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.

Accepted for publication in ApJ

References in corpus (6)

• The cometary composition of a protoplanetary disk as revealed by complex cyanides
• A New Reference Chemical Composition for TMC-1
• Non-Detection of HC${}_{11}$N toward TMC-1: Constraining the Chemistry of Large Carbon-Chain Molecules
• Detection of Interstellar HC${}_5$O in TMC-1 with the Green Bank Telescope
• Precise observations of the 12C/13C ratios of HC3N in the low-mass star-forming region L1527
• Detection of ${}^{13}C$ Isotopic Species of $H{C}_{7}N$

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• Discovery of the acetyl cation, CH3CO+, in space and in the laboratory
• Detection of HC${}_5$N and HC${}_7$N Isotopologues in TMC-1 with the Green Bank Telescope
• Discovery of HCCCO and C5O in TMC-1 with the QUIJOTE line survey
• Gas-grain model of carbon fractionation in dense molecular clouds
• Machine Learning of Interstellar Chemical Inventories
• Discovery of CH3CHCO in TMC-1 with the QUIJOTE line survey
• A Computational Study of the Reaction Cyanoacetylene and Cyano Radical leading to 2-Butynedinitrile and Hydrogen Radical