Spitzer mid-infrared spectroscopic observations of planetary nebulae

TL;DR

This study uses Spitzer mid-infrared spectroscopy to analyze planetary nebulae, revealing consistent H2 excitation temperatures, ionic line detections, and insights into nebular composition and stellar evolution.

Contribution

First mid-IR spectroscopic analysis of planetary nebulae covering key emission lines, providing new data on molecular and ionic conditions.

Findings

H2 rotational temperature ~900 K across sources

Detection of ionic lines [Ar III], [S IV], [Ne II]

Line ratio trends suggest neon enrichment or high-density regions

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 {\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.