Issue |
A&A
Volume 511, February 2010
|
|
---|---|---|
Article Number | A82 | |
Number of page(s) | 10 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/200912484 | |
Published online | 16 March 2010 |
H2CO and CH3OH
maps of the Orion Bar photodissociation region![[*]](/icons/foot_motif.png)
S. Leurini1,2 - B. Parise2 - P. Schilke2,3 - J. Pety4 - R. Rolffs2
1 - ESO, Karl-Scharzschild-Strasse 2, 85748 Garching-bei-München,
Germany
2 - Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121
Bonn, Germany
3 - Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77,
50937 Köln, Germany
4 - Institut de Radioastronomie Millimétrique, 300 rue de la Piscine,
38406 Saint-Martin d'Hères, France
Received 13 May 2009 / Accepted 4 December 2009
Abstract
Context. A previous analysis of methanol and
formaldehyde towards the Orion Bar concludes that the two molecular
species may trace different physical components, methanol the clumpy
material, and formaldehyde the interclump medium.
Aims. To verify this hypothesis, we performed
multi-line mapping observations of the two molecules to study their
spatial distributions.
Methods. The observations were performed with the
IRAM-30 m telescope at 218 and 241 GHz, with
an angular resolution of .
Additional data for H2CO from the Plateau de
Bure array are also discussed. The data were analysed using an LVG
approach.
Results. Both molecules are detected in our
single-dish data. Our data show that CH3OH peaks
towards the clumps of the Bar, but its intensity decreases below the
detection threshold in the interclump material. When averaging over a
large region of the interclump medium, the strongest CH3OH
line is detected with a peak intensity of 0.06 K. Formaldehyde also peaks on the
clumps, but it is also detected in the interclump gas.
Conclusions. We verified that the weak intensity of
CH3OH in the interclump medium is not caused by
the different excitation conditions of the interclump material, but
reflects a decrease in the column density of methanol. The abundance of
CH3OH relative to H2CO
decreases by at least one order of magnitude from the dense clumps to
the interclump medium.
Key words: ISM: individual objects: Orion Bar - ISM: abundances - ISM: molecules - ISM: structure
1 Introduction
Given its proximity and nearly edge-on orientation, the so-called Orion Bar is one of the clearest examples of a photon-dominated region (PDR). For this reason, the Orion Bar has been extensively observed in past decades to test theoretical models of PDR structure, chemistry, and energetics. Its nearly edge-on orientation make direct observations of the gas stratification possible, from ionised gas to neutral atomic gas to molecular gas as a function of the increasing distance from the ionisation source (e.g., van der Werf et al. 1996; van der Wiel et al. 2009; Tielens et al. 1993). These studies show that the molecular gas consists of clumpy molecular cores embedded in an interclump gas. While the clumps have densities of several 106 cm-3 (Lis & Schilke 2003; Young Owl et al. 2000), the interclump material has lower densities (104-105 cm-3, Hogerheijde et al. 1995; Young Owl et al. 2000). Such observational results are successfully reproduced by theoretical works (e.g., Gorti & Hollenbach 2002).Observations of methanol (CH3OH) and formaldehyde (H2CO) in the Orion Bar have been reported in the past (Jansen et al. 1995; Leurini et al. 2006; Hogerheijde et al. 1995). Leurini et al. (2006) studied the excitation of both molecular species in the line of sight of one molecular clump through a multi-line analysis at 290 GHz. Their findings (high density and relatively low temperature for CH3OH, high temperature, and relatively low density for H2CO) suggest that methanol and formaldehyde do not trace the same material, but the first is found in the dense gas associated with the clumps, the second in the warmer and less dense gas of the interclump material. Since both molecular species can efficiently form on grain surfaces (Hidaka et al. 2004), the authors suggested that photodissociation of methanol to form formaldehyde (Le Teuff et al. 2000) takes place in the interclump medium, while the high density shields CH3OH in the clumps and prevents its photodissociation. Whereas grain surface reactions are the only viable route to methanol, formaldehyde can also form in the gas phase via the reaction CH3+O (e.g., Le Teuff et al. 2000), which could contribute to the interclump abundance of H2CO.
![]() |
Figure 1:
In grey scale, the integrated intensity of the H2CO
(
30,3-20,2)
line ( left) and of the CH3OH-A
(50-40)transition
(
right) observed with the
IRAM-30 m telescope. Black contours (4, 8, and 12
times 0.032 Jy beam-1 km s-1)
show the distribution of the H13CN
(1-0) emission observed with the Plateau de Bure interferometer
(Lis & Schilke 2003).
