Dense cores in dark clouds. IV - HC5N observations

Detections or possible detections of the J = 9-8 rotational transition of HC5N in 10 new dark cloud sources are reported. All of the strong sources and most of the known sources are in Taurus. In contrast to maps of TMC-1, maps of NH3 and HC5N emission in L1544 and in TMC-1C show no relative shift of peak positions. Column densities of HC5N and (C-18)O are roughly corelated, but N(HC5N) increases faster than N(C-18)O, suggesting that the abundance of HC5N ranges from about 10 to the -10th in the weakest sources to about 10 to the -9th in the strongest sources. Comparison with predictions of chemical models indicates that this abundance variation is unlikely to be due to variations in cloud density, but it may be due to diferences in the degree of chemical evolution from cloud to cloud. If so, TMC-1 is more evolved than L183.