WEAVE imaging spectroscopy of NGC 6720: an iron bar in the Ring

TL;DR

This study uses the WEAVE LIFU to obtain spatially resolved optical spectroscopy of NGC 6720, revealing a previously unidentified linear 'iron bar' across the nebula center emitting [Fe v] 4227 and select [Fe vi] lines. Detailed analysis yields bar-specific physical conditions (n$_e$≈460 cm$^{-3}$, Te≈$11{,}300$ K) and iron abundances (Fe$^{4+}$+Fe$^{5+}$/H$^+$≈$1.3\times10^{-7}$) with a substantial depletion relative to solar, though less severe than in the bright ring, suggesting partial dust destruction. Kinematic measurements show Fe lines are redshifted by ~$20$–$50$ km s$^{-1}$ and not consistent with a jet origin; JWST imaging indicates the bar lies near a central dust lane with surrounding H$_2$ emission, implying a complex interaction between dust processing and highly ionized gas. The results illustrate WEAVE's capability to uncover new structural components in planetary nebulae and motivate higher-resolution follow-up to resolve the bar's formation and line-of-sight geometry.

Abstract

We present spatially resolved spectroscopic observations of the planetary nebula NGC 6720, the Ring Nebula, taken during the science verification phase of WEAVE, a new instrument mounted on the William Herschel Telescope on La Palma. We use the instrument's Large Integral Field Unit (LIFU) to obtain spectra of the Ring Nebula, covering its entire optically bright inner regions as well as parts of its much fainter outer molecular halo. We report the discovery of emission from [Fe~{\sc v}] and [Fe~{\sc vi}] confined to a narrow ``bar'' extending across the central regions of the nebula. No lines of other elements share this morphology or, at the spectral resolving power used ($R \sim 2500$), the same radial velocity. The extent to which iron in this bar is depleted is presently unclear; comparison with JWST-detected dust continuum emission suggests that some dust grain destruction may be occurring in the region, but there is currently no observational evidence for the $>$ 50~km\,s$^{-1}$ shock waves or $T > 10^6$~K X-ray emitting gas needed to enable this. Where the bar is located along the line of sight through the nebula, and how it was created, are new puzzles to be solved for this iconic planetary nebula.

Figures (6)

• Figure 1: Colour composite image of the Ring Nebula, reconstructed from WEAVE LIFU emission line maps of [O i] 6300 Å (red), H$\beta$ 4861 Å (green), and He ii 4686 Å (blue). Gaps in the reconstructed image arise from LIFU fibres which were not operational at the time of the observations.
• Figure 2: (left) [Fe v] 4227 Å emission line map, (right) summed emission of [Fe vi] emission lines at 5147, 5177, 5425 and 5678 Å. White crosshairs indicate the position of the central star. Apparent emission in the outer regions of the right-hand panel is an artefact of the low signal-to-noise ratio of the [Fe vi] lines.
• Figure 3: Selected WEAVE emission-line maps, ordered by the minimum photon energy required to create the species being traced: the relevant ion is the detected ion in the case of collisionally-excited lines, and the recombining ion in the case of recombination lines. The emitting species and the associated creation energy are indicated on each map. Contours derived from the [Fe v] map are overlaid in each panel. [Fe v] is produced by collisional excitation of Fe$^{4+}$ (54.8 eV ionization potential). All images are displayed on a linear surface brightness scale with cuts at intensity percentiles of 0.1% and 99.9%.
• Figure 4: (top) Spectrum of the bar region, showing the clear detection of [Fe v] 4227 Å near the upper wavelength limit of the top panel. The spectrum of [Fe v] predicted for a temperature of 10 000 K and an electron density of 1000 cm$^{-3}$ is shown in black. [Fe v] 4227 Å is by far the strongest [Fe v] line in the wavelength region observed, and a number of the already much fainter transitions lie close to other strong emission lines. (bottom) Detections of [Fe vi] lines.
• Figure 5: Heliocentric radial velocities measured from individual spectral lines in the blue spectral region, for the bar regions east (left panel) and west (right) of the central star. The [Fe v] 4227 Å and [Fe vi] 5679 Å lines (plotted in orange) stand out as redshifted with respect to the median of the other lines.