Deep K-band observations of TMC-1 with the Green Bank Telescope: Detection of HC7O, non-detection of HC11N, and a search for new organic molecules

Abstract

The 100 m Robert C. Byrd Green Bank Telescope K-band (KFPA) receiver was used to perform a high-sensitivity search for rotational emission lines from complex organic molecules in the cold interstellar medium towards TMC-1 (cyanopolyyne peak), focussing on the identification of new carbon-chain-bearing species as well as molecules of possible prebiotic relevance. We report a detection of the carbon-chain oxide species HC${}_7$O and derive a column density of $(7.8\pm 0.9)\times 10^{11}$~cm${}^{-2}$. This species is theorized to form as a result of associative electron detachment reactions between oxygen atoms and C${}_7$H${}^{-}$, and/or reaction of C${}_6$H${}_2$${}^{+}$ with CO (followed by dissociative electron recombination). Upper limits are given for the related HC${}_6$O, C${}_6$O and C${}_7$O molecules. In addition, we obtained the first detections of emission from individual ${}^{13}$C isotopologues of HC${}_7$N, and derive abundance ratios HC${}_7$N/HCCC${}^{13}$CCCCN = $110\pm 16$ and HC${}_7$N/HCCCC${}^{13}$CCCN = $96\pm 11$, indicative of significant ${}^{13}$C depletion in this species relative to the local interstellar elemental ${}^{12}$C/${}^{13}$C ratio of 60-70. The observed spectral region covered two transitions of HC${}_{11}$N, but emission from this species was not detected, and the corresponding column density upper limit is $7.4\times 10^{10}$ cm${}^{-2}$ (at 95% confidence). This is significantly lower than the value of $2.8\times 10^{11}$ cm${}^{-2}$ previously claimed by Bell et al. (1997) and confirms the recent non-detection of HC${}_{11}$N in TMC-1 by Loomis et al. (2016). Upper limits were also obtained for the column densities of malononitrile and the nitrogen heterocycles quinoline, isoquinoline and pyrimidine.

arXiv commentAccepted for publication in ApJ

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