Deep K-band Observations of TMC-1 with the Green Bank Telescope: Detection of HC₇O, Nondetection of HC₁₁N, and a Search for New Organic Molecules

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 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 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.