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