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We study the physical properties and molecular excitation of the differentwarm gas components found in the protoplanetary nebula CRL 618. We revise ourprevious Herschel/HIFI observations, which consist of several 12CO and 13COlines in the far-infrared/sub-mm band. These data have been re-analyzed indetail by improving calibration, the signal-to-noise-ratio, and baselinesubstraction. We identify the contributions of the different nebular componentsto the line profiles. We have used a spatio-kinematical model to betterconstrain the temperature, density, and kinematics of the molecular componentsprobed by the improved CO observations. The 12CO and 13CO J=16-15, J=10-9, andJ=6-5 transitions are detected in this source. The line profiles present acomposite structure showing spectacular wings in some cases, which becomedominant as the energy level increases. Our analysis of the high-energy COemission with the already known low-energy J=2-1 and J=1-0 lines confirms thatthe high-velocity component, or fast bipolar outflow, is hotter than previouslyestimated with a typical temperature of ~300 K. This component may then be anexample of a very recent acceleration of the gas by shocks that has not yetcooled down. We also find that the dense central core is characterized by avery low expansion velocity, ~5 km/s, and a strong velocity gradient. Weconclude that this component is very likely to be the unaltered circumstellarlayers that are lost in the last AGB phase, where the ejection velocity isparticularly low. The physical properties of the other two nebular components,the diffuse halo and the double empty shell, more or less agrees with theestimations derived in previous models.

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