The molecular envelope of CRL 618: A new model based onHerschel/HIFI observations

Abstract

We study the physical properties and molecular excitation of the different warm gas components found in the protoplanetary nebula CRL 618. We revise our previous Herschel/HIFI observations, which consist of several 12CO and 13CO lines in the far-infrared/sub-mm band. These data have been re-analyzed in detail by improving calibration, the signal-to-noise-ratio, and baseline substraction. We identify the contributions of the different nebular components to the line profiles. We have used a spatio-kinematical model to better constrain the temperature, density, and kinematics of the molecular components probed by the improved CO observations. The 12CO and 13CO J=16-15, J=10-9, and J=6-5 transitions are detected in this source. The line profiles present a composite structure showing spectacular wings in some cases, which become dominant as the energy level increases. Our analysis of the high-energy CO emission with the already known low-energy J=2-1 and J=1-0 lines confirms that the high-velocity component, or fast bipolar outflow, is hotter than previously estimated with a typical temperature of ~300 K. This component may then be an example of a very recent acceleration of the gas by shocks that has not yet cooled down. We also find that the dense central core is characterized by a very low expansion velocity, ~5 km/s, and a strong velocity gradient. We conclude that this component is very likely to be the unaltered circumstellar layers that are lost in the last AGB phase, where the ejection velocity is particularly low. The physical properties of the other two nebular components, the diffuse halo and the double empty shell, more or less agrees with the estimations derived in previous models.