High-resolution observations of the J=1-0 transition of cyanoacetylene in Sgr B2

We present Australia Telescope Compact Array observations of the 9-GHz, J=1 -0, rotational transition of cyanoacetylene (HC3N) arising in the dense mol ecular clouds associated with the prominent Hn region Sgr B2. Images with a synthesized beam of 4.4 x 9.9 arcsec(2) show that the line emission is con centrated towards the 9-GHz continuum emission, and arises as a result of w eak 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 sh own by detailed excitation calculations. The overall velocity distribution of HC3N is consistent with the presence of three clouds centred near 50, 65 and 80kms(-1) self-absorption associated with the 65 km s(-1) cloud toward s Sgr B2(M) accounts for the apparently anomalous behaviour of the F = 2-1 and F = 1-1 hyperfine transitions in this direction.