Planetary Nebulae in the UWISH2 survey
  Near-infrared imaging in the 1 - 0 S(1) emission line of molecular hydrogen
is able to detect planetary nebulae (PNe) that are hidden from optical emission
line surveys. We present images of 307 objects from the UWISH2 survey of the
northern Galactic Plane, and with the aid of mid-infrared colour diagnostics
draw up a list of 291 PN candidates. The majority, 183, are new detections and
85 per cent of these are not present in H$\alpha$ surveys of the region. We
find that more than half (54 per cent) of objects have a bipolar morphology and
that some objects previously considered as elliptical or point-source in
H$\alpha$ imaging, appear bipolar in UWISH2 images. By considering a small
subset of objects for which physical radii are available from the H$\alpha$
surface brightness-radius relation, we find evidence that the H2 surface
brightness remains roughly constant over a factor 20 range of radii from 0.03
to 0.6 pc, encompassing most of the visible lifetime of a PN. This leads to the
H$\alpha$ surface brightness becoming comparable to that of H2 at large radius
(> 0:5 pc). By combining the number of UWISH2 PNe without H$\alpha$ detection
with an estimate of the PN detection efficiency in H2 emission, we estimate
that PN numbers from H$\alpha$ surveys may underestimate the true PN number by
a factor between 1.5 and 2.5 within the UWISH2 survey area.
