Exploring the Ionized Core of the Proto-Planetary Nebula CRL 618 and Its Vicinity with ALMA

Abstract

Proto- and young planetary nebulae comprise dense circumstellar envelopes made of molecular gas and dust, some of which hide compact ionized cores that host stellar systems with hot objects, and show high-velocity bipolar outflows launched from inside their cores by means of still unknown mechanisms. We present high-angular-resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations (HPBW ≃ 30–50 mas) of CRL 618 at 1.35 mm covering the H30α recombination line as well as ≃150 molecular lines. The ionized core is resolved, showing a size of ≃0.″8×0.″5 and is elongated along the east–west direction. This region exhibits a remarkable incomplete ring-like structure with two bright spots to the north and south that are separated by ≃0.″2 and shows deprojected velocity gradients ranging from 0.2 to 0.6 km s−1au−1. The 1 mm wavelength continuum emission is mostly produced by free–free emission with a small contribution from dust with an average spectral index of 0.28 (Sν∝να). The ionized core can roughly be modeled as a tilted hollow cylinder with a denser, incomplete equatorial band lacking its back side. Molecular emission traces the neutral component of the same structures enclosing the ionized matter.

Keywords

• stars: AGB and post-AGB
• stars: individual: CRL 618
• techniques: interferometric
• radio continuum: stars
• radio lines: stars
• stars: jets