We used the Effelsberg 100 m and IRAM 30 m telescopes to observe vibrationally excited cyanoacetylene ( \mathrm { HC _ { 3 } N } ) in several rotational transitions toward the proto-planetary nebula CRL 618 .
Lines from 9 different vibrationally excited states with energies ranging up to 1600 K above ground were detected .
The lines show P Cygni profiles indicating that the \mathrm { HC _ { 3 } N } emission originates from an expanding and accelerating molecular envelope .
The \mathrm { HC _ { 3 } N } rotational temperature varies with velocity , peaks at 520 K , 3 \mathrm { km s ^ { -1 } } blue-shifted from the systemic velocity and decreases with higher blueshift of the gas .
The column density of the absorbing \mathrm { HC _ { 3 } N } is 3–6 \times 10 ^ { 17 } \hbox { { cm } } ^ { -2 } .
We modeled spectra based on spherical models of the expanding envelope which provide 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 \pm 2 .