapezium stars, is partly located in front of the KL region and the interfacing PDR region (the "Orion Bar'', cf. Wilson et al. 2001; Larsson et al. 2003, in this issue) adds to the richness and complexity from a spectroscopic point of view.
In this Letter, we add further important clues to the Orion KL enigma, in the
form of core region mapping observations at 1
spacing of the
ortho-H2O and ortho-H218O ground state submm
rotational lines, performed by Odin, a Swedish-led astronomy and aeronomy
satellite observatory
.
Odin was launched on 20 February 2001 by a Start-1 rocket, from Svobodny in
eastern Russia (Nordh et al. 2003, in this issue).
The observing runs discussed here took place on 24 September 2001,
15/17 April 2002
(H2O,
map,
15 h on-source integration time),
and on 27 September, 15 October 2001 (H218O, three positions,
20 h
on-source).
Two different tunable submm receivers were used for the
water lines (H216O at 556.936 GHz and H218O at 547.67644 GHz), having average SSB system temperatures
of 3200 K and 3500 K, respectively.
The spectra were recorded with a hybrid autocorrelator spectrometer
(AC) configured for a channel spacing of 0.5 MHz (working bandwidth 300 MHz),
and an AOS (1 MHz resolution and 1.1 GHz bandwidth).
A sky switching scheme was employed (5 or 10 s cycle),
either looking at the main beam (1.1 m aperture, FWHM circular
beam size
2
1 at 557 GHz) or at one of the two available sky
beams (of 4
size,
40
offset from the telescope axis),
cf. Frisk et al. (2003), in this issue.
The pointing reconstruction uncertainty is very small (around 5
)
in
the large majority of the data. After correcting for absolute beam offsets
using dedicated Jupiter observations close in time to our Orion measurements,
we estimate that any residual disalignment between the two Orion maps is no
larger than 10
.
From the Jupiter data,
we also find that our beam efficiency is close to 90% (cf. Hjalmarson et al.
2003, in this issue).
Data were calibrated using the standard chopper wheel method, switching between an internal hot load and the main beam (Olberg et al. 2003, in this issue).
Furthermore, the H218O data have been subtracted with calibrated off-source AOS measurements and a polynomial baseline, while the H216O map data (where the amount of line-free baseline region in the 300 MHz AC spectrometer band is limited at positions of broad emission) have merely been subtracted with one common sinusoidal fit (Fig. 1, lower right insert) applied to an average of the available spectra off source (together with peripheral parts of the map where the lines are narrow). We have benefitted in this procedure from the ability to cross check our results with the broad band AOS data from the complementary water receiver.
A general discussion about Odin's performance
can be found in Hjalmarson et al. (2003),
and a technical description of the spacecraft is given in
Frisk et al. (2003).
![]() |
Figure 1:
The Odin 557 GHz H2O map![]() ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
Figure 1 shows a position-averaged version of the
entire ortho-H2O spectra map. Individual scans were observed
around grid points slightly different from those shown (due to systematic
attitude reconstruction scatter, and to pointing model adjustments).
In order to visualise the data, we have chosen to "regrid'' the map
so that it retains the nominal map spacing of 1
and to centre the
map at the Orion KL position. This method implies an additional spatial
smearing so that the effective resolution becomes 130-140
.
We emphasise that the original data set was used in the
preliminary map analysis in Sect. 3.2.2.
As briefly discussed at the end of this Letter, a more detailed analysis
based on a deconvolved Odin water map will appear in a subsequent A&A paper.
As is evident, we detect water emission at nearly all map points,
although the velocity integrated flux density is
heavily dominated by the KL outflow source, as expected from
the lower resolution map by Snell et al. (2000), obtained
using the SWAS space telescope
(
beam, cf. Melnick et al. 2000b).
There is pronounced
velocity structure present, both in the Extended Ridge/Bar
narrow line emission, and in the KL outflow high velocity wings.
