2017ApJ...850..187C

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 toward 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 HC₇O and derive a column density of (7.8±0.9)×10¹¹ cm^–2. This species is theorized to form as a result of associative electron detachment reactions between oxygen atoms and C₇H^–, and/or reaction of C₆H₂⁺ with CO (followed by dissociative electron recombination). Upper limits are given for the related HC₆O, C₆O, and C₇O molecules. In addition, we obtained the first detections of emission from individual ¹³C isotopologues of HC₇N, and derive abundance ratios HC₇N/HCCC¹³CCCCN = 110 ± 16 and HC₇N/HCCCC¹³CCCN = 96 ± 11, indicative of significant ¹³C depletion in this species relative to the local interstellar elemental ¹²C/¹³C ratio of 60-70. The observed spectral region covered two transitions of HC₁₁N, but emission from this species was not detected, and the corresponding column density upper limit is 7.4×10¹⁰ cm^–2 (at 95% confidence). This is significantly lower than the value of 2.8×10¹¹ cm^–2 previously claimed by Bell et al. and confirms the recent nondetection of HC₁₁N in TMC-1 by Loomis et al. Upper limits were also obtained for the column densities of malononitrile and the nitrogen heterocycles quinoline, isoquinoline, and pyrimidine.