For H2CO the contours start from
1.8 K km s-1 in steps
of 1.8 (equal to 3 |
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However, the study of Leurini
et al. (2006) on CH3OH and
H2CO is based on low angular resolution (
), single-pointing data. To
confirm that the two species indeed trace different
media, we mapped a large area of the Orion Bar in formaldehyde and
methanol at 218 and 241 GHz, respectively, with an angular
resolution
of
.
We also analysed existing interferometric data for the
218 GHz formaldehyde lines, to compare the observed fluxes
from single-dish and interferometer and derive constraints on the size
of the
emitting region. This work represents a first observational attempt to
study the
distribution of CH3OH and H2CO
in the Orion Bar
and to verify their different origin.
2 Observations
2.1 IRAM-30 m telescope
The observations were performed in February and March 2007 using the HERA multi-beam receiver (Schuster et al. 2004) at the IRAM-30 m telescope. The HERA1 and HERA2 pixels were tuned to 241.850 GHz in lower side band (LSB), to detect the CH3OH (5k-4k) band. On February 23 and 24, the HERA2 receivers were tuned to 218.349 GHz (LSB) to detect the H2CO ( 3Ka,Kc-2Ka,Kc-1) band. The backend used was VESPA with a bandwidth of 160 MHz and a resolution of 0.3125 MHz for the CH3OH setup (corresponding to a velocity resolution of

We used the on-the-fly mode with a rotation of the multi-beam
system
of 9.7,
to ensure a Nyquist sampling between the rows. The
reference position used as the centre of the map was
,
,
corresponding to the ``Orion Bar (HCN)'' position of
Schilke et al.
(2001), the most massive clump seen in H13CN
(Lis & Schilke 2003),
as well as the target of the spectral
survey of Leurini
et al. (2006). The OFF position was
chosen to be
(500'', 0'') from the centre of the map.
The pointing was checked on several continuum sources
and found to be accurate within 2''.
The observations were done
under varying weather conditions, and the
span the range
540-1200 K for HERA1 and 700-1600 K for HERA2. The
half-power beam width of the IRAM-30 m telescope at
218 GHz is
and
at 242 GHz. We used a beam efficiency of 0.55 at
218 GHz, and of 0.50 at 242 GHz to convert
antenna
temperatures
into main-beam temperatures
.
The region covered by our observations is shown in Fig. 1.
2.2 Plateau de Bure interferometer
We observed the H2CO
30,3-20,2
line at 218.2 GHz with the Plateau de Bure
interferometer in March, April, and December 2004. We observed a
7-field
mosaic centred on
,
.
The field positions followed a compact hexagonal
pattern to ensure Nyquist sampling in all directions. The imaged
field-of-view is almost a circle with a radius of 22.5''. On March
28 and 29, the six antennas were used in the 6 Cp
configuration with 3 and 1.5 mm precipitable water vapour,
which translated into system
temperatures of 450 K and 250 K. On April 22, the six antennas
were used in
the more compact 6Dp configuration with 6 to 10 mm PWV,
leading to a system temperature of about 1000 K. On
December 12, the six
antennas were used in the 6Cp configuration, with 3.5 mm PWV,
leading to a system temperature of about 700 K. Taking the
time for calibration and data filtering into account, this translates
into an on-source integration time of useful data
of 5.3 h for a full
6-antenna array. The typical 1 mm resolution is 2.4''. We used
the 30 m
data described above to produce the missing short spacings.
The data processing was done with the GILDAS software suite (Pety 2005). Standard calibration
methods implemented in the GILDAS/CLIC program were applied using close
bright quasars (0528+134 and 0607-157) as phase and amplitude
calibrators. The absolute flux was calibrated using simultaneous
measurements of the PdBI primary flux calibrator MWC349. uv tables
were
then produced at a relatively coarse velocity resolution of
1.7 km s-1.
All other processing was performed with the GILDAS/MAPPING
software. The
single-dish map from the IRAM-30 m was used to create the
short-spacing
pseudo-visibilities unsampled by the Plateau de Bure
interferometer (Rodríguez-Fernández
et al. 2008). These were then merged with the
interferometric observations. Each mosaic field was imaged
independently, and a dirty mosaic was built through a linear
combination of these dirty
images (Gueth et al.
