A 4–6 GHz Spectral Scan and 8–10 GHz Observations of the Dark Cloud TMC‐1

We present the results of the lowest frequency spectral survey carried out toward a molecular cloud and sensitive observations at selected frequencies. The entire Arecibo C band (4-6 GHz) was observed toward the cyanopolyyne peak of TMC-1, with an rms sensitivity of about 17-18 mK (about 2-2.5 mJy). In addition, a number of selected frequency ranges within the C band and X band (8-10 GHz) were observed with longer integration times and rms sensitivities of 7-8 mK (≈2 mJy) or higher. In the spectral scan itself, already-known H 2 CO and HC 5 N lines were detected. However, in more sensitive observations at selected frequencies, lines of C 2 S, C 3 S, C 4 H, C 4 H 2 , HC 3 N and its 13 C substituted isotopic species, as well as HC 5 N, HC 7 N, and HC 9 N were found, about half of them detected for the first time. The rotational temperatures of the detected molecules fall in the range 4-9 K. Cyanopolyyne column densities vary from 5.6 × 10 13 cm -2 for HC 5 N to 2.7 × 10 12 cm -2 for HC 9 N. Our results show that, for molecular observations at low frequencies (4-10 GHz) to be useful for studying dark clouds, the sensitivity must be of the order of 5-10 mK or better. To date, observations at ~10 GHz have been more productive than those at lower frequencies.