2. Observations and results
The observations have been carried out with the LWS spectrometer (Clegg et al. 1996) on board the ISO satellite as part of our open time programs. The LWS grating spectra of SgrB2 were obtained during revolution 287 using the AOT LWS01. The spectra consist of 3 grating scans taken with 0.5 sec integration ramps at each commanded grating position and were oversampled at 1/4 of a resolution element. 1 The flux calibration of the spectra is relative to Uranus (Swinyard et al. 1996). The total on-target time was 2775 sec for each raster (north-south and east-west and centered on SgrB2) of 7 points each with a spacing. The data have been corrected for the interference pattern which is systematically seen in the LWS grating spectra of extended sources or point sources which are offset from the optical axis (Swinyard et al. 1996) - see Figs. 1a and b. The central position was also observed at higher spectral resolution in two H2 O, one H2 18 O and two OH lines using the LWS Fabry-Perot (AOT LWS04) during revolution 322. The spectra were over-sampled at 1/2 of a resolution element yielding a spectral resolution of 30 kms-1. All data used in this Letter have been processed through the LWS Pipeline Version 6.0.
The grating spectrum toward the central position shows the continuum of the dust emission plus a series of absorption lines (Fig. 1b). The strongest absorption at 179.5 µm corresponds to the water vapour 212 -101 ortho transition. To illustrate the line content of the spectra, Fig. 1c displays the 160 to 196 µm line over continuum ratio of SgrB2. The grating spectra at positions offset from SgrB2 are shown in the central part of Fig. 1. The most striking result from the raster map toward SgrB2 is the presence of the water vapour 179.5 µm line in absorption in all the positions observed. At the resolution of the grating ( 1000 km s-1), the absorption remains strong and roughly constant over the raster with an absorption depth of 15%, consistent with saturated water vapour lines with a line width of 150 kms-1. The 303 -212 H2 O transition at 174.5 m is also present in all the raster positions with an absorption depth of 1%. The absorption at 180.5 m (detected in each position) could be a blend of the 221 -212 H2 O transition at 180.488 µm, the CH line at 180.478 µm and the 212 -101 H O line at 181.05 µm. The other absorption features present in the spectra are seen in all the individual scans and observed positions, although many of them are stronger towards the central position (Fig. 1c). Some of them could be real but further analysis is required to assess their reality. We note that transitions of light molecular species and low energy bending modes of polyatomic molecules are expected at these wavelengths.
The Fabry-Perot spectra obtained toward the central position of SgrB2 are presented in Fig. 2. The upper panel shows the H2 O 212 -101 and 313 -212 transitions and the lower panel the 212 -101 transition of the isotope H O. For comparison, the middle panel displays the 119 and 84 µm transitions of OH. The OH spectra will be discussed in detail in a forthcoming paper. The 179.5 µm transition of water vapour, whose lower energy level is the o-H2 O ground state (Fig. 1e), is seen in absorption at all velocities between -150 and 100 km s-1. The line is saturated as expected from the grating spectrum. The H2 18 O transition is also detected over a wide velocity range (-50 to 80 ). Water vapour is thus detected in absorption not only at the velocities corresponding to the SgrB2 complex but also at more negative velocities which trace the gas along the line of sight toward SgrB2. This is similar to the OH transitions at 119 and 84 µm (Fig. 2d- see also Storey et al. 1981). In contrast, the 174.6 µm H2 O is only detected at 60 km s-1, the velocity of SgrB2.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998