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

TL;DR

This study presents detailed spectral analysis of the planetary nebula CRL 618, revealing an active symbiotic central star, evolving line ratios, and complex nebular structures, indicating dynamic changes over recent decades.

Contribution

It provides new insights into the nature of the central star as a symbiotic system and documents temporal spectral variations in CRL 618.

Findings

Central star is an active symbiotic with a WC8 companion.

Line ratios such as [O III]/Hβ and He I/Hβ have increased over time.

Hα/Hβ ratio is decreasing, indicating changes in nebular conditions.

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. Dust-scattered 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 H{\alpha} emission from the symbiotic star. Comparing our line ratios with those observed historically shows that (1) the [O III]/H\b{eta} and He I/H\b{eta} ratios have been steadily rising by large amounts throughout the nebula, (2) the H{\alpha}/H\b{eta} ratio is steadily decreasing while H{\gamma}/H\b{eta} 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 H{\alpha} line has faded significantly in the past 20 years.