Abstract
Pre-planetary nebulae (PPNs) are objects in transition between the asymptotic giant branch (AGB) phase and the planetary nebula (PN) phase in the evolution of low- to intermediate-mass stars. Most of them were found to have highly aspherical shapes, with a significant fraction having highly collimated bipolar or multipolar lobes. However, the formation mechanism of those structures is still uncertain.
CRL 618 is a relatively nearby well-studied PPN. It has multiple highly collimated optical lobes, fast molecular outflows along the optical lobes, and an extended molecular envelope that consists of a dense torus in the equator and a tenuous round halo. We have mapped it with the Submillimeter Array at 350 GHz at unprecedented resolutions of upto 0.3" in continuum and molecular lines. The dense core is dusty and it has a mass of ∼0.47 Msun and a dynamical age of ∼400 yr. It could result from a recent enhanced heavy mass-loss episode that ends the asymptotic giant branch phase. In the dense core, the isotopic ratios of 12C/13C and 14N/15N are 9 ± 4 and 150 ± 50, respectively, both lower than the solar values, suggesting a presence of a hot CNO cycle in the past. A compact emission is detected at the center tracing the dense inner part of the HII region previously detected in a 23 GHz continuum and it may trace a fast ionized wind recently launched at the base. We spatially resolve the fast molecular outflow region previously detected in CO near the central star and find it to be composed of multiple outflows that have similar dynamical ages and are oriented along the different optical lobes. We also detect fast molecular outflows further away from the central star near the tips of the extended optical lobes and a pair of equatorial outflows inside the dense torus. We find that two episodes of bullet ejections in different directions are needed, one producing the fast molecular outflows near the central star and one producing the fast molecular outflows near the tips of the extended optical lobes. One possibility to launch these bullets is a magneto-rotational explosion of the stellar envelope.
Coffee and tea will be served 20 minutes prior to the seminar.