Spitzer mid-infrared spectroscopic observations of planetary nebulae
  We present Spitzer Space Telescope archival mid-infrared (mid-IR)
spectroscopy of a sample of eleven planetary nebulae (PNe). The observations,
acquired with the Spitzer Infrared Spectrograph (IRS), cover the spectral range
5.2-14.5 {\mu}m that includes the H2 0-0 S(2) to S(7) rotational emission
lines. This wavelength coverage has allowed us to derive the Boltzmann
distribution and calculate the H2 rotational excitation temperature (Tex). The
derived excitation temperatures have consistent values ~900+/-70 K for
different sources despite their different structural components. We also report
the detection of mid-IR ionic lines of [Ar III], [S IV], and [Ne II] in most
objects, and polycyclic aromatic hydrocarbon (PAH) features in a few cases. The
decline of the [Ar III]/[Ne II] line ratio with the stellar effective
temperature can be explained either by a true neon enrichment or by high
density circumstellar regions of PNe that presumably descend from higher mass
progenitor stars.
