The unusual hydrocarbon emission from the early carbon star HD 100764: The connection between aromatics and aliphatics

TL;DR

This study analyzes the unusual infrared emission features from the carbon star HD 100764, revealing a connection between aromatic and aliphatic hydrocarbons influenced by a weak radiation environment.

Contribution

It provides new insights into the nature of PAH emissions in low-radiation fields, linking spectral shifts to the presence of both aromatic and aliphatic hydrocarbons.

Findings

PAH features are shifted to longer wavelengths in HD 100764.

The spectral characteristics suggest a mixture of aromatic and aliphatic hydrocarbons.

Weak UV radiation allows fragile aliphatic materials to survive.

Abstract

We have used the Infrared Spectrograph (IRS) on the Spitzer Space Telescope to obtain spectra of HD 100764, an apparently single carbon star with a circumstellar disk. The spectrum shows emission features from polycyclic aromatic hydrocarbons (PAHs) that are shifted to longer wavelengths than normally seen, as characteristic of ``class C'' systems in the classification scheme of Peeters et al. All seven of the known class C PAH sources are illuminated by radiation fields that are cooler than those which typically excite PAH emission features. The observed wavelength shifts are consistent with hydrocarbon mixtures containing both aromatic and aliphatic bonds. We propose that the class C PAH spectra are distinctive because the carbonaceous material has not been subjected to a strong ultraviolet radiation field, allowing relatively fragile aliphatic materials to survive.