1995). The dirty mosaic was then deconvolved using the
standard Högbom CLEAN algorithm. While all these procedures are
standard
and usually easy to use in the GILDAS/MAPPING software, we had to take
special precautions with this data set because the short usable
integration
time at PdBI implied 1) a low signal-to-noise ratio of the
interferometric
data and 2) dirty beams for each field with large secondary side-lobes
(see
Fig. 2).
As the 30 m data show that the extended signal fills
most
of the field-of-view observed with the interferometer, we subtracted
the spectrum averaged over the observed field-of-view from the
30 m data before any
processing and we added this averaged spectrum again after the
deconvolution
of the 30 m+PdBI hybrid data set to recover the correct flux
scale. This
simplifies the deconvolution considerably by the usual CLEAN algorithms
(i.e. much fewer CLEAN components are needed) because it avoids the
deconvolution of extended uniform intensities. Second, we tapered the
visibility weights with an axis-symmetrical Gaussian of 80 m FWHM
to get a
more symmetrical beam (going from
for natural weighting
to
after tapering) and to improve the signal-to-noise
ratio for the extended structures. The final
noise rms measured at the centred of the mosaic is about 0.3 K
in channels
of 1.7 km s-1 width. This
leads to a maximum signal-to-noise ratio of 18 for
the hybrid data cube. However, the produced data cube has its
brightness
dynamical range limited by the poor dirty beam. The cleaned data cube
was finally scaled
from Jy/beam to
temperature
scale using the synthesised beam
size.
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Figure 2: Top: uv coverage for one field of the hybrid 30 m+PdBI mosaic. Bottom: associated dirty beam. Please note that the dirty beam has many secondary side-lobes as high as 40% of the main lobe. |
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3 Observational results
![]() |
Figure 3: Spectrum of the H2CO ( 3Ka,Kc-2Ka,Kc-1) band towards clump 1 ( top) and averaged over the interclump medium ( bottom) from the IRAM-30 m telescope. |
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Figure 4:
Spectrum of the CH3OH (5k-4k)
band towards clump 1 at a resolution of
|
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3.1 Single-dish data
Figures 3a
and 4a
show the spectra at 218.4 and 241.8 GHz towards the central
position of the map. Both formaldehyde transitions are detected. Only
the 50-40-A
and 51-41-E
methanol transitions are detected at the original resolution
of .
However, when increasing the signal-to-noise ratio by smoothing to a
lower resolution of
,
CH3OH transitions with upper level energies
lower than 85 K are also detected (Fig. 4b). The
detected lines of both molecular species are reported in Table 1 (Müller
et al. 2005,2001).
Table 1: Detected methanol and formaldehyde transitions in the IRAM-30 m telescope data.
The region mapped with the
IRAM-30 m telescope is presented in
Fig. 1,
where in the left panel we show the integrated
intensity of the strongest detected H2CO line,
while the right
panel shows the emission from the strongest CH3OH
transition along
the Orion Bar alone. For comparison, the integrated intensity of the
H13CN (1-0) from Lis & Schilke (2003)
is overlaid on our
data. Both molecules clearly trace the molecular medium associated
with the Orion Bar, and peak to the southwest of it. This peak
probably belongs to the low-intensity bridge that connects Orion South
to the Orion Bar. This structure was detected in previous observations
at velocities between 7 and 9 km s-1
(e.g., Tauber
et al. 1995). The southwest peak of CH3OH
and H2CO
detected in our data has a peak velocity of 8 km s-1.
In
addition, formaldehyde is detected towards the north of the map, at a
position that spatially coincides with continuum emission at
m (Lis et al. 1998)
and with a velocity between 9
and 11 km s-1.
Along the Bar, both molecules trace the elongated region where
molecular clumps are located, with an absolute emission peak on
clump 3 of Lis
& Schilke (2003). Moreover, both species are detected
on a secondary peak, northeast of clump 1
(
,
),
not covered by the observations of Lis
& Schilke (2003), but
close to a peak of CO(6-5) (Fig. 5), which could
represent an additional clump of dense gas along the Orion Bar. To
better visualise the overall morphology of the Orion Bar, in
Fig. 5
we show the 20 cm continuum emission from
Yusef-Zadeh (1990),
which traces the
ionisation front; the CO(6-5) integrated emission (Lis et al. 1998),
which shows the temperature distribution
of the molecular gas; and the H13CN (1-0)
emission, which traces the dense clumps.
Table 2: Line parameters.
