The Dustiest Galactic S Stars: Mid-Infrared Spectra from SOFIA/FORCAST

TL;DR

This study presents mid-infrared spectra of the dustiest S stars in the Milky Way, revealing unique dust and molecular features that differ from typical oxygen- or carbon-rich AGB stars, indicating complex and less processed hydrocarbon chemistry.

Contribution

First mid-infrared spectral analysis of the dustiest S stars, highlighting their unique dust and molecular features and expanding understanding of their chemical compositions.

Findings

Most stars show strong 10-11 μm dust emission features with atypical shapes.

Detection of crystalline alumina and complex hydrocarbons in several stars.

Absence of strong 11.3 μm hydrocarbon feature suggests predominance of aliphatic hydrocarbons.

Abstract

We present spectra of 12 of the reddest, and hence dustiest, S stars in the Milky Way, observed with the FORCAST grisms on SOFIA. S stars are asymptotic giant branch (AGB) stars with C/O$\sim$1, so their molecular and dust chemistries are dominated by neither O nor C, often leading to atypical spectral features from their molecules and dust grains. All of the stars in our sample have strong dust emission features at 10--11 $\mu$m, but the shape of the feature in most of the stars differs from the shapes commonly observed in either oxygen-rich or carbon-rich AGB stars. Two stars also show the 13 $\mu$m feature associated with crystalline alumina. Two have a water absorption band at $\sim$6.5--7.5 $\mu$m, and a third has a tentative detection, but only one of these three has the more common SiO absorption band at 7.5 $\mu$m. Three others show a red 6.3 $\mu$m emission feature from complex hydrocarbons consistent with ``Class C'' objects, and in a fourth it appears at 6.37 $\mu$m, redder than even the standard Class C hydrocarbon feature. Class C spectra typically indicate complex hydrocarbons which have been less processed by UV radiation, resulting in more aliphatic bonds relative to aromatic bonds. None of the S stars shows a strong 11.3 $\mu$m hydrocarbon feature, which is also consistent with the presence of aliphatic hydrocarbons.