The E-W trend across the Extended Ridge into the Bar is
illustrated in Fig. 2 by a sequence of spectra at
DEC offset, and the S-N structure across KL is shown
in a velocity-declination diagram (Fig. 3). In the former
figure, water emission is indeed seen at Orion Bar velocities
(
10.5
,
Wilson et al. 2001), where Odin also has
detected NH3 (Larsson et al. 2003). Apart from the
outflow appearance in Fig. 3 (discussed further below),
the emission centroids shifts from
8-9
in the south to
9-10
in the north,
in agreement with the two cloud components at these velocities
previously detected by e.g. Rydbeck et al. (1981) and
Friberg (1984).
![]() |
Figure 2:
H2O spectra
![]() ![]() ![]() |
Open with DEXTER |
![]() |
Figure 3:
H2O declination-velocity diagram through Orion KL. The colour coding goes from black (![]() ![]() ![]() |
Open with DEXTER |
The Odin Orion KL water spectrum is shown separately in Fig. 4,
together with a ,
spectrum of SiO (observed with
a 43
beam using the Onsala 20 m telescope), and
the Odin H218O spectrum (described further below).
Compared with
the corresponding water spectrum observed by SWAS (Melnick
et al. 2000a), the Odin spectrum clearly has resolved out
the outflow emission better from the ambient cloud narrow emission.
In fact, a convolution of our Odin H2O map (Fig. 1) to the
SWAS resolution results in an H2O spectrum almost identical to
the one observed by SWAS (as demonstrated by Hjalmarson et al.
2003).
The high velocity portions of the H2O and SiO profiles are very similar,
suggesting an outflow/shock origin of the water line as well.
However, an important difference between the two is the water central
asymmetry - the bluewards absorption-like feature (further discussed in
Sect. 3.2).
This peculiarity (which is confirmed by data from the complementary water
frontend/AOS backend system; see Fig. 1 of Hjalmarson et al.
2003) also seem to be slightly more emphasised
1
northwards from the KL position.
![]() |
Figure 4:
From top to bottom: H2O (Odin beam 2.1![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
Our Orion KL H218O AOS spectrum (Fig. 4) - at first reduced
in the manner described in Sect. 2 - was additionally
treated for a residual sinusoidal-like interference, a measure
that may formally underestimate the strength by 4%.
Since the baseline in the AOS spectrum extends far beyond what is
shown here (and is now largely flat), we find no reason to believe that
the process has distorted the line shape further.
There are a few features immediately worth pointing out (some of which
are discussed more in Sect. 3.2):
In view of the rather low SWAS limit for this line
(Snell et al. 2000, centred 3
2 south of Orion KL),
we infer that the H218O source size should be compact,
which may be expected if the emission primarily comes from a
previously detected warm massive clump in the Orion ridge
(NH3: Source 6 in Batrla et al. 1983;
CS condensation 4 in the high resolution map of Mundy et al. 1988).
Since a detection of an ambient cloud H218O line is very important
for a reliable determination of the water abundance, we aim at verifying
this probable detection, and also at observing additional ambient cloud
positions, in the near future.
![]() |
Figure 5:
H218O towards 2![]() ![]() |
Open with DEXTER |
To illuminate similarities and differences in our H2O, H218O, and SiO spectra, we show them together in Fig. 6 scaled so that their red line wings overlap. We note first that all three lines seem to lack evidence for emission features characteristic for the Hot Core or the Compact Ridge.
![]() |
Figure 6:
The spectra of Fig. 4 scaled
![]() |
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The H2O line has, compared to the SiO line, a central asymmetry
that takes the form of an emission deficiency from the line centre into
the blue wing of the water line. We tentatively consider the H2O profile to
show evidence of broad outflow self-absorption, consistent with what is
sometimes seen in optically thick lines from expanding regions such as
circumstellar shells (CO; Crosas & Menten 1997), and cometary comae
(Odin H2O; Lecacheux et al. 2003, in this issue).
The steep change in the
proposed H2O KL self-absorption occurs at the systemic velocity of the
ambient cloud (8 to 10
). A similar behaviour - narrow absorptions
at systemic
velocities - is found to an even larger extent in most of the outflow
sources observed by Odin and SWAS. We tentatively suggest that such water
self-absorption,
occuring in the line-of-sight lower-density outer layer of the quiescent cloud
component, is an intrinsic radiative transfer property of water in warm
GMC cores.
