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 ~19 400 yr for a distance of 4 kpc. The high He abundance, and N/O ratio indicate a progenitor of 5 M_⊙ 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] λ5200 and [O I] λ6300 emission lines indicating possible shock interactions. Abell 14 appears to be a member of a small group of highly evolved, extreme type-I planetary nebulae (PNe). The members of this group lie at the lower-left corner of the PNe regime on the [NII]/Hα versus [S II]/Hα 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 photoionization rate, which can make any contribution of shock interaction easily perceptible, even for small velocities.