Detection of HC11N in the Cold Dust Cloud TMC-1

Two consecutive rotational transitions of the long cyanopolyyne HC₁₁N, J = 39 --> 38 and 38 --> 37, have been detected in the cold dust cloud TMC-1 at the frequencies expected from recent laboratory measurements by Travers and coworkers, and at about the expected intensities. The astronomical lines have a mean radial velocity of 5.8(1) km s^-1, in good agreement with the shorter cyanopolyynes HC₇N and HC₉N observed in this very sharp line source [5.82(5) and 5.84(5) km s^-1, respectively]. The column density of HC₁₁N is calculated to be 2.8 × 10¹¹ cm^-2. The abundance of the cyanopolyynes decreases smoothly with length to HC₁₁N, the decrement from one to the next being about six for the longer carbon chains.