2014ApJ...783...36M

Pety et al. have reported the detection of eight transitions of a closed-shell, linear molecule (B11244) in observations toward the Horsehead photodissociation region (PDR), which they attribute to the l-C₃H⁺ cation. Recent high-level ab initio calculations have called this assignment into question; the anionic C₃H^– molecule has been suggested as a more likely candidate. Here, we examine observations of the Horsehead PDR, Sgr B2(N), TMC-1, and IRC+10216 in the context of both l-C₃H⁺ and C₃H^–. We find no observational evidence of K_a= 1 lines, which should be present were the carrier indeed C₃H^–. Additionally, we find a strong anticorrelation between the presence of known molecular anions and B11244 in these regions. Finally, we discuss the formation and destruction chemistry of C₃H^– in the context of the physical conditions in the regions. Based on these results, we conclude there is little evidence to support the claim that the carrier is C₃H^–.