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Description
Laboratory detection of four isomers of C_5_H_2_ molecule have been reported by Travers et al. (1997ApJ...483L.135T), McCarthy et al. (1997, Science, 275, 518), and Gottlieb et al. (1998ApJ...509L.141G). They suggested for detection of the ring-chain isomer of C_5_H_2_ (c-C_5_H_2_) in cosmic objects, as it is the most stable one in comparison to the others. Two transitions 3_13_-2_12_ and 3_03_-2_02_ at 19.147GHz and 19.606GHz, respectively, of c-C_5_H_2_ have been detected in TMC-1. We suggest that the c-C_5_H_2_ may be identified in cool cosmic objects through its transition 3_13_-4_04_ at 4.3GHz in absorption against the cosmic microwave background. Since in absence of availability of the collisional rates, we have used scaled values for them, we have checked the sensitivity of the lines on the rates by enhancing the rate for the transitions with {Delta}k_a_=0 by a factor of 10. Though the transitions are not found sensitive, our results ma y be treated as qualitative in nature. This absorption line may play an important role for identification of c-C_5_H_2_ in cosmic objects.
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