Numerical simulations of wind-driven protoplanetary nebulae. II.
  signatures of atomic emission
  We follow up on our systematic study of axisymmetric hydrodynamic simulations
of protoplanetary nebula. The aim of this work is to generate the atomic
analogues of the $\mathrm H_2$ near-infrared models of Paper I with the ZEUS
code modified to include molecular and atomic cooling routines. We investigate
stages associated with strong $\mathrm {[Fe II]}$ 1.64 $\mathrm {{\mu m}}$ and
$\mathrm {[S II]}$ 6716 {\AA} forbidden lines, the $\mathrm {[O I]}$ 6300 {\AA}
airglow line, and H$\mathrm \alpha\, 6563$ {\AA} emission. We simulate
($\mathrm{80\sim200\,km\,s^{-1}}$) dense ($\mathrm{\sim10^{5}\,cm^{-3}}$)
outflows expanding into a stationary ambient medium. In the case of an atomic
wind interacting with an atomic medium, a decelerating advancing turbulent
shell thickens with time. This contrasts with all other cases where a shell
fragments into a multitude of cometary-shaped protrusions with weak oblique
shocks as the main source of gas excitation. We find that the atomic
wind-ambient simulation leads to considerably higher excitation, stronger peak
and integrated atomic emission as the nebula expands. The weaker emission when
one component is molecular is due to the shell fragmentation into fingers so
that the shock surface area is increased and oblique shocks are prevalent.
Position-velocity diagrams indicate that the atomic-wind model may be most easy
to distinguish with more emission at higher radial velocities. With post-AGB
winds and shells often highly obscured and the multitude of configurations that
are observed, this study suggests and motivates selection criteria for new
surveys.
