Numerical simulations of wind-driven protoplanetary nebulae. I.
  near-infrared emission
  To understand how the circumstellar environments of post-AGB stars develop
into planetary nebulae, we initiate a systematic study of 2D axisymmetric
hydrodynamic simulations of protoplanetary nebula (pPN) with a modified ZEUS
code. The aim of this first work is to compare the structure of prolate
ellipsoidal winds into a stationary ambient medium where both media can be
either atomic or molecular. We specifically model the early twin-shock phase
which generates a decelerating shell. A thick deformed and turbulent shell
grows when an atomic wind expands into an atomic medium. In all other cases,
the interaction shell region fragments into radial protrusions due to molecular
cooling and chemistry. The resulting fingers eliminate any global slip parallel
to the shell surface. This rough surface implies that weak shocks are prominent
in the excitation of the gas despite the fast speed of advance. This may
explain why low excitation molecular hydrogen is found towards the front of
elliptical pPN. We constrain molecular dissociative fractions and timescales of
fast $\mathrm H_2$ winds and the pPN lifetime with wind densities
$\mathrm{\sim10^{5}cm^{-3}}$ and shock speeds of
$\mathrm{80\sim200\,km\,s^{-1}}$. We identify a variety of stages associated
with thermal excitation of H$_2$ near-infrared emission. Generated line
emission maps and position-velocity diagrams enable a comparison and
distinction with post-AGB survey results. The $\mathrm{1\to0 \, S(1)}$ $\&$
$\mathrm{2\to1 \, S(1)}$ lines are lobe-dominated bows rather than bipolar
shells.
