IFU spectroscopy of Southern PNe: VII Photo-ionization modelling of
  intermediate excitation class objects
  We present integral field unit spectroscopic observations of southern
Galactic planetary nebulae (PNe), IC 2501, Hen 2-7, and PB4. The goal of
studying these objects together is that, although they have roughly similar
intermediate excitation and evolution of central stars (CSs), they display very
different evolution in their nebular structure which needs to be understood.
The morphologies and ionisation structures of the objects are investigated
using a set of emission-line maps representative of the different ionisation
zones. We use those in order to construct two-zone self-consistent
photoionisation models for each nebula to determine new model-dependent
distances, progenitor luminosities, effective temperatures and CS masses. The
physical conditions, chemical compositions, and expansion velocities and ages
of these nebulae are derived. In Hen 2-7 we discover a strong poleward-directed
jet from the presumed binary CS. Oxygen and nitrogen abundances derived from
both collisionally excited and recombination lines reveal that PB4 displays an
extreme abundance discrepancy factor, and we present evidence that this is
caused by uorescent pumping of the OII ion by the EUV continuum of an
interacting binary CS, rather than by recombination of the OIII ion. Both IC
2501 and PB4 were classified by others as Weak Emission Line Stars (WELS).
However, our emission line maps show that their recombination lines are
spatially extended in both objects, and are therefore of nebular rather than CS
origin. Given that we have found this result in a number of other PNe, this
result casts further doubt on the reliability, or even the reality, of the WELS
classification.
