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

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 ∼ 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 μ 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 μ m feature associated with crystalline alumina. Two have a water absorption band at ∼6.5–7.5 μ m, and a third has a tentative detection, but only one of these three has the more common SiO absorption band at 7.5 μ m. Three others show a red 6.3 μ m emission feature from complex hydrocarbons consistent with “Class C” objects, and in a fourth it appears at 6.37 μ m, redder than even the standard Class C hydrocarbon feature. Class C spectra typically indicate complex hydrocarbons that have been less processed by UV radiation, resulting in more aliphatic bonds relative to aromatic bonds. None of the S stars show a strong 11.3 μ m hydrocarbon feature, which is also consistent with the presence of aliphatic hydrocarbons.