Spitzer IRS spectroscopy of planetary nebulae

Abstract

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 μm 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.