Imaging the elusive H-poor gas in the high ADF planetary nebula NGC 6778

TL;DR

This study provides the first direct imaging of H-poor, high-metallicity gas in planetary nebula NGC 6778, revealing its central concentration and spatial separation from other emission lines, which informs the understanding of nebular composition and origin.

Contribution

First direct imaging of H-poor gas in a planetary nebula, showing its spatial distribution and relation to nebular structures, advancing the understanding of abundance discrepancies.

Findings

H-poor gas is centrally concentrated in NGC 6778.

O II ORLs are spatially distinct from [O III] and H alpha emissions.

The distribution suggests a link to the nebula's binary central star.

Abstract

We present the first direct image of the high-metallicity gas component in a planetary nebula (NGC 6778), taken with the OSIRIS Blue Tunable Filter centered on the O II 4649+50 angstroms optical recombination lines (ORLs) at the 10.4m Gran Telescopio Canarias. We show that the emission of these faint O II ORLs is concentrated in the central parts of the planetary nebula and is not spatially coincident either with emission coming from the bright [O III] 5007 angstroms collisionally excited line (CEL) or the bright H alpha recombination line. From monochromatic emission line maps taken with VIMOS at the 8.2m Very Large Telescope, we find that the spatial distribution of the emission from the auroral [O III] 4363 line resembles that of the O II ORLs but differs from nebular [O III] 5007 CEL distribution, implying a temperature gradient inside the planetary nebula. The centrally peaked distribution of the O II emission and the differences with the [O III] and H I emission profiles are consistent with the presence of an H-poor gas whose origin may be linked to the binarity of the central star. However, determination of the spatial distribution of the ORLs and CELs in other PNe, and a comparison of their dynamics is needed to further constrain the geometry and ejection mechanism of the metal-rich (H-poor) component and hence, understand the origin of the abundance discrepancy problem in PNe.