The Slowly Expanding Envelope of CRL 618 Probed with HC3N 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 CRL 618. The complete sequence of HC 3 N rotational lines from J = 9-8 to J = 30-29 has been observed with the IRAM 30 m 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 Caltech Submillimeter Observatory 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 CRL 618 high-velocity outflow. Vibrationally excited HC 3 N rotational lines exhibit P Cygni profiles at 3 mm, evolving to pure emission line shapes at shorter wavelengths. This evolution of the line profile shows little dependence on the vibrational state from which the rotational lines arise. The absorption features are formed against the continuum emission, which has been successfully characterized in this work as a result of the large frequency coverage. The fluxes range from 1.75 to 3.4 Jy in the frequency range 90-240 GHz. These values translate to an effective continuum source with a size between 0 22 and 0 27, an effective temperature at 200 GHz ranging from 3900 to 6400 K, and a 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 with an expanding shell around the central star and its associated H II region, assuming that LTE prevails for HC 3 N. The simulations show that the slowly expanding inner envelope has expansion and turbulence velocities of ~5-18 and ~3.5 km s -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 continuum source has been determined by comparing the output of an array of models with the data. The best fits are obtained for column densities in the range × 10 17 cm -2 , consistent with previous estimates from Infrared Space Observatory ( ISO ) data, and T K in the range 250-275 K, in very good agreement with estimates made from the same ISO data.