Near-infrared photometry of Galactic planetary nebulae with the VVV
  Survey
  Planetary nebulae (PNe) are powerful tracers of evolved stellar populations.
Among the 3000 known PNe in the Galaxy, about 600 are located within the 520
square-degree area covered by the VVV survey. The VVV photometric catalogue
provides an important new dataset for the study of PNe, with high-resolution
imaging in five near-infrared bands. Aperture photometry of known PNe in the
VVV area was retrieved from source catalogues. Care was taken to minimise any
confusion with field stars. The colours of the PNe we are determined for H-Ks,
J-H, Z-Y, and Y-J, and compared to stars and to other types of emission line
objects. Cloudy photo-ionisation models were used to predict colours for
typical PNe. We present near-infrared photometry for 353 known PNe. The best
separation from other objects is obtained in the H-Ks vs. J-H diagram. We
calculated the emission-line contribution to the in-band flux based on a model
for NGC 6720: we find that this is highest in the Z and Y bands at over 50%,
lower in the J band at 40%, and lowest in the H and Ks bands at 20%. A new view
of PNe in the wavelength domain of the Z and Y bands is shown. Photo-ionisation
models are used to explore the observed colours in these bands. The Y band is
shown to be dominated by HeI 1.083 mu and HeII 1.012 mu, and colours involving
this band are very sensitive to the temperature of the ionizing star. The VVV
survey represents a unique dataset for studing crowded and obscured regions in
the Galactic plane. The diagnostic diagrams presented here allow one to study
the properties of known PNe and to uncover objects not previously classified.
