Astronomy and Astrophysics, volume 383, 603-613 (2002/2-4)
ISO ammonia line absorption reveals a layer of hot gas veiling Sgr B2.
CECCARELLI C., BALUTEAU J.-P., WALMSLEY M., SWINYARD B.M., CAUX E., SIDHER S.D., COX P., GRY C., KESSLER M. and PRUSTI T.
Abstract (from CDS):
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 500cm–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 104cm–3. The total NH3 column density in the layer is (3±1)x1016cm–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.
infrared: ISM: lines and bands - ISM: individual: Sgr B2 - stars: formation - techniques: spectroscopic
in ref list MARTIN 1972ApL....11..219M instead of ApJ.11.
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