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 (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(-1) blueshifted from the systemic velocity, and decreases with higher blueshift of the gas. The column density of the absorbing HC3N is (3-6) x 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 C-13 substituted cyanoacetylene were observed. They can be used to constrain the C-12/C-13 ratio in this source to 10 +/- 2.