ISO ammonia line absorption reveals a layer of hot gas veiling Sgr B2^*

¹ Observatoire de Bordeaux, BP 89, 33270 Floirac, France
² Laboratoire d'Astrophysique, Observatoire de Grenoble – BP 53, 38041 Grenoble Cedex 09, France
³ Laboratoire d'Astrophysique de Marseille, CNRS & Université de Provence, BP 8, 13376 Marseille Cedex 12, France
⁴ Osservatorio di Arcetri, 5 L. go E. Fermi, 50125 Firenze, Italy
⁵ Rutherford Appleton Laboratory, Chilton, Didcot OX1 3RH, UK
⁶ CESR CNRS-UPS, BP 4346, 31028 Toulouse Cedex 04, France
⁷ Institut d'Astrophysique Spatiale, 91405 Orsay, France
⁸ ISO Data Center, Astrophysics Division, Space Science Department of ESA, Villafranca del castillo, PO Box 50727, I28080 Madrid, Spain
⁹ Space Science Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands

Corresponding author: C. Ceccarelli, Ceccarelli.Cecilia@observ.u-bordeaux.fr

Received: 25 September 2001
Accepted: 20 November 2001

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 () K and has a density lower than 10⁴ cm^-3. The total NH₃ column density in the layer is  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.