[HWC99] 5 , the SIMBAD biblio

We present Australia Telescope Compact Array observations of the 9-GHz, J=1-0, rotational transition of cyanoacetylene (HC₃N) arising in the dense molecular clouds associated with the prominent H II region Sgr B2. Images with a synthesized beam of 4.4x9.9 arcsec² show that the line emission is concentrated towards the 9-GHz continuum emission, and arises as a result of weak masing. The J=1-0 transition consists of three hyperfine transitions with relative intensities of 5:3:1 under conditions of local thermodynamic equilibrium. In Sgr B2, however, the intensity ratios are found to be 10:3:1 overall, with more extreme ratios being found towards the main continuum peaks. These ratios are consistent with the presence of maser action, as shown by detailed excitation calculations. The overall velocity distribution of HC₃N is consistent with the presence of three clouds centred near 50, 65 and 80 km.s^–1; self-absorption associated with the 65 km.s^–1 cloud towards Sgr B2(M) accounts for the apparently anomalous behaviour of the F=2-1 and F=1-1 hyperfine transitions in this direction.