Sulfur-Bearing Polycyclic Aromatic Hydrocarbons in Interstellar Environments
Polycyclic aromatic hydrocarbons (PAHs) are widely studied in an astrophysical context. Studies have also been performed on nitrogen-bearing PAH species (Hudgins+2005) and unsuccessful searches have been performed for interstellar nitrogen-bearing PAHs (Ali-Haïmoud+2015). Laboratory irradiation of ices containing aromatic hydrocarbons has recently been found to produce oxygen and nitrogen heterocycles (Materese+2015), though previous astronomical searches for such heterocycles have proven unsuccessful (Charnley+2005; Dezafra+1972; Kutner+1980).
However, astrochemical studies on interstellar sulfur are quite neglected. Sulfur heterocycles have received little astrochemical attention, despite having been detected inside meteorites (Krishnamurthy+1992; Sephton+2002). Thiophenes are expected to be stable at interstellar medium (ISM) temperatures (Guillemin+2010). Observed sulfur depletion does not follow the same pattern as other elements in dust grains (White+2011), and small sulfur compounds like H₂CS are detected in outflows where they are expected to form into more complex species (Minh+2011).
Given that sulfur and carbon have similar electronegativities and both have extensive chemistries, it is logical to consider heterocyclic sulfur-bearing PAH molecules in astrochemical studies, both experimentally and observationally. Vibrational wavelengths are expected to fall in the 6-9 µm region and at longer wavelengths, where it is well known that many observed emission bands are still poorly understood, with several unidentified or tentatively assigned lines remaining.