Deciphering the bipolar planetary nebula Abell 14 with 3D ionization and
  morphological studies
  Abell 14 is a poorly studied object despite being considered a born again
planetary nebula. We performed a detailed study of its 3D morphology and
ionization structure using the SHAPE and MOCASSIN codes. We found that Abell 14
is a highly evolved, bipolar nebula with a kinematical age of $\sim$19,400 yr
for a distance of 4 kpc. The high He abundance, and N/O ratio indicate a
progenitor of 5 $M_{\odot}$ that has experienced the third dredge-up and hot
bottom burning phases. The stellar parameters of the central source reveal a
star at a highly evolved stage near to the white dwarf cooling track, being
inconsistent with the born again scenario. The nebula shows unexpectedly strong
[N I] $\lambda 5200$ and [O I] $\lambda 6300$ emission lines indicating
possible shock interactions. Abell 14 appears to be a member of a small group
of highly evolved, extreme Type-I PNe. The members of this group lie at the
lower-left corner of the PNe regime on the [N II]/H$\alpha$ vs. [S
II]/H$\alpha$ diagnostic diagram, where shock--excited regions/objects are also
placed. The low luminosity of their central stars, in conjunction with the
large physical size of the nebulae, result in a very low photo-ionization rate,
which can make any contribution of shock interaction easily perceptible, even
for small velocities.
