The Shaping of the Multipolar Pre-Planetary Nebula CRL 618 by Multi-directional Bullets

TL;DR

This study uses 3D hydrodynamic simulations to demonstrate how multi-directional bullets can shape the complex multipolar structure of the pre-planetary nebula CRL 618, matching observed features and suggesting episodic ejections.

Contribution

The paper introduces a multi-episode bullet ejection model in 3D simulations to explain the formation of multipolar structures in CRL 618, incorporating molecular chemistry and cooling effects.

Findings

Multi-epoch bullets can produce observed multipolar lobes.

Simulated CO maps match observed intensity and velocity structures.

Bullet ejections may result from magneto-rotational or nova-like explosions.

Abstract

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.