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- J. R. Pardo, J. Cernicharo, J.R. Goicoechea, T.G. Phillips
"The slowly expanding envelope of CRL618 probed with HC3N rotational ladders"
Astrophysical Journal, 615/1, 495-505 (2004).
Lines from HC3N 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 HC3N 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 HC3N 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 HC3N, in its ground vibrational state, display evidence of the well known
CRL618 high velocity velocity outflow. Vibrationally excited
HC3N 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 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 HC3N. The simulations
show that the inner slowly expanding envelope has expansion and turbulence
velocities of ~ 5-18
km/s and ~ 3.5 km/s respectively, and
that it is possibly elongated. Its inclination with respect to the line of sight
has also been explored. The HC3N 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 x 10**17 cm**(-2), consistent with previous estimates
from ISO data, and TK$ in the range 250 to 275 K, in very good agreement
with estimates made from the same ISO data.
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