Collimated Fast Wind in the Pre-Planetary Nebula CRL 618

TL;DR

This study models collimated fast winds in the pre-planetary nebula CRL 618, comparing simulations with observations to understand wind structures and their role in nebula shaping, revealing limitations of current models and suggesting cylindrical jets as a possible solution.

Contribution

The paper advances understanding of CFW structures in PPNs by comparing detailed models with multi-wavelength observations and proposing cylindrical jets to explain high-velocity molecular emissions.

Findings

Limb-brightened shells match low-velocity observations

Models fail to reproduce high-velocity molecular emissions

Cylindrical jet structure may explain high-velocity features

Abstract

Collimated fast winds (CFWs) have been proposed to operate during the post-AGB evolutionary phase (and even earlier during the late AGB phase), responsible for the shaping of pre-planetary nebulae (PPNs) and young planetary nebulae (PNs). This paper is a follow-up to our previous study of CFW models for the well-studied PPN CRL 618. Previously, we compared our CFW models with optical observations of CRL 618 in atomic and ionic lines and found that a CFW with a small opening angle can readily reproduce the highly collimated shape of the northwestern (W1) lobe of CRL 618 and the bow-like structure seen at its tip. In this paper, we compare our CFW models with recent observations of CRL 618 in CO J=2-1, J=6-5, and H2 1-0 S(1). In our models, limb-brightened shell structures are seen in CO and H2 at low velocity arising from the shocked AGB wind in the shell, and can be identified as the low-velocity (LV) components in the observations. However, the shell structure in CO J=2-1 is significantly less extended than that seen in the observations. None of our models can properly reproduce the observed high-velocity (HV) molecular emission near the source along the body of the lobe. In order to reproduce the HV molecular emission in CRL 618, the CFW is required to have a different structure. One possible CFW structure is the cylindrical jet, with the fast wind material confined to a small cross section and collimated to the same direction along the outflow axis.