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

¹ Observatorio Astronómico Nacional, c/ Alfonso XII 3, 28014 Madrid, Spain
e-mail: r.soria@oan.es
² Observatorio Astronómico Nacional, Ap 112, 28803 Alcalá de Henares, Spain

Received: 25 June 2013
Accepted: 10 September 2013

Abstract

Aims. We study the physical properties and molecular excitation of the different warm gas components found in the protoplanetary nebula CRL 618. The proper study of the nebular structure and its implications on the dynamics and kinematics of the molecular gas are of particular importance for understanding the evolution of these objects.

Methods. We revise our previous Herschel/HIFI observations, which consist of several ¹²CO and ¹³CO lines in the far-infrared/sub-mm band in the nebula CRL 618. These data have been re-analyzed in detail by improving calibration, the signal-to-noise-ratio, and baseline substraction. Due to the high performance of Herschel, it was possible to identify the contributions of the different nebular components to the line profiles. Previous optical imaging and mm-wave interferometric mapping revealed that CRL 618 shows a complex molecular structure composed of a large and diffuse spherical halo, a compact central core, double shells, and a fast bipolar outflow. 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.

Results. 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 the fast bipolar outflow, is hotter than previously estimated with a typical temperature of ~300 K. This very fast 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^-1, 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.