Anomalous H 2 CO Absorption in the L1204/S140 Region and a Comparison with the CO (1-0) Emission Mónica I. Rodríguez 1,2, Laurent Loinard 1, Ron J. Allen 2, Vladimir Escalante and Tommy Wiklind 2 1: Centro de Radiostronomía y Astrofísica, Universidad Nacional Autónoma de México, Apartado Postal (Xangari), Morelia, Michoacán, México m.rodriguez, l.loinard, 2: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD US monica, rjallen, 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 ( ) transition of H 2 CO. 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 H 2 CO 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 H 2 CO and CO are similar, but there are some notable differences including the fact that the maximum H 2 CO 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 H 2 CO and CO to local excitation conditions, we conclude that the CO emission in this region is bright because of local heating. The H 2 CO 6-cm line apparently traces the bulk of the molecular gas, which is located behind S140. The H 2 CO spectra show a complex velocity structure with at least two main parts to the L1204 molecular cloud. Introduction : H 2 lacks a permanent dipole moment, therefore is a poor emitter of radiation  Emission lines of simple non-symmetric polar molecules are needed to study cold gas CO (J = 1-0), at 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 ( ) 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 < 10 5 cm -3 ) gas 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 : Lynds 1204, is centered at l = 107 o.47, b = +4 o.82. Covers a 2.5 square degree area (Lynds, 1962). 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 HD The distance to the region is 910 pc (Crampton & Fisher 1974). 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 H 2 CO was first detected in L1204 near S140 by Blair et al. (1978), and unexpectedly by Evans et al. (1987) in north west of S140. Interestingly, both authors found that the peak of H 2 CO absorption was not strongest where the CO is brightest, but deeper into the dust cloud. Bibliography : Blair, G.N., Evans, N.J., Vanden Bout, P.A., & Peters W.L., 1978, ApJ, 219, Crampton, D., & Fisher, W.A., 1974, Pub. Dom. Astrophys. Obs., 14, 283 Evans, N..J., II, Kutner, M.L., & Mundy, L.G., 1987, ApJ, 323, 145 Helfer, T.T., & Blitz, L., 1997, ApJ, 478, 233 Heyer, M..H., Carpenter, J..M., & Ladd, E.F., 1996, ApJ, 463, 630 Lynds, B.T., 1962, ApJS, 7, 1 Fig. 1.- Square grid of H 2 CO (left) and CO (rigth) spectra at 77 positions toward S140. The (0,0) position correspond to l = 107 o and b = +5.3 o and the offsets are in l and b. Fig. 2.-The first panel shows the comparison between the CO emission (white contourns) and H 2 CO absorption (red contourns) on the optical DSS-red image of S140 at -12 km s -1 < V LSR < -10 km s -1. The second panel shows the comparison between CO emission (white contourns) and H 2 CO absorption (red contourns) on the optical DSS-red image of the principal component of S140 at -10 km s -1 < V LSR < -5 km s -1. Observations :  January, September-October 2004  25.6 m telescope (OSO)  Angular resolution 10’  Frequency : MHz  Bandwidth of 96 km s -1  Velocity resolution 0.12 km s -1  V lsr = − 8.0 km s -1  77 positions centered at l = 107 o, b = +5.3 o  The system temperature 33 to 36 K  Typical final noise level of 3 mK (T A *) Results : 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 H 2 CO absorption exhibit similar morphologies, but there are some notable differences including the fact that the maximum H 2 CO absorption is clearly separated by a full 10' (~ 3 pc) from the CO peak (Fig 1). Thus, we confirm that the strongest H 2 CO absorption does not occur near the Hα arc where the CO emission is brightest. How can we explain these differences? 207th AAS Meeting Washington, DC 8-12 January 2006 Fig. 3.- Brightnees temperature minus backgroung continuum for different kinetic temperatures T of a 1-pc thick slab. Fig. 4.- Brightnees temperature minus backgroung continuum for different slab thicknesses at T = 40 K. Discussion : As we can see in Fig. 1 and Fig. 2, in general the distribution of CO emission and H 2 CO 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 H 2 CO absorption to S140 region (Fig. 2). The maximum H 2 CO 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. 2.- The different shielding of carbon monoxide and formaldehyde affects the CO/H 2 CO 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 NH 3 observations and IRAS data Tafalla et al. (1993) have suggested that the most dense cores (n(H 2 ) > 10 5 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 H 2 CO emission in the S140 core which was not detected. For these reason we expect the H 2 CO 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 H 2 CO absorption is weak or absent. Conclusions :  Formaldehyde absorption was clearly detected in 16 of our 77 observed positions (Fig. 1)  In general the CO emission and the H 2 CO absorption exhibit similar morphologies  The maximum H 2 CO 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 H 2 CO is dissociated close to photodissociation front.  Toward south west of the figure, the region seems to be low density. Palmer, P. Zuckerman, B. Buhl, D. & Snyder, L.E ApJ 156, 147 Tafalla, M., Bachiller, R. & Martin-Pintado, J. 1993, ApJ, Sharpless, S. 1959, ApJS, 4, 257 Ungerechts, H., Winnewisser, G. & Walmsley, C. M., 1986, A&A