In Figs. 3
and 4
we compare the
H2CO and CH3OH spectrum
towards clump 1 to the corresponding
spectra averaged over the region of the interclump gas outlined in
Fig. 1
(left). The integrated intensity
of the CH3OH
(50-40)-A
transition in the interclump medium is a factor
9 lower than
in the clump, the one of H2CO
32,2-22,1 line
a factor of 7 (see Table 2). For a
better visualisation of the formaldehyde and methanol morphologies, we
show in Fig. 6
the variation in the integrated
intensities of the CH3OH-A (50-40)
and H2CO
(32,2-22,1)
lines for three strips in the map; the exact
location of the strips is shown in Fig. 1. The strip
across clump 1 (Fig. 6a) suggests a
larger extension of
the H2CO emission with respect to CH3OH,
while the variation in
the integrated intensity of the two species along the Bar is very
similar (Fig. 6c).
Although we present data for
the weakest of the two H2CO lines, the
signal-to-noise ratio in the
methanol data is still lower than for H2CO and
could bias our
results. From these strips and from the comparison between the
H2CO emission and the CO(6-5) map, it also
emerges that
formaldehyde extends to a greater distance from the clumps in the
molecular cloud than towards the ionisation front. We also smoothed
the data to lower resolutions, to verify whether the CH3OH
and H2CO emission are similarly affected by beam
dilution. At a
resolution of 50'', the H2CO emission is still
37% of the
intensity of the original data, while at the same resolution the
CH3OH peak intensity drops down to
18% of the
original
value. These tests suggest that formaldehyde emission is associated
with the clumps but also with the interclump medium, while methanol
emission is only found in the dense molecular clumps. In Sect. 4, we
investigate whether the different
morphologies of the two molecular species stem from an excitational
or observational bias, or whether they imply different abundances of
H2CO and CH3OH in the two
media.
Finally, the spectral resolution used for the methanol
observations
allows us to study the velocity field along the Bar. The channel maps
of the (50-40)-A
line (Fig. 7,
smoothed to
0.77 km s-1) show that
clump 1 peaks around 9.5 km s-1
and
clump 3 around 10.5, as for H13CN
(Lis & Schilke 2003).
The secondary peak detected to the
northeast of clump 1 has a peak velocity around
11 km s-1, while
the molecular cloud to the south of the Bar is red-shifted with
respect to the clumps (
km s-1).
This trend
in velocity (blue-shifted velocities in the northeast, red-shifted
values in the southwest) was also found by
Young Owl
et al. (2000) in their HCN maps.
![]() |
Figure 5:
Distribution of the CO(6-5) peak brightness temperature
(colour image). The black contours (from 40 |
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3.2 Hybrid interferometric+single-dish data
Figure 8 compares the 30 m (top panels) and hybrid 30 m+PdBI (bottom panels) data cubes. The left column displays the images integrated between 8.3 and 11.7 km s-1. The middle column displays the spectra averaged over the whole interferometric field-of-view. The hybrid spectra are identical to the 30 m ones within the noise level, confirming that all the extended emission filtered out by the PdBI is correctly recovered from the single-dish data. The right column of Fig. 8 displays the spectra averaged over the central clump marked in the left column. The hybrid spectrum is brighter than the 30 m spectrum implying beam dilution in the single-dish data.
From the PdBI data and the hybrid 30 m+PdBI data, we can therefore conclude that the H2CO emission comes from both an extended component and a compact one, still unresolved at the resolution of the IRAM-30 m telescope.
![]() |
Figure 6: Variation in the integrated intensity of the CH3OH-A(50-40) and H2CO 32,2-22,1 lines observed with the IRAM-30 m telescope across clumps 1 (a), the interclump medium between clump 1 and 2 (b) and along the Bar (c). In Fig. 1 we outline the strips used to extract the plots. |
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![]() |
Figure 7: Channel maps of the CH3OH-A (50-40) emission between 11.3 and 7.4 km s-1 observed with the IRAM-30 m telescope. |
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4 Discussion
4.1 Formaldehyde
Line ratios of formaldehyde lines can be used to infer the physics of
the gas in dense molecular regions (Mangum
& Wootten 1993).
Because of the coarse spectral resolution of our observations (
km s-1),
the line profiles of the H2CO transitions
are poorly resolved. As a result, we do not have a correct measure of
the
peak intensity of the lines, diluted over the beam, but our values are
a lower limit to the
true main-beam line intensities. However, the areas are conserved
quantities
and can be used to study the excitation of H2CO
in the Orion Bar.
