Odin observations of H2O in the Galactic Centre

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

Assimilation of Odin/SMR O₃ and N₂O measurements in a three-dimensional chemistry transport model.

A method for assimilating observations of long-lived species such as ozone (O3) and nitrous oxide (N2O) in a three-dimensional chemistry transport model (3D-CTM) is described. The model is forced by the temperature and wind analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF). The O3 and N2O fields used in this study are obtained from the Sub-Millimeter Radiometer (SMR) aboard the Odin satellite. The assimilation technique used is the sequential statistical interpolation approach. The parametrization of the error covariance matrix of the model forecast field is described. A sensitivity study of the system parameters is done in terms of the OMF (observation minus forecast) vector also called “innovation” vector and in terms of the χ2 (chi-square) test. The effect of the correlation distances is critical for the assimilated field. The RMS (root mean square) of the OMF for the correlation distances is minimal for values of 1500 km in the meridional direction and 500 km in the zonal direction for both O3 and N2O. The treatment of the meridional distance as a function of latitude does not reveal an important improvement. The χ2 diagnostic shows that the asymptotic value of the model error (the model error of saturation) is optimal for the value of 12.5% for O3 and 18% for N2O. We demonstrate the applicability of the developed assimilation method for the Odin/SMR data. We also present first results of the assimilation of Odin/SMR ozone and nitrous oxide for the period from 22 December 2001 to 17 January 2002

Low upper limits on the O2 abundance from the Odin satellite

For the first time, a search has been conducted in our Galaxy for the 119 GHz transition connecting to the ground state of O2, using the Odin satellite. Equipped with a sensitive 3 mm receiver (Tsys(SSB) = 600 K), Odin has reached unprecedented upper limits on the abundance of O2, 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(O2)/N(H2) ≤ 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 O2 abundances. Goldsmith et al. (2002) recently reported a SWAS tentative detection of the 487 GHz transition of O2 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 O2 column density is 7.3 × 1015 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

Odin water mapping in the Orion KL region

New results from water mapping observations of the Orion KL region using the submm/mm wave satellite Odin (2.1′ beam size at 557 GHz), are presented. The ortho-H2O JK(+)K(-) = l1.0 → l0.1 ground state transition was observed in a 7′ × 7′ rectangular grid with a spacing of 1′, while the same line of H2 18O was measured in two positions, Orion KL itself and 2′ south of Orion KL. In the main water species, the KL molecular outflow is largely resolved from the ambient cloud and it is found to have an extension of 60″-110″. The H2O outflow profile exhibits a rather striking absorption-like asymmetry at the line centre. Self-absorption in the near (or "blue") part of the outflow (and possibly in foreground quiescent halo gas) is tentatively suggested to play a role here. We argue that the dominant part of the KL H2 18O outflow emission emanates from the compact (size ∼15″) low-velocity flow and here estimate an H2O abundance of circa 10-5 compared to all H2 in the flow - an order of magnitude below earlier estimates of the H2O abundance in the shocked gas of the high-velocity flow. The narrow ambient cloud lines show weak velocity trends, both in the N-S and E-W directions. H2 18O is detected for the first time in the southern position at a level of ∼0.15 K and we here estimate an H2O abundance of (1-8) × 10-8. (Note: Some of the scientific symbols can not be represented correctly in the abstract. Please read with caution and refer to the original publication.

Highlights from the first year of Odin observations

Key Odin operational and instrumental features and highlights from our sub-millimetre and millimetre wave observations of H2O, H2 18O, NH3, 15NH3 and O2 are presented, with some insights into accompanying Odin Letters in this A&A issue. We focus on new results where Odin's high angular resolution, high frequency resolution, large spectrometer bandwidths, high sensitivity or/and frequency tuning capability are crucial: H2O mapping of the Orion KL, W3, DR 21, S 140 regions, and four comets; H2O observations of Galactic Centre sources, of shock enhanced H2O towards the SNR IC 443, and of the candidate infall source IRAS 16293-2422; H2 18O detections in Orion KL and in comet Ikeya-Zhang; sub-mm detections of NH3 in Orion KL (outflow, ambient cloud and bar) and ρ Oph, and very recently, of 15NH3 in Orion KL. Simultaneous sensitive searches for the 119 GHz line of O2 have resulted in very low abundance limits, which are difficult to accommodate in chemical models. We also demonstrate, by means of a quantitative comparison of Orion KL H2O results, that the Odin and SWAS observational data sets are very consistently calibrated