The Emerging Planetary Nebula CRL 618 and Its Unsettled Central Star(s)

Abstract

We report deep long-slit emission-line spectra, the line flux ratios, and Doppler profile shapes of various bright optical lines. The low-ionization lines (primarily [N I], [O I], [S II], and [N II]) originate in shocked knots, as reported by many previous observers. Dustscattered lines of higher ionization are seen throughout the lobes but do not peak in the knots. Our analysis of these line profiles and the readily discernible stellar continuum shows that (1) the central star is an active symbiotic (whose spectrum resembles the central stars of highly bipolar and young PNe such as M2-9 and Hen2-437) whose compact companion shows a WC8-type spectrum, (2) extended nebular lines of [O III] and He I originate in the heavily obscured nuclear H II region, and (3) the Balmer lines observed throughout the lobes are dominated by reflected Hα emission from the symbiotic star. Comparing our line ratios with those observed historically shows that (1) the [O III]/Hβ and He I/Hβ ratios have been steadily rising by large amounts throughout the nebula, (2) the Hα/Hβ ratio is steadily decreasing while Hγ/Hβ remains nearly constant, and (3) the low-ionization line ratios formed in the shocked knots have been in decline in different ways at various locations. We show that the first two of these results might be expected if the symbiotic central star has been active and if its bright Hα line has faded significantly in the past 20 years.

Exotic Carbon Chemistry in a Planetary Nebula: The Unusual Case of K4-47

“…Huang et al (2016) stated that a nova-like origin for the ratios in CRL 618 would necessitate a binary system with a secondary white dwarf, though the helium shell flash phenomenon would not. While the central star of CRL 618 cannot be directly observed at optical wavelengths, Balick et al (2014) found evidence of an active symbiotic system with a donor AGB and WC8 stars. There is currently no evidence for a symbiotic system for K4-47.…”

Kinematical investigation of possible fast collimated outflows in twelve planetary nebulae

“…As for Hen 2-47, the outflow does not seem particularly collimated and the data presented here fail to detect emission from a compact knot expanding ballistically at the tip of the V-shaped lobe, e.g. as in CRL 618 (Balick et al 2013(Balick et al , 2014. This feature has a projected distance from the CSPN 2×10 17 cm (0.069 pc).…”

Planetary nebulae in 2014: A review of research

“…CRL 618 shows evidence for an increasing central star temperature (Balick et al 2014), from its increasing radio flux and evolving line ratios (see Fig. 2).…”

“…CRL 618 gained a star (Balick et al 2014) when the central system was shown to be a symbiotic-like system with a [WC8] companion, and the central star of K 3-18, previously suspected to be a symbiotic, was found to be a [WC9] star (Miszalski & Miko lajewska 2014). The possible relation between symbiotic stars and [WC] stars deserves further investigation, for those wishing to complicate stellar evolution even further.…”