Figure 9 shows the ratio of the integrated intensities of the 30,3-20,2 line to the 32,2-22,1 line. The analysis is limited to the region where the signal-to-noise ratio of the 32,2-22,1 integrated intensity is at least equal to 3. The ratio increases from the northeast, where the clumps are, to the southwest.
We ran LVG calculations for column densities of H2CO
between
1012 and 1015 cm-2,
temperatures in the range 20-200 K,
and densities of 104-108 cm-3,
and calculated the behaviour
of the
value of the observed 30,3-20,2
and
32,2-22,1
line intensities as function of these quantities. Two possible
regimes are found (see Fig. 10),
independent of
temperature. At low densities (n<106 cm-3),
the
value
depends only on the product of the H2 density
and H2CO column
density (low-density branch). For higher densities, the
value
varies only slowly with H2 density and
is mostly a function of
H2CO column density (high-density branch).
For the values in the interclump gas, assuming a beam filling
factor of unity, we find a nominal minimum for the interclump medium at
T=180 K,
cm-3,
cm-2.
However, as can be seen in Fig. 10,
these parameters are not well constrained. Using the
confidence level, we derived a lower limit to the kinetic temperature
of the interclump equal to 76 K, and the product
is equal to
cm-5
for densities smaller than 106 cm-3.
By comparison with PDR models, Young Owl
et al. (2000) found densities of hydrogen nuclei of
104-105 cm-3
in the interclump; similarly, Hogerheijde
et al. (1995) found an interclump H2
density of
cm-3
by modelling mm line observations. Thus, by using a molecular hydrogen
density between
and
cm-3
we can constrain the column density of H2CO to
values in the range
cm-2.
The high-density branch does not appear to be relevant for the
interclump medium.
In Sect. 3.2
we analysed the combined PdBI+30 m data and concluded that the
H2CO emission is composed of two components, one
extended and one unresolved at the resolution of the
30 m telescope at this frequency. However from these
data, it is difficult to infer the exact size of clump 1 and
the contribution of
the interclump gas to the total emission of H2CO.
In Fig. 8,
we marked clump 1 with an ellipse of
(
)
and showed that the IRAM-30 m data are affected by beam
dilution at this scale, implying a smaller size for the clump.
Therefore, for clump 1, we corrected the main-beam line
intensities for the
contribution from the interclump and for the beam dilution assuming a
typical size for the clump of 7'' (see Lis & Schilke 2003).
As expected given the similar value of the
line ratio on clump 1 and in the interclump, the values
derived are
similar to those of the interclump: T=170 K,
cm-3,
cm-2.
The
contour gives a value of
cm-5
for the product
for
cm-2
and
cm-3.
For the low-density branch, we derive a
lower limit of 50 K for the kinetic temperature. For the
high-density
branch, more appropriate for the clump, the column density is
constrained to values
cm-2.
A lower limit of 80 K can be derived for the
kinetic temperature. It may appear surprising that the temperature is
so little constrained, although the ratio of these lines has been used
as a temperature tracer
(Mangum
& Wootten 1993; Hogerheijde et al. 1995).
However, for the
values of the line ratio we observe, the LVG predictions show a more
complex behaviour with temperature (see also Fig. 13
of Mangum &
Wootten 1993). Taking the measurement errors into
account by calculating the
value, as we do, and
using the
line intensities and not just the line ratios then constrains only a
lower limit to the temperature.
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Figure 8: Top line: IRAM-30 m data. Bottom line: hybrid 30 m+PdBI deconvolved data cube. Left column: images of the integrated line intensity of the H2CO ( 30,3-20,2) transition between 8.3 and 11.7 km s-1. The values of the contour levels are shown on the colour scale. Middle column: spectra averaged over the whole interferometric field-of-view marked as the red circle on the images. Right column: spectra averaged over the central clump marked with the blue ellipse on the images. |
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Table 3: Summary of the model results.
The results of the LVG analysis of H2CO are presented in Table 3.
4.2 Methanol
Leurini et al. (2004) have studied the excitation of the CH3OH (5k-4k) band as function of the temperature, density, and methanol column density of the gas for a range of physical conditions applicable to the Orion Bar. They concluded that line ratios within this band are not sensitive to the temperature of the gas for temperatures higher than 30 K, but only depend on the density.The methanol emission from clumps 1 and 3 is studied in a
separate
paper (Parise et al. 2009):
clump 1 is found to be warmer than
clump 3, although the errors are large (
ranges:
K,
K),
but with similar
column densities and densities (
cm-2,
cm-3).
