  We present the Spitzer/IRS spectra of 157 compact Galactic PNe. These young
PNe provide insight on the effects of dust in early post-AGB evolution, before
much of the dust is altered or destroyed by the hardening stellar radiation
field. Most of the selected targets have PN-type IRS spectra, while a few
turned out to be misclassified stars. We inspected the group properties of the
PN spectra and classified them based on the different dust classes
(featureless, carbon-rich dust; oxygen-rich dust; mixed-chemistry dust) and
subclasses (aromatic and aliphatic; crystalline and amorphous). All PNe are
characterized by dust continuum and more than 80% of the sample shows solid
state features above the continuum, in contrast with the Magellanic Cloud
sample where only ~40% of the entire sample displays solid state features; this
is an indication of the strong link between dust properties and metallicity.
The Galactic PNe that show solid state features are almost equally divided
among the CRD, ORD, and MCD. We analyzed dust properties together with other PN
properties and found that (i) there is an enhancement of MCD PNe toward the
Galactic center; (ii) CRD PNe could be seen as defining an evolutionary
sequence, contrary to the ORD and MCD PNe; (iii) C- and O-rich grains retain
different equilibrium temperatures, as expected from models; (iv) ORD PNe are
highly asymmetric and CRD PNe highly symmetric; point-symmetry is statistically
more common in MCD. We find that the Galactic Disk sample does not include MCD
PNe, and the other dust classes are differently populated from high to low
metallicity environments. The MCPNe seem to attain higher dust temperatures at
similar evolutionary stages, in agreement with the observational findings of
smaller dust grains in low metallicity interstellar environments.
