Context. Molecular oxygen, O 2 , has been expected historically to be an abundant component of the chemical species in molecular clouds and, as such, an important coolant of the dense interstellar medium. However, a number of attempts from both ground and from space have failed to detect O 2 emission. Aims. The work described here uses heterodyne spectroscopy from space to search for molecular oxygen in the interstellar medium. Methods. The Odin satellite carries a 1.1 m sub-millimeter dish and a dedicated 119 GHz receiver for the ground state line of O 2 . Starting in 2002, the star forming molecular cloud core ρ Oph A was observed with Odin for 34 days during several observing runs. Results. We detect a spectral line at v LSR = +3.5 km s −1 with ∆v FWHM = 1.5 km s −1 , parameters which are also common to other species associated with ρ Oph A. This feature is identified as the O 2 (N J = 1 1 −1 0 ) transition at 118 750.343 MHz. Conclusions. The abundance of molecular oxygen, relative to H 2 , is 5 × 10 −8 averaged over the Odin beam. This abundance is consistently lower than previously reported upper limits.

We have analyzed the properties of dust in the high galactic latitude translucent cloud Lynds 1780 using ISOPHOT maps at 100 µm and 200 µm and raster scans at 60 µm, 80 µm, 100 µm, 120 µm, 150 µm and 200 µm. In far-infrared (FIR) emission, the cloud has a single core that coincides with the maxima of visual extinction and 200 µm optical depth. At the resolution of 3.0 , the maximum visual extinction is 4.0 mag. At the cloud core, the minimum temperature and the maximum 200 µm optical depth are 14.9 ± 0.4 K and 2.0 ± 0.2 × 10 −3 , respectively, at the resolution of 1.5 . The cloud mass is estimated to be 18 M . The FIR observations, combined with IRAS observations, suggest the presence of different, spatially distinct dust grain populations in the cloud: the FIR core region is the realm of the "classical" large grains, whereas the very small grains and the PAHs have separate maxima on the Eastern side of the cold core, towards the "tail" of this cometary-shaped cloud. The color ratios indicate an overabundance of PAHs and VSGs in L1780. Our FIR observations combined with the optical extinction data indicate an increase of the emissivity of the big grain dust component in the cold core, suggesting grain coagulation or some other change in the properties of the large grains. Based on our observations, we also address the question, to what extent the 80 µm emission and even the 100 µm and the 120 µm emission contain a contribution from the small-grain component.

We present spectroscopic results of 146 water maser outbursts in W49 N, obtained with the Metsahovi radio telescope at 22 GHz. Combining this data with Gwinn's VLBI results, we were able to fix the free parameters in the shock model of Hollenbach and McKee and the maser model of Elitzur, Hollenbach, and McKee. This enabled a straightforward determination of some 20 shock and maser parameters, among others, the kinetic temperature, postshock density, water density, water abundance, preshock and postshock magnetic field strengths as well as the inclination of the mean field with respect to the line of sight. A step-by-step presentation of our diagnostic method is given and the relation between observations and model parameters is discussed. One uniquely powerful outburst feature, referred to as the "big flare feature", showed also the narrowest linewidth. Observations indicate that the velocity of this feature lies in the plane of the sky, whereas preshock and postshock magnetic fields are directed nearly along the line of sight. Consequently, Alfvenic wave fluctuations along the line of sight, and linewidth, are minimal, and a very high aspect ratio is achieved. Furthermore, the big flare feature stands out through its low space velocity, higher temperature (480 K), and larger preshock magnetic field strength (8.2 mG).Comment: 23 pages (html), 2 tables (html), 15 figures. To be published in ApJ 534, No 2 (May 10, 2000

Abstract. For the first time, a search has been conducted in our Galaxy for the 119 GHz transition connecting to the ground state of O 2 , using the Odin satellite. Equipped with a sensitive 3 mm receiver (T sys (SSB) = 600 K), Odin has reached unprecedented upper limits on the abundance of O 2 , especially in cold dark clouds where the excited state levels involved in the 487 GHz transition are not expected to be significantly populated. Here we report upper limits for a dozen sources. In cold dark clouds we improve upon the published SWAS upper limits by more than an order of magnitude, reaching N(O 2 )/N(H 2 ) ≤ 10 −7 in half of the sources. While standard chemical models are definitively ruled out by these new limits, our results are compatible with several recent studies that derive lower O 2 abundances. Goldsmith et al. (2002) recently reported a SWAS tentative detection of the 487 GHz transition of O 2 in an outflow wing towards ρ Oph A in a combination of 7 beams covering approximately 10 ×14 . In a brief (1.3 hour integration time) and partial covering of the SWAS region (≈65% if we exclude their central position), we did not detect the corresponding 119 GHz line. Our 3 sigma upper limit on the O 2 column density is 7.3 × 10 15 cm −2 . We presently cannot exclude the possibility that the SWAS signal lies mostly outside of the 9 Odin beam and has escaped our sensitive detector.

The Odin satellite has been used to detect emission and absorption in the 557-GHz H2O line in the Galactic Centre towards the Sgr A* Circumnuclear Disk (CND), and the Sgr A +20 km/s and +50 km/s molecular clouds. Strong broad H2O emission lines have been detected in all three objects. Narrow H2O absorption lines are present at all three positions and originate along the lines of sight in the 3-kpc Spiral Arm, the -30 km/s Spiral Arm and the Local Sgr Spiral Arm. Broad H2O absorption lines near -130 km/s are also observed, originating in the Expanding Molecular Ring. A new molecular feature (the ``High Positive Velocity Gas'' - HPVG) has been identified in the positive velocity range of ~ +120 to +220 km/s, seen definitely in absorption against the stronger dust continuum emission from the +20 km/s and +50 km/s clouds and possibly in emission towards the position of Sgr A* CND. The 548-GHz H2_18O isotope line towards the CND is not detected at the 0.02 K (rms) level.Comment: 5 pages, 3 figures, accepted by A&A, special Odin Letters issu