Detection of 13C isotopomers of the molecule HC7N

The C-13 substitutions of the molecule HC7N were observed in TMC-1 using the J=12-11, J=13-12 rotational transitions in the frequency range 12.4-13.6 GHz. We present the first detection of the C-13 isotopic species of HC7N in the interstellar medium, based on the average of a number of weak rotational transitions. This paper describes the calibration and data-averaging process that is also used in a search for large cyanopolyyne molecules in TMC-1 using the 100 m Robert C. Byrd Green Bank Telescope (GBT). The capabilities of the GBT 11-15 GHz observing system are described, along with a discussion of numerical methods for averaging observations of a number of weak spectral lines to detect new interstellar molecules.