The shaping of multipolar pre-planetary nebulae by multi-epoch bullets

Po-Sheng Huang, Chin-Fei Lee, Anthony Moraghan

Academia Sinica Institute of Astronomy and Astrophysics, Taiwan

We propose a multi-epoch ejection model of bullets for shaping the collimated lobes of multipolar pre-planetary nebulae (PPNe). In order to investigate the physical properties of the bullets, we used the ZEUS-3D hydrodynamics code to simulate the shaping process and compared the results to the multipolar PPN, CRL 618. Five bullets were ejected to produce the major eastern lobes of CRL 618. The launch procedure of the bullets is derived from the ages of these lobes. We assumed that bullets are cylinders with a radius of 100 AU and a length of 100 AU. They have initial velocities of 165-350 km/s and mass of ~(0.005-0.011) M_☉. The total mass of the five bullets is ~0.036 M_☉. Our results show the following. (1) In order to collimate the outflow lobes, a dense core is needed, which in turn requires a high mass loss of the asymptotic giant branch (AGB) wind, i.e., a superwind at the end of the AGB phase. (2) In our simulated CO intensity maps, the velocity of outflows linearly decreases behind the tips of the outflow lobes, consistent with the CO observations of CRL 618. (3) If the circumstellar envelope near the central star has a torus-like density distribution, we can reproduce the U-shaped CO cavity walls as seen in the low-velocity CO maps of CRL 618. We also discuss the possibility of a nova outburst to launch the bullets, triggered by the accretion of hydrogen gas onto a white dwarf in a close binary system.

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