The slowly expanding envelope of CRL 618 probed with HC 3 N rotational ladders

Lines from HC 3 N and isotopic substituted species in ground and vibrationally excited states produce crowded millimeter and submillimeter wave spectra in the C-rich protoplanetary nebula CRL618. The complete sequence of HC 3 N rotational lines from J=9–8 to J=30–29 has been observed with the IRAM 30m telescope toward this object. Lines from a total of 15 different vibrational states (including the fundamental), with energies up to 1100 cm −1 , have been detected for the main HC 3 N isotopomer. In addition, the CSO telescope has been used to complement this study in the range J=31–30 to J=39–38, with detections in five of these states, all of them below 700 cm −1. Only the rotational lines of HC 3 N, in its ground vibrational state, display evidence of the well known CRL618 high velocity velocity outflow. Vibrationally excited HC 3 N rotational lines exhibit P-Cygni profiles at 3 mm, evolving to pure emission lineshapes at shorter wavelengths. This evolution of the line profile shows little dependence on the vibrational state from which they rotational lines arise. The absorption features are formed against the continuum emission, which has been successfully characterized in this work due to the large frequency coverage. The fluxes range – 2 – from 1.75 to 3.4 Jy in the frequency range 90 to 240 GHz. These values translate to an effective continuum source with a size between 0.22 " and 0.27 " and effective temperature at 200 GHz ranging from 3900 to 6400 K with spectral index between-1.15 and-1.12. We have made an effort to simultaneously fit a representative set of observed HC 3 N lines through a model for an expanding shell around the central star and its associated HII region assuming that LTE prevails for HC 3 N. The simulations show that the inner slowly expanding envelope has expansion and turbulence velocities of ∼ 5-18 kms −1 and ∼ 3.5 kms −1 respectively, and that it is possibly elongated. Its inclination with respect to the line of sight has also been explored. The HC 3 N column density in front of the the continuum source has been determined by comparing the output of an array of models to the data. The best fits are obtained for column densities in the range 2.0-3.5·10 17 cm −2 , consistent with previous estimates from ISO data, and T K in the range 250 to 275 K, …