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

Abstract

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 (approximate to2 mJy) or higher. In the spectral scan itself, already-known H2CO and HC5N lines were detected. However, in more sensitive observations at selected frequencies, lines of C2S, C3S, C4H, C4H2, HC3N and its C-13 substituted isotopic species, as well as HC5N, HC7N, and HC9N 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 x 10(13) cm(-2) for HC5N to 2.7 x 10(12) cm(-2) for HC9N. 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 similar to10 GHz have been more productive than those at lower frequencies.