A LABORATORY SEARCH FOR THE CARRIER OF U-LINES ATTRIBUTED TO $l$-C$_3$H$^+$ IN THE HORSEHEAD NEBULA PDR

Author Institution: Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138Recent radio observations of the Horsehead nebula PDR in the millimeter-wave band by Pety \textit{et al.} \textbf{548}, A68 (2012).} detected a series of unidentified lines which have been attributed to $l$-C$_3$H$^+$, a molecular cation for which no high-resolution laboratory data presently exist. We have detected a pair of rotational lines in the centimeter-wave band at frequencies predicted from their derived spectroscopic constants using Fourier transform microwave and microwave-microwave double resonance spectroscopies. Experimental evidence strongly suggests that the laboratory and astronomical lines arise from a common carrier, and that the carrier is a closed-shell, reactive hydrocarbon containing a linear or nearly-linear three carbon atom backbone