J-PLUS: Tools to identify compact planetary nebulae in the Javalambre
  and southern photometric local universe surveys
  From the approximately $\sim$3,500 planetary nebulae (PNe) discovered in our
Galaxy, only 14 are known to be members of the Galactic halo. Nevertheless, a
systematic search for halo PNe has never been performed. In this study, we
present new photometric diagnostic tools to identify compact PNe in the
Galactic halo by making use of the novel 12-filter system projects, J-PLUS
(Javalambre Photometric Local Universe Survey) and S-PLUS (Southern-Photometric
Local Universe Survey). We reconstructed the IPHAS (Isaac Newton Telescope
(INT) Photometric H${\alpha}$ Survey of the Northern Galactic Plane) diagnostic
diagram and propose four new ones using i) the J-PLUS and S-PLUS synthetic
photometry for a grid of photo-ionisation models of halo PNe, ii) several
observed halo PNe, as well as iii) a number of other emission-line objects that
resemble PNe. All colour-colour diagnostic diagrams are validated using two
known halo PNe observed by J-PLUS during the scientific verification phase and
the first data release (DR1) of S-PLUS and the DR1 of J-PLUS. By applying our
criteria to the DR1s ($\sim$1,190 deg$^2$), we identified one PN candidate.
However, optical follow-up spectroscopy proved it to be a H II region belonging
to the UGC 5272 galaxy. Here, we also discuss the PN and two H II galaxies
recovered by these selection criteria. Finally, the cross-matching with the
most updated PNe catalogue (HASH) helped us to highlight the potential of these
surveys, since we recover all the known PNe in the observed area. The tools
here proposed to identify PNe and separate them from their emission-line
contaminants proved to be very efficient thanks to the combination of many
colours, even when applied -like in the present work- to an automatic
photometric search that is limited to compact PNe.
