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A LABORATORY SEARCH FOR THE CARRIER OF U-LINES ATTRIBUTED TO l-C₃H⁺ IN THE HORSEHEAD NEBULA PDR.

MICHAEL C. MCCARTHY, KYLE N. CRABTREE AND OSCAR MARTINEZ JR., Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138.

Recent radio observations of the Horsehead nebula PDR in the millimeter-wave band by Pety et al. detected a series of unidentified lines which have been attributed to l-C₃H⁺, 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.