Unusual Dust Emission from Planetary Nebulae in the Magellanic Clouds

TL;DR

This study uses Spitzer infrared spectroscopy to analyze dust features in 25 planetary nebulae in the Magellanic Clouds, revealing diverse dust compositions and unusual features, especially silicon carbide presence, at low metallicity environments.

Contribution

First detailed infrared spectral analysis of Magellanic Cloud planetary nebulae highlighting unusual dust compositions and features not commonly observed in similar environments.

Findings

Over half show polycyclic aromatic hydrocarbons.

Silicon carbide features are unusually common.

Oxygen-rich dust is rare at low metallicity.

Abstract

We present a Spitzer Space Telescope spectroscopic study of a sample of 25 planetary nebulae in the Magellanic Clouds. The low-resolution modules are used to analyze the dust features present in the infrared spectra. This study complements a previous work by the same authors where the same sample was analyzed in terms of neon and sulfur abundances. Over half of the objects (14) show emission of polycyclic aromatic hydrocarbons, typical of carbon-rich dust environments. We compare the hydrocarbon emission in our objects to those of Galactic HII regions and planetary nebulae, and LMC/SMC HII regions. Amorphous silicates are seen in just two objects, enforcing the now well-known-fact that oxygen-rich dust is less common at low metallicities. Besides these common features, some planetary nebulae show very unusual dust. Nine objects show a strong silicon carbide feature at 11um and twelve of them show magnesium sulfide emission starting at 25um. The high percentage of spectra with silicon carbide in the Magellanic Clouds is not common. Two objects show a broad band which may be attributed to hydrogenated amorphous carbon and weak low-excitation atomic lines. It is likely that these nebulae are very young. The spectra of the remaining eight nebulae are dominated by the emission of fine-structure lines with a weak continuum due to thermal emission of dust, although in a few cases the S/N in the spectra is low, and weak dust features may not have been detected.