The Shaping of the Multipolar Pre-Planetary Nebula CRL 618 by
  Multi-directional Bullets
  In order to understand the formation of the multipolar structures of the
pre-planetary nebula (PPN) CRL 618, we perform 3D simulations using a
multi-directional bullet model. The optical lobes of CRL 618 and fast molecular
outflows at the tips of the lobes have been found to have similar expansion
ages of ~ 100 yr. Additional fast molecular outflows were found near the source
along the outflow axes with ages of ~ 45 yr, suggesting a second episode of
bullet ejections. Thus, in our simulations, two episodes of bullet ejections
are assumed. The shaping process is simulated using the ZEUS-3D hydrodynamics
code that includes molecular and atomic cooling. In addition, molecular
chemistry is also included to calculate the CO intensity maps. Our results show
the following: (1) Multi-epoch bullets interacting with the toroidal dense core
can produce the collimated multiple lobes as seen in CRL 618. The total mass of
the bullets is ~ 0.034 solar mass, consistent with the observed high-velocity
CO emission in fast molecular outflows. (2) The simulated CO J=3-2 intensity
maps show that the low-velocity cavity wall and the high-velocity outflows
along the lobes are reasonably consistent with the observations. The
position-velocity diagram of the outflows along the outflow axes shows a linear
increase of velocity with distance, similar to the observations. The ejections
of these bullets could be due to magneto-rotational explosions or nova-like
explosions around a binary companion.
