Detailed Studies of IPHAS sources. II. Sab 19, a true planetary nebula
  and its mimic crossing the Perseus Arm
  The INT Photometric H$\alpha$ Survey (IPHAS) has provided us with a number of
new-emission line sources, among which planetary nebulae (PNe) constitute an
important fraction. Here we present a detailed analysis of the IPHAS nebula
Sab\,19 (IPHASX\,J055242.8+262116) based on radio, infrared, and optical images
and intermediate- and high-dispersion longslit spectra. Sab\,19 consists of a
roundish 0.10 pc in radius double-shell nebula surrounded by a much larger 2.8
pc in radius external shell with a prominent H-shaped filament. We confirm the
nature of the main nebula as a PN whose sub-solar N/O ratio abundances, low
ionized mass, peculiar radial velocity, and low-mass central star allow us to
catalog it as a type III PN. Apparently, the progenitor star of Sab\,19 became
a PN when crossing the Perseus Arm during a brief visit of a few Myr. The
higher N/O ratio and velocity shift $\simeq$40 \kms\ of the external shell with
respect to the main nebula and its large ionized mass suggest that it is not
truly associated with Sab\,19, but it is rather dominated by a Str\"omgren zone
in the interstellar medium ionized by the PN central star.