In the case of H218O, there is an apparent shortage of emission
compared to SiO out to a very low velocity (-30
), possibly
below the H218O baseline. If real (although currently hard to
reconcile in view of our KL H2O spectrum), the necessary continuum that
is absorbed should come from the Hot Core and/or the Compact Ridge
which are located behind the blue - or near - part of the outflow.
Another possibility is that the narrower width of the H218O line indicates that we see a different kinematical component in this species. A comparison then can be made with the submillimetre HDO lines observed by Pardo et al. (2001) which have similar widths and centre velocities. If the emission from these two molecules does indeed originate in the same gas, then the H218O profile is probably dominated by emission from the inner part of the outflow (low-velocity flow, or the "Plateau''), although the apparant presence of a red high velocity wing (which HDO clearly lacks) may indicate a minor contribution from the shocked gas (the proportions being determined by the optical depths involved). We will examine this interesting alternative more quantitatively in Sect. 3.2.3.
An important clue to the interpretation of the water outflow/shock
emission is the size and position of the source. Using the integrated
intensity across the whole band (excluding only the very central part),
we find that the peak is located 10
north of our map centre.
This location of the outflow centre is identical to
the one determined by high resolution ground based observations
(cf. Wilson et al. 2001). To estimate the source size,
we have used three methods: i), a 2D Gaussian fit to the map
integrated intensity corrected for the beam response, ii), a preliminary
deconvolution of the integrated intensities in the red and blue line wings,
and iii), through comparing the centre KL water spectra of Odin and SWAS
(Hjalmarson et al. 2003).
The Gaussian sizes all fall within the range 60
-110
.
In contrast to H2O, Hjalmarson et al. concluded, through a direct
Odin/SWAS comparison, that the H218O source size is pointlike
in the Odin beam.
Governed by the similarities between the H218O and HDO spectra
(see Sect. 3.2.1), we will hereafter assume that the H218O sources size is 15
,
i.e., equal to extent of the submillimetre HDO emission from the low-velocity Plateau (Pardo et al. 2001).
The H2O source size in the southern position (2
south of KL)
is simply taken to be
larger than the Odin beam since the narrow line emission is obviously
extended (cf. Fig. 1).
We will herein attempt a very simple analysis and bring forth a few basic comments in connection with our observational results. A detailed treatment is planned to appear in an A&A paper in the near future, the prospects of which are discussed at the end of this Letter.
To derive the water content, we have used the optically thin LTE approximation applied to the H218O line.
In this limit, the expression for the water column density takes the form:
Table 1: Integrated intensities, column densities, and abundances.
Using an
corresponding to our adopted H218O source size
of 15
,
a rotation temperature
T=72 K (
s K-1 km-1 cm-2 in Eq. (1))
as found by Wright et al. (2000), we obtain
cm-2. This value is about a factor of four lower than the column density obtained from pure rotational H2O lines
in the 25-45
m band (Wright et al. 2000).
When estimating the water abundance,
,
it is not obvious which
to use.
For instance, if we adopt the H2 column density of
cm-2 estimated by Watson et al. (1985)
from observations of far-infrared rotational CO lines, we obtain a
water abundance of
.
Alternatively, if we assume that
the H218O gas is essentially the same component as seen in HDO,
we instead find a Plateau water abundance of
(now
using the total outflow H2 column density of about
cm-2 from Masson et al. 1988),
which is more than an order of
magnitude lower than our previous estimate. Both our estimates are listed
in Table 1.
Previous observations of broad water line components have all resulted in
high abundances, e.g.
(2-5)
(Wright et al. 2000, ISO mid-IR absorption lines),
(Harwit et al. 1998, ISO far-IR emission lines),
and
(Melnick et al. 2000a, SWAS sub-mm emission line). As advocated by these authors, such high abundances are expected
in shock-heated gas (cf. Kaufman & Neufeld 1996). Our water
abundance may be underestimated in case the H218O line is optically
thick in the Plateau. From the H218O column density and the observed
line width, we find
for T=72 K. This optical
depth corresponds to an expected peak antenna temperature of about 0.5 K for a 15
source size, i.e., consistent with the observed
peak antenna temperature of 0.8 K. An optical depth of 0.9 implies an
increase of our deduced water column density, and hence our water abundances,
by a factor of
1.5.