These values were
derived by modelling the 5k-4k
and 6k-5k bands
observed with
the IRAM-30 m (observations presented in this paper) and the
APEX
telescopes, respectively. For the models, the IRAM data were smoothed
to the resolution of the APEX data (FWHM beam
). A
source size of 10'' was used, since several clumps fall in the beam
of the observations. This is equivalent to assuming that all clumps
have
similar physical conditions. Since the CH3OH
emission was found to
be optically thin, we can correct the results for a source size of 7''
(as assumed for the model of H2CO) and derive
the equivalent column density. This
corresponds to a total column density of
cm-2,
or to
cm-2
for
CH3OH-A, assuming that the
two symmetric states have the same
column density. The
confidence level is
cm-2.
For the interclump medium, we analysed the CH3OH
spectrum averaged
over the area of the interclump medium outlined in
Fig. 5.
We ran models for the excitation of methanol
in the range of densities 104-108 cm-3,
column densities
1012-1018 cm-2,
and temperatures 20-200 K. The observed
main-beam brightness temperature of the
(50-40)-A
line in the
interclump medium is K.
Assuming that the emission
comes from an extended source (e.g., beam filling factor
),
we find a situation similar to the one described for H2CO
and we can
identify two different regimes. In the first (n<106 cm-2),
a
fit to the data is found for
cm-2:
the column density decreases with increasing
density, but their product is almost constant (between
cm-5,
,
and
cm-5,
).
For higher densities,
the 50-40 line
thermalises. In this regime, the column density
increases slowly (
cm-2).
For both regimes, the dependence on the kinetic
temperature is not strong and the results presented are valid for
temperatures in the range 20-200 K. However, increasing the
temperature of the gas implies a decrease in the column density of
methanol for the low-density case (
K,
cm-2,
see Fig. 11).
As already discussed in the previous section, from the
literature we
know that the density of the interclump gas is less than
106 cm-3, and
therefore that the low-density regime applies to
the modelling of the interclump. The best fit to the data is found for
cm-2,
K,
cm-3.
For densities in the range
cm-3,
and for the values of the product
given above,
we derive column densities in the range
cm-2.
However, if we allow only
temperatures above 50 K, as found in our data and by other
authors,
the inferred methanol column density in the interclump is
cm-2
(see Table 3).
5 Abundances of methanol and formaldehyde in the Orion Bar
From the analysis performed in the previous section, we can compute the abundance of methanol relative to formaldehyde in the interclump medium and in the dense clumps, and verify that the drop of intensity of the methanol emission in the interclump medium is the result of the different physics of the interclump relative to the dense clumps, or whether it reflects a real decrease in the CH3OH abundance.
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Figure 9: Ratio of the integrated intensity of the H2CO 30,3-20,2 transition to the 32,2-22,1. The solid black contours show the integrated intensity of the 30,3-20,2 line (levels are from 1.8 K km s-1 in steps of 1.8). Typical errors along the Bar are of the order of 0.3. |
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Figure 10:
|
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Figure 11: Results of the statistical equilibrium calculations for CH3OH. The solid and dashed lines show the line intensity of the 50-40-A line as measured towards the interclump medium (0.06 K) for temperatures of 20 and 80 K, respectively, as functions of density and column density of CH3OH-A. |
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Given an estimate of H2 column density, we could
also infer the
abundance of both molecules relative to molecular hydrogen. An
estimate of the H2 column density can be derived
from the continuum
emission at (sub)millimetre wavelengths, assuming a given temperature
for the dust and optically thin emission. The continuum emission at
350 m
of the Orion Bar was studied by
Lis et al. (1998).
These authors defined an average
temperature for the dust in the OMC-1 region of 55 K on the
basis of
its FIR colours (Werner
et al. 1976) and excluded colder
temperatures for the Orion Bar given the good agreement of the
350
m
continuum emission and the CO(6-5) emission, which
originated in the outer PDR layers. However,
Werner et al.
(1976) found an average FIR colour temperature of
75 K in the Orion Bar, which could imply that the Orion Bar is
significantly warmer than the rest of OMC-1. For a dust temperature
of 55 K in the interclump medium, and for the mean value of
the
350
m
emission over the region used to derive the average spectra
of the interclump medium (Fig. 1), we derive a
column
density of H2 of
cm-2.
This translates into
to
cm-2
for
K. A value of
0.101 g cm-2 was used for the
dust opacity
(Ossenkopf &
Henning 1994). For clump 1,
for a temperature of 45 K (as derived from methanol), and
under the assumption that the dust and the gas are
thermally coupled at
the high density of the clumps (e.g., Krügel & Walmsley 1984),
the H2column density is
cm-2.
This is not the average
column density on the beam, but it was corrected for a source size of
7'' and for the contribution of the interclump medium at the
position of clump 1. These values translate into abundances of
H2CO
in the range
for the interclump,
for the clumps. For methanol, we obtain an
abundance relative to H2 of
for the
interclump,
for the clumps.
However, these estimates are affected by large uncertainties:
a difference of 20 K in
the dust temperature implies an uncertainty of a factor of 2 in the
estimate of the H2 column density in
the interclump, uncertainty
that could be even greater if the average temperature in the region of
the interclump used in our analysis is higher than 75 K.
Similarly,
the estimate of the H2 column density
on the clumps is affected by
large uncertainties because of the assumption that the dust is
thermally
coupled to the gas at high densities and because of the large errors on
the
temperature of the gas from the methanol spectrum (28<T1<92 K,
20<T3<52 K).
On the other hand, the uncertainties in the determination of
the
abundance of CH3OH relative to H2CO
are related only to the
errors in the estimate of the column density of the two molecules,
which already include the uncertainties in the temperature.
Therefore, we believe
to be more
solidly estimated than
or
.
Using the column densities derived in the previous section, we infer
an abundance of CH3OH relative to H2CO
of the order 0.4-1.1 in
the dense clumps, and
in the interclump. If we
assume that the temperature of the interclump medium is higher than
50 K, then abundance of methanol drops down to
.
Since it is a reasonable assumption that
the temperature of the interclump medium is 50 K or even
higher, we conclude
that the abundance of methanol relative to formaldehyde decreases by
at least one order of magnitude in the interclump medium in comparison
to the dense clumps.
A possible explanation for the decrease in abundance of methanol relative to formaldehyde from the clumps to the interclump medium is photodissociation in a low-density enviroment. Although the photodissociation rates of CH3OH and H2CO are of the same order of magnitude (Le Teuff et al. 2000), one of the products of the photodissociation of methanol is formaldehyde. Therefore, both molecules can be destroyed by the radiation field in the interclump medium, but H2CO may be replenished through the photodissociation of CH3OH. However, large uncertainties affect the measurement of the photodissociation rates, and so more accurate models would be required to test this hypothesis. Alternatively, as already discussed by Leurini et al. (2006), H2CO could be formed in the interclump medium through gas phase reactions, which are not efficient for the formation of CH3OH (Luca et al. 2002).
6 Conclusion
In a previous analysis of methanol and formaldehyde towards the Orion Bar, we suggested that the two molecules might trace different environments and that, while CH3OH is associated with the clumpy molecular cores of the Bar, H2CO is found in the interclump material. To test this hypothesis, we mapped the Orion Bar in both molecular species with the IRAM-30 m telescope, with a spatial resolution slightly higher than the expected size of the clumps. Additional data were taken with the IRAM Plateau de Bure Interferometer in H2CO.
Both molecules are detected in the Orion Bar in our
single-dish
data. Our data show that CH3OH peaks towards the
clumps of the Bar,
but its intensity decreases below the detection threshold in the
interclump at individual positions. By averaging over a large region
of the interclump medium, the strongest of the CH3OH
lines in our
setup (50-40-A)
is detected with a peak intensity of 0.06 K.
On the other hand, contrary to the hypothesis formulated in our
previous study, formaldehyde is detected towards the clumps and the
interclump gas.
Using an LVG program, we studied the excitation of H2CO
and CH3OH
in the Orion Bar. We suggest that formaldehyde is present in both
components of the Bar (clumps and interclump material), with column
densities up to cm-2
in the clumps and between
cm-2
in the interclump. These values only
refer to para-formaldehyde. From the analysis of methanol, we
concluded that the reason for the drop of intensity of CH3OH
in the
interclump medium is not the different physical conditions
with respect to the clumps, but a real drop in its column density
compared to the clumps (down to
cm-2
for
temperatures above 50 K). The abundance of methanol relative
to
formaldehyde decreases by at least one order of magnitude in the
interclump medium compared to that in the dense clumps.
Despite the large errors on our estimates, our observations reveal that the column density of methanol and formaldehyde in the clumps are of the same order of magnitude, while the column density of methanol in the interclump is lower than that of formaldehyde. This may be a result of photodissociation of CH3OH in the unshielded interclump gas, or it may reflect that H2CO can be produced in the gas phase more efficiently than CH3OH. Definitive observational conclusions would require a much deeper integration towards the interclump gas and a better characterisation of its density and temperature. More detailed chemical models of PDRs, including grain surface reactions, are clearly needed to properly interpret these observational results.
AcknowledgementsWe are grateful to Helmut Wiesemeyer for his support before and during the observations, and to the observers who obtained part of the data presented in this work during pooled observations at the IRAM-30 m telescope.
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Footnotes
- ... region
- Based on observations carried out with the IRAM-30 m telescope and the Plateau de Bure interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).
- ...
- http://www.iram.es/IRAMES/telescope.html
- ... GILDAS
- See http://www.iram.fr/IRAMFR/GILDAS for more information about the GILDAS softwares.
All Tables
Table 1: Detected methanol and formaldehyde transitions in the IRAM-30 m telescope data.
Table 2: Line parameters.
Table 3: Summary of the model results.
All Figures
![]() |
Figure 1:
In grey scale, the integrated intensity of the H2CO
(
30,3-20,2)
line ( left) and of the CH3OH-A
(50-40)transition
(
right) observed with the
IRAM-30 m telescope. Black contours (4, 8, and 12
times 0.032 Jy beam-1 km s-1)
show the distribution of the H13CN
(1-0) emission observed with the Plateau de Bure interferometer
(Lis & Schilke 2003).
For H2CO the contours start from
1.8 K km s-1 in steps
of 1.8 (equal to 3 |
Open with DEXTER | |
In the text |
![]() |
Figure 2: Top: uv coverage for one field of the hybrid 30 m+PdBI mosaic. Bottom: associated dirty beam. Please note that the dirty beam has many secondary side-lobes as high as 40% of the main lobe. |
Open with DEXTER | |
In the text |
![]() |
Figure 3: Spectrum of the H2CO ( 3Ka,Kc-2Ka,Kc-1) band towards clump 1 ( top) and averaged over the interclump medium ( bottom) from the IRAM-30 m telescope. |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Spectrum of the CH3OH (5k-4k)
band towards clump 1 at a resolution of
|
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Distribution of the CO(6-5) peak brightness temperature
(colour image). The black contours (from 40 |
Open with DEXTER | |
In the text |
![]() |
Figure 6: Variation in the integrated intensity of the CH3OH-A(50-40) and H2CO 32,2-22,1 lines observed with the IRAM-30 m telescope across clumps 1 (a), the interclump medium between clump 1 and 2 (b) and along the Bar (c). In Fig. 1 we outline the strips used to extract the plots. |
Open with DEXTER | |
In the text |
![]() |
Figure 7: Channel maps of the CH3OH-A (50-40) emission between 11.3 and 7.4 km s-1 observed with the IRAM-30 m telescope. |
Open with DEXTER | |
In the text |
![]() |
Figure 8: Top line: IRAM-30 m data. Bottom line: hybrid 30 m+PdBI deconvolved data cube. Left column: images of the integrated line intensity of the H2CO ( 30,3-20,2) transition between 8.3 and 11.7 km s-1. The values of the contour levels are shown on the colour scale. Middle column: spectra averaged over the whole interferometric field-of-view marked as the red circle on the images. Right column: spectra averaged over the central clump marked with the blue ellipse on the images. |
Open with DEXTER | |
In the text |
![]() |
Figure 9: Ratio of the integrated intensity of the H2CO 30,3-20,2 transition to the 32,2-22,1. The solid black contours show the integrated intensity of the 30,3-20,2 line (levels are from 1.8 K km s-1 in steps of 1.8). Typical errors along the Bar are of the order of 0.3. |
Open with DEXTER | |
In the text |
![]() |
Figure 10:
|
Open with DEXTER | |
In the text |
![]() |
Figure 11: Results of the statistical equilibrium calculations for CH3OH. The solid and dashed lines show the line intensity of the 50-40-A line as measured towards the interclump medium (0.06 K) for temperatures of 20 and 80 K, respectively, as functions of density and column density of CH3OH-A. |
Open with DEXTER | |
In the text |
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