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Abstract. We report the first results of the unbiased spectral high resolution survey obtained towards Sgr B2 with the Long Wavelength Spectrometer on board ISO. The survey detected more than one hundreds lines from several molecules. Ammonia is the molecule with the largest number (21) of detected lines in the survey. We detected NH3 transitions from levels with energies from 45 to 500 cm −1 . The detected transitions are from both para and ortho ammonia and metastable and non-metastable levels. All the ammonia lines are in absortion against the FIR continuum of Sgr B2. With such a large number of detected lines in such a large range of energy levels, we could very efficiently constrain the main parameters of the absorbing gas layer. The gas is at (700 ± 100) K and has a density lower than 10 4 cm −3 . The total NH3 column density in the layer is (3 ± 1) × 10 16 cm −2 , equally shared between ortho and para ammonia. Given the derived relatively high gas temperature and ammonia column density, our observations support the hypothesis previously proposed of a layer of shocked gas between us and Sgr B2. We also discuss previous observations of far infrared line absorption from other molecules, like H2O and HF, in the light of this hot absorbing layer. If the absorption is done by the hot absorbing layer rather than by the warm envelope surrounding Sgr B2, as was previously supposed in order to interpret the mentioned observations, the derived H2O and HF abundances are one order of magitude larger than previously estimated. Yet, the present H2O and HF observations do not allow one to disentangle the absorption from the hot layer against the warm envelope. Our conclusions are hence that care should be applied when interpreting the absorption observations in Sgr B2, as the hot layer clearly seen in the ammonia transitions may substantially contribute to the absorption.

Abstract. We present a clear detection of CH 2 in absorption towards the molecular cloud complexes Sagittarius B2 and W49 N using the ISO Long Wavelength Spectrometer. These observations represent the first detection of its low excitation rotational lines in the interstellar medium. Towards Sagittarius B2, we detect both ortho and para transitions allowing a determination of the total CH 2 column density of N(CH 2 ) = (7.5± 1.1)× 10 14 cm −2 . We compare this with the related molecule, CH, to determine [CH/CH 2 ] = 2.7±0.5. Comparison with chemical models shows that the CH abundance along the line of sight is consistent with diffuse cloud conditions and that the high [CH/CH 2 ] ratio can be explained by including the effect of grain-surface reactions.

Abstract. The giant molecular cloud Sagittarius B2, located near the Galactic Centre, has been observed in the far-infrared by the ISO Long Wavelength Spectrometer. Wavelengths in the range 47-196 µm were covered with the high resolution Fabry-Pérot spectrometer, giving a spectral resolution of 30-40 km s −1 . The J = 1 → 0 and J = 2 → 1 rotational transitions of HD fall within this range at 112 µm and 56 µm. A probable detection was made of the ground state J = 1 → 0 line in emission but the J = 2 → 1 line was not detected above the noise. This allowed us to calculate an upper limit on the temperature in the emitting region of approximately 80 K and a value for the deuterium abundance in the Sgr B2 envelope of D/H = (0.2−11) × 10 −6 .

Broad emission lines λλ 682.5, 708.2 nm, observed for years in the spectrum of symbiotic stars, have been identified by Schmid (1989) as due to Raman scattering of O VI 103.2, 103.6 nm by H0. In one symbiotic star, van Groningen (1993) detected Raman scattered He II.

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