Detection of c-C₃H₂, NO, and CH₃CN towards the outer edges of the galaxy: defining the organic zone

In a previous study, methanol, CH₃OH, was observed towards 20 molecular clouds located in the outer Galaxy (R_GC = 12.9-23.5). The abundance of CH₃OH did not appear to decrease significantly with distance from the Galactic Center and apparently is not affected by the decline in metallicity. These detections suggest an “Organic Zone,” a region of the Galaxy where organic chemistry is active. In order to further define the Organic Zone, we have been searching for c-C₃H₂, NO, and CH₃CN in the “edge” molecular clouds where CH₃OH has been detected. Part of these observations are being conducted with a new 2 mm receiver at the 12m telescope of the Arizona Radio Observatory (ARO). Detections of the J=4→3 transitions of c-C₃H₂, as well as hyperfine components of the J=3/2→1/2 transition of NO, have been made in almost all 20 clouds of the sample, including WB89-640, WB89-380, and 19423+2541. Multiple K components of the J =5→4 transition of CH₃CN have also been identified in WB89-640. From a radiative transfer analysis, fractional abundances of the three molecules, relative to H₂, are f ~ 10^-11, f ~ 10^-10, and f ~10^-9 for CH₃CN, c-C₃H₂, and NO, respectively. Preliminary results suggest that abundances of these species are comparable to those in the inner Galaxy. The detection of c-C₃H₂, NO, and CH₃CN at the Galactic edge suggest that organic chemistry is still active despite the decrease in metallicity. Additional observations of CH₃CN are currently being conducted.