2003ApJ...586..344W

We used the Effelsberg 100 m and IRAM 30 m telescopes to observe vibrationally excited cyanoacetylene (HC₃N) in several rotational transitions toward the proto-planetary nebula CRL 618. Lines from nine different vibrationally excited states, with energies ranging up to 1600 K above ground, were detected. The lines show P Cygni profiles indicating that the HC₃N emission originates from an expanding and accelerating molecular envelope. The HC₃N rotational temperature varies with velocity, peaks at 520 K, 3 km/s blueshifted from the systemic velocity, and decreases with higher blueshift of the gas. The column density of the absorbing HC₃N is (3-6)x10¹⁷ cm^–2. We modeled spectra based on spherical models of the expanding envelope that provided an excellent fit to the observations and discuss the implications of the models. Additionally, lines from ¹³C substituted cyanoacetylene were observed. They can be used to constrain the ¹²C/¹³C ratio in this source to 10±2.