Widespread far-infrared absorption of CH, OH and H₂O in Sgr B2

M.A. Orlandi , J. Cernicharo , E. González-Alfonso , P. Najarro , S. Perez , M.J. Sempere 

Consejo Superior de Investigaciones Cientificas. Instituto de Estructura de la Materia. Dpto Fisica Molecular. C/Serrano 121. 28006 Madrid. Spain

We present LWS-grating and LWS-FP data of the SgrB2 molecular cloud. Two full LWS-grating rasters, centered on SgrB2 and giving a total of 19 positions, have been obtained. In addition, a 7x7 positions raster, with an angular separation of 90", has been performed with the LWS-FP in the 179.5 m line of H₂O and the 119.5 m line of OH.

In the grating data we observed H₂O in absorption at positions as far as 15 arcminutes from SgrB2. Taking into account the H₂O map analyzed by Cernicharo et al. (1997), the present data indicates that water vapour is abundant in the direction of SgrB2 in a region as large as half a degree (i.e., about 80 parsecs in size). In all observed positions there is a line in absorption at 181 m that we belive to be the 2₁₂-1₀₁ of H₂¹⁸O at 181.053 m. The water abundance derived from these data is . The FP map around SgrB2 indicates that the line profile changes from point to point suggesting important variations in the excitation conditions. The OH lines absorb completely the continuum emission in all observed positions.

CH has been detected in all the positions observed with the grating. In addition, in the central position we have perfomed a search of several rotational lines of this molecule with the FP. Only the fundamental line was detected suggesting that CH is only abundant in the external parts of the cloud where collisions are not important enough to populate the high-J levels of CH. The averaged CH column density in our map is very similar to that found by Stacey et al (1987) with the KAO in the direction of SgrB2.