Physical Conditions in the Proto–Planetary Nebula CRL 618 Derived from Observations of Vibrationally Excited HC3N

We used the Effelsberg 100 m and IRAM 30 m telescopes to observe vibrationally excited cyanoacetylene (HC 3 N) 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 HC 3 N emission originates from an expanding and accelerating molecular envelope. The HC 3 N rotational temperature varies with velocity, peaks at 520 K, 3 km s -1 blueshifted from the systemic velocity, and decreases with higher blueshift of the gas. The column density of the absorbing HC 3 N is (3-6) × 10 17 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 13 C substituted cyanoacetylene were observed. They can be used to constrain the 12 C/ 13 C ratio in this source to 10 ± 2.