Collimated Fast Wind in the Pre-Planetary Nebula CRL 618
  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.
