Cyanodiacetylene /HC5N/ in Sagittarius B2

Results are reported for observations of the J = 2-1 and 3-2 rotational lines of HC5N at 5.325 and 7.988 GHz, respectively, in Sgr B2. Statistical-equilibrium calculations are performed to model the physical conditions implied by the observations. A consistent model of the Sgr B2 molecular cloud and continuum sources is obtained which includes an extensive molecular cloud of very low density surrounding the core and halo of the model proposed by Morris et al. (1976); this cloud is assumed to have a diameter of 30 pc, an H2 density of about 100 per cu cm, and a kinetic temperature of approximately 20 K. A maximum column density of 1.6 x 10 to the 14th per sq cm is estimated for HC5N and shown to be equal to the column density for HC3N in the same source.