DETECTION OF C3N IN THE SPIRAL ARM GAS CLOUDS IN THE DIRECTION OF CASSIOPEIA-A

We report detection of the strongest component of the N = 1 --> 0 tran sition of the free radical C3N in absorption against Cas A. The inferr ed column densities are NL(C3N) = 10(12)-10(13) cm-2 depending on whet her the absorbing molecules are in the low-density interstellar medium or in denser molecular gas (cloudlets) in the Orion and Perseus arms. After considering both cases and comparing the results to published a bundances in TMC-1 we find no clear evidence that either C3N or C4H ar e located in diffuse gas and conclude that both are more likely locate d in dense gas clouds. Our results do suggest, at least in the directi on of Cas A, that the C(n)H chains, like the C(n)H chains, may be suff iciently abundant for the longer members to be carriers of the DIBs if they can survive in unprotected regions. Unlike the case for C4H, how ever, where the absorption line is relatively strong in the Orion arm gas, there is evidence in the C3N case for stronger absorption in the Perseus arm. We suggest that this abundance difference between the two arms might be related to the fact that different families of DIBs hav e been reported whose relative abundance also varies from source to so urce.