Transcript H2CO and CO in S140

Anomalous H2CO Absorption in the L1204/S140 Region
and a Comparison with the CO (1-0) Emission
Mónica I. Rodríguez1,2, Laurent Loinard1, Ron J. Allen2,
Vladimir Escalante and Tommy Wiklind2
1: Centro de Radiostronomía y Astrofísica, Universidad Nacional Autónoma de México, Apartado Postal 72--3 (Xangari), 58089 Morelia, Michoacán, México
m.rodriguez, l.loinard, [email protected]
2: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 US
monica, rjallen, [email protected]
207th AAS Meeting
Washington, DC
8-12 January 2006
Abstract :
We report observations of the region including the S140 H arc and the molecular/dust cloud L1204 with the Onsala 25-m telescope in the 6 cm (111-110)
transition of H2CO. This spectral line is seen here in absorption against the cosmic microwave background, and is a tracer for the presence of relatively cold
molecular gas of intermediate density. We have detected H2CO absorption in 16 pointing positions. We compare our results to earlier maps of the region in
the CO(1-0) line. In general the distributions of H2CO and CO are similar, but there are some notable differences including the fact that the maximum H2CO
absorption is clearly separated by a full 10' (~ 3 pc) from the CO peak. This CO peak is nearly coincident with the H arc S140. From a consideration of a
different sensitivities of H2CO and CO to local excitation conditions, we conclude that the CO emission in this region is bright because of local heating. The
H2CO 6-cm line apparently traces the bulk of the molecular gas, which is located behind S140. The H2CO spectra show a complex velocity structure with at
least two main parts to the L1204 molecular cloud.
Introduction :
Results :
H2 lacks a permanent dipole moment, therefore is a poor emitter of
radiation
Formaldehyde absorption was clearly detected in 16 of our 77 observed positions (Fig. 1). We compare our results to earlier maps of the
region in the CO(1-0) line (Fig. 2). In general the CO emission and the H2CO absorption exhibit similar morphologies, but there are
some notable differences including the fact that the maximum H2CO absorption is clearly separated by a full 10' (~ 3 pc) from the CO
peak (Fig 1). Thus, we confirm that the strongest H2CO absorption does not occur near the Hα arc where the CO emission is brightest.
How can we explain these differences?
 Emission lines of simple non-symmetric polar molecules are
needed to study cold gas
CO (J = 1-0), at 115.27 GHz is usually optically thick, so its
intensity depends on the kinetic temperature, of the emitting gas. All
molecular traces share these disadvantages
 Absorption lines can be detected even in very cold gas

The 6-cm (111-110) transition of ortho-formaldehyde has been
observed in absorption against the CMB (Palmer et al. 1969).

The line is an indicator of the presence of relatively low temperature
(8 K < T < 30 K) and intermediate density (100 cm-3< n < 105 cm-3)
gas
Fig. 2.-The first panel shows the comparison between the CO emission (white
contourns) and H2CO absorption (red contourns) on the optical DSS-red image of S140
at -12 km s-1 < VLSR < -10 km s-1. The second panel shows the comparison between
CO emission (white contourns) and H2CO absorption (red contourns) on the optical
DSS-red image of the principal component of S140 at -10 km s-1 < VLSR < -5 km s-1.
Our study is aimed in order to test the hypothesis that clouds of
cold, dense molecular gas may exist in regions where the emission
tracers (such as CO (1-0)) are weak or absent
The L1204/S140 Region :
Fig. 1.- Square grid of H2CO (left) and CO (rigth) spectra at 77 positions toward S140. The (0,0) position
correspond to l = 107o and b = +5.3o and the offsets are in l and b.
Lynds 1204, is centered at l = 107o.47, b = +4o.82. Covers a 2.5
square degree area (Lynds, 1962).
Discussion :
Located at the southwest edge there is a prominent compact HII
region (S140 − Sharpless 1959).
S140 appears as an Hα emission arc. The ionization is driven by the
nearby star HD211880.
The distance to the region is 910 pc (Crampton & Fisher 1974).
Fig. 3.- Brightnees temperature minus backgroung continuum
for different kinetic temperatures T of a 1-pc thick slab.
As we can see in Fig. 1 and Fig. 2, in general the distribution of CO emission and
H2CO absorption match rather well, it seems that both molecules traces the same
region. There is a disagreement in detailed way. When we compared the CO
emission and H2CO absorption to S140 region (Fig. 2). The maximum H2CO
absorption is a full 10' beam away from the CO peak. A possible explanation of this
disagreement could be for two reasons:
1.- The peak brightness of the optically thick CO line increases monotonically with
the kinetic temperature of the emitting gas. We argue that in S140, the CO is bright
because the gas is warm (Timmermann et al. 1996) due to photon heating the nearly
B star and due to close star formation region. Therefore the CO molecule looks like
a temperature tracer.
CO emission peaks on the infrared sources regions (Helfer & Blitz
1997, Heyer et al. 1996, Evans et al. 1987, Blair et al. 1978).
The 6-cm line of H2CO was first detected in L1204 near S140 by
Blair et al. (1978), and unexpectedly by Evans et al. (1987) in north
west of S140.
Fig. 4.- Brightnees temperature minus backgroung continuum
for different slab thicknesses at T = 40 K.
Interestingly, both authors found that the peak of H2CO absorption
was not strongest where the CO is brightest, but deeper into the dust
cloud.
Conclusions :
Observations :
 Formaldehyde absorption was clearly detected
 January, September-October 2004
 25.6 m telescope (OSO)
 Angular resolution 10’
 Frequency : 4829.660 MHz
 Bandwidth of 96 km s-1
 Velocity resolution 0.12 km s-1
 Vlsr = − 8.0 km s-1
 77 positions centered at l = 107o, b = +5.3o
 The system temperature 33 to 36 K
 Typical final noise level of 3 mK (TA*)
in 16 of our 77 observed positions (Fig. 1)
 In general the CO emission and the H2CO
absorption exhibit similar morphologies
 The maximum H2CO absorption is clearly
separated by a full 10' (~ 3 pc) from the CO peak
(Fig. 1 and Fig. 2)
 The CO molecule looks like a temperature
tracer, and the H2CO is dissociated close to
photodissociation front.
 Toward south west of the figure, the region
seems to be low density.
2.- The different shielding of carbon monoxide and formaldehyde affects the
CO/H2CO abundance. We see in the Fig. 2 that the formaldehyde and carbon
monoxide lines peak at different locations. Ungerechts et al. (1986) determined the
kinetic temperature varying from 40K to 20K within the distance 0.5pc from the
infrared sources in the S140 core. Using CS and NH3 observations and IRAS data
Tafalla et al. (1993) have suggested that the most dense cores (n(H2) > 105 cm-3) in
the complex are associated with the very red IRAS sources. In this case follow the
model in the Fig. 3 and Fig. 4 we would expect H2CO emission in the S140 core
which was not detected.
For these reason we expect the H2CO to be concentrated far from the
photodissociation front and dissociated close to the star formation region. Toward
south east of the figure, the region seems to be low density since there is CO
emission but the H2CO absorption is weak or absent.
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