Pardo et al. (2001) argue that the gas phase water
in the low-velocity Plateau is more likely the result of evaporation of
icy grain mantles by radiative heating (in turn explaining their high HDO abundance).
Hence, we find no obvious contradiction in suggesting
that the Plateau water abundance may be smaller than
the high value found in the post-shock gas.
It is also worth noting that
the Plateau column densities of HDO by Olofsson (1984)
and Pardo et al., in combination with the H2O column density found
here, would imply a high degree of deuteration (HDO/H2O
0.025-0.1)
in agreement with earlier findings for other deuterated molecules in
Orion KL sources (Charnley et al. 1997;
Mauersberger et al. 1988; Roberts et al. 2002).
The H2O/H218O line ratio of emission in the red wings (the only part
of the spectral range where the profiles look similar, see
Fig. 6) is about 15, indicative of
.
Using the estimated size of 60
for H2O and the observed peak
antenna temperature (8.1 K), we then find that the excitation temperature
(for thermal emission) must be
56 K. This is close to the H2O rotation temperature of 72 K (Wright et al. 2000).
However, should the excitation temperature be substantially higher,
e.g.
150 K (González-Alfonso et al. 2002),
we need to introduce an additional source filling factor of about 0.4,
as could be the case if a considerable part of the emission arises
in small, dense, optically thick post-shock clumps.
Such a filamentary and clumpy structure is seen
over a
1
region in H2
maps by
Salas et al. (1999)
. Should the H2O profile
suffer strongly from self-absorption (see Sect. 3.2.1),
a higher excitation temperature is indeed required.
In the southern position (2
south of KL), we again employ the
optically thin LTE approximation to estimate an ambient cloud water abundance
and note that the peak brightness temperature ratio is
,
confirming that the H218O line is optically thin.
For an LTE temperature of 25 K
(
s K-1 km-1 cm-2) we obtain
cm-2. Using an
H2 column density of
1022 cm-2 (Dutrey et al. 1993; Goldsmith et al. 1997),
we find
.
This low
value may be considered a lower limit since (as suggested in
Sect. 3.1) the H218O beam filling factor may be as low
as 10-15% judging from the clump size observed by
Mundy et al. (1988).
As a comparison, Snell et al. (2000) have demonstrated a method
using the H2O line itself to arrive at an abundance, provided that
the line is optically thick and arising in strongly subthermal conditions.
In the 2
S position,
is estimated to be
.
If we use the integrated intensity of our narrow component H2O line
from Table 1 together with the Eq. (2) of Snell et al.
(here corrected for an ortho-para ratio of 3) valid
for a kinetic temperature of 40 K and assume an H2 density
of 106 cm-3 and the same H2 column density as before
(1022 cm-2), we instead arrive at a water abundance
of
.
Both our results are inconsistent with the high water abundance
(10-5) estimated for gas adjascent to (but outside) the KL centre of activity by Cernicharo et al. (1994) using the 183 GHz water maser line. At present time, we concur with the opinion
of Snell et al. that the maser line, which originates from energy levels
near 200 K, must trace a denser and warmer gas
component.
On the whole and at this stage, it appears that unequivocal estimates of the water abundances remain elusive, but the current work will be continued in a forthcoming paper where we aim at including all Orion water mapping data at our disposal (from past and future observations, including spectra from both Odin water receivers), in combination with new H218O and H217O observations. This will make feasable a more elaborate analysis and modelling, particularly in view of our ambition to produce a deconvolved water data cube (using the MEM-algorithm) that may reveal further details on, e.g., the Orion KL H2O outflow absorption reported on above.
Acknowledgements
Generous financial support from the Research Councils and Space Agencies in Canada, Finland, France and Sweden is gratefully acknowledged. We also want to thank the referee for constructive criticism which has strengthened our presentaion.