Where Have All the Sulfur Atoms Gone? Polycyclic Aromatic Hydrocarbon as a Possible Sink for the Missing Sulfur in the Interstellar Medium. I. The C--S Band Strengths

TL;DR

This study investigates sulfurated polycyclic aromatic hydrocarbons (PASHs) as potential reservoirs for missing sulfur in the interstellar medium, using computational spectra to identify characteristic infrared features that could be observed by space telescopes.

Contribution

It introduces sulfurated PAHs as a novel S reservoir candidate and computes their vibrational spectra to guide future astronomical detection.

Findings

PASH molecules show a distinct 10 micron C--S stretching band.

Weak C--S deformation bands are found at 15 and 25 microns.

Detection of these bands could indