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Anomalous formaldehyde 6-cm CMB absorption in
the L1204/S140 region Mónica I. Rodríguez1,
Laurent Loinard1, Ron J. Allen2 and Tommy
Wiklind 1 Centro de Radiostronomía y
Astrofísica, Universidad Nacional Autónoma de
México, Apartado Postal 72--3 (Xangari), 58089
Morelia, Michoacán, México m.rodriguez,
l.loinard_at_astrosmo.unam.mx 2 Space
Telescope Science Institute, 3700 San Martin
Drive, Baltimore, MD 21218 US rjallen,
wiklind_at_stsci.edu
Submillimeter Astronomy in the era of the SMA
Harvard University, Cambridge, MA USA
Harvard-Smithsonian Center for Astrophysics June
13-16, 2005
Abstract The region surrounding the
Sharpless 140 HII region and the associated
dust/molecular cloud Lynds 1204 has been observed
with the Onsala 25-m telescope in the H2CO(111-
110) transition. This spectral line is seen here
in absorption against the microwave background,
and is a tracer for the presence of relatively
cold and moderately dense molecular material. We
have detected absorption in 16 pointing
positions, and confirmed that the strongest
absorption occurs deep in the cloud rather than
on its surface where the CO(J 1-0) emission is
brightest. We argue that the CO emission is
strong on the edge of the cloud because of local
heating by a nearby B0 star, but that the H2CO
6-cm line may be a better tracer of the bulk of
molecular gas, located behind the PDR.
Introduction Since H2 lacks a permanent
dipole moment, it is difficult to detect
directly. Thus, the structure and properties of
cool molecular clouds are usually studied using
emission lines of simple non-symmetric polar
molecules, the first rotational transition (J
1-0) of carbon monoxide (CO), at 115.27 GHz being
the most popular. However, this transition is
well-known to be nearly always optically thick,
so its intensity is expected to increase
monotonically with the kinetic temperature of the
emitting gas. Clearly, this could have adverse
effects on the distribution of molecular gas
deduced from CO observations. On the other hand,
absorption lines can be detected even in very
cold gas, but sufficiently bright background
continuum sources need be present. The scarcity
of such sources at the wavelenghts of the common
molecular tracers has limited the usefulness of
absorption measurements. The 6-cm (111-110)
transition of ortho-formaldehyde H2CO offers an
interesting alternative, because it can be
observed in absorption against the cosmic
microwave background (Palmer et al. 1969).
Although the absorption line is weak and its
strength depends on physical conditions, the line
is an indicator of the presence of relatively low
temperature (8 K lt T lt 30 K) and intermediate
density (100 cm-3lt n lt 105 cm-3) gas.
The L1204/S140 Region The dark dust cloud
Lynds 1204, is centered at l 107o.47, b
4o.82 and covers a 2.5 square degree area
according to Lynds (1962). Located at the
southwest edge there is a prominent compact HII
region (S140 - Sharpless 1959). When seen with
sufficient angular resolution, S140 appears as an
Ha emission arc. The ionization is driven by the
nearby star HD211880. The distance to the region
is 910 pc (Crampton Fisher 1974). S140 has been
the subject of many observational studies, which
have focused on the PDR on the edge of L1204, and
on the embedded infrared sources located right
behind it (e.g. Preibisch et al. 2001 Hayashi et
al. 1987 Preibisch Smith 2002 Bally et al.
2002). While the dust cloud is seen as an
extended dark feature covering more than two
square degrees, the CO emission peaks on the
infrared sources regions (Helfer Blitz 1997,
Heyer et al. 1996, Evans et al. 1987, Blair et
al. 1978). Remarkably, only relatively dim
emission extends further into the cloud (Fig. 1).
The 6-cm line of H2CO was first detected in L1204
near S140 by Blair (1978), and unexpectedly by
Evans et al. (1987) in north west of S140.
Interestingly, both authors found that the peak
of H2CO absorption was not strongest where the CO
is brightest, but deeper into the dust cloud.
Observations The data were obtained during
two observing sessions (January,
September-October 2004, respectively) with the
25.6 m telescope of the Onsala Space Observatory
(OSO) in Sweden. At 6 cm, the angular resolution
of the 25 m is about ___
10, and our pointing precision is better than
20. Frequency- switching was used. At the
observed frequency of 4829.660 MHz, our setup
provided a total bandwidth of 96 km s-1 for a
velocity resolution 0.12 km s-1. The spectrometer
was centered at the systemic velocity of S140,
Vlsr - 8.0 km s-1. The system temperature
during our observations varied from 33 to 36
K. In order to map the entire region behind S140,
we observed 36 positions centered at l 107o, b
5.3o. The total integration time for each of
these positions was 10 hours, yielding a typical
final noise level of 3 mK (TA) per 0.12 km s-1
velocity channel. The data processing was done
with CLASS.
Fig. 2.- Square grid of H2CO spectra at 36
positions toward S140. The (0,0) position
correspond to l 107o and b 5.3o and the
offsets are in l and b.
Fig. 1.-The first panel shows the comparison
between the CO and the optical DSS-red image of
S140. The second panel shows the comparison
between the H2CO and the optical DSS-red image of
S140. Note the decrease of the CO brightness
moving away from the Ha arc, while the H2CO
absorption covers a big region which extends
further, and resembles more the dust absorption
feature.
Results Formaldehyde absorption was clearly
detected in 16 of our 36 observed positions (Fig.
2). The maximum absorption is located near the
center of our grid, 10' from the CO peak (Fig.
1). Thus, we confirm that the strongest H2CO
absorption does not occur near the Ha arc where
the CO emission is brightest, but deeper into the
dust cloud. Also, the H2CO absorption feature is
more than half a degree across, significantly
more extended than the CO peak (Fig. 1). In
summary, although both are acknowledged tracers
of molecular material, the CO emission and the
H2CO absorption exhibit quite different
morphologies. How can we explain these
differences?
Discussion In the molecular ISM, the CO J
1-0 line is nearly always optically thick, and is
therefore expected to be a better tracer of
temperature than of column density. Thus, CO(1-0)
observations of the molecular ISM will tend to be
biased toward warmer regions (Tilanus Allen
1993). From the SWAS detection of the CO(5-4)
transition toward the bright CO(1-0) peak just
behind the S140 arc, it is known that the
molecular gas there is at a kinetic temperature
above 60 K (Ashby et al. 2000, Ashby et al.
2000). The molecular gas located deeper into the
cloud, however, is expected to be much cooler
(typically 10 K). But at Tkin 60 K, the
brightness of the CO(1-0) transition is expected
to be about 60 K, whereas at Tkin 10 K, it is
expected to be about 7 K, ten times lower. Thus,
we naturally expect the CO(1-0) emission to be
much brighter just behind the Ha arc than in the
rest of the dust cloud, independently of where
the column density is highest. For the H2CO
absorption to be detectable, on the other hand,
the molecular gas must be cool and moderately
dense, but the exact conditions will not greatly
affect the optical depth of the line the 6 cm
line of formaldehyde is a poor diagnostic of
physical conditions, but a good tracer of the
mere presence of molecular gas. Consequently, we
argue that the H2CO gives a fair picture of the
overall distribution of molecular gas in the
L1204/S140 region, whereas the CO(1-0)
distribution provides a picture of the
temperature distribution in the region.
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Conclusions We have obtained a large scale map
of the L1204/S140 region in the 6-cm line of
formaldehyde. The morphology of the formaldehyde
absorption feature is quite different from that
of the associated CO(1-0) emission, and we argue
that the H2CO provides a better description of
the total extent of the molecular gas
distribution, whereas the CO is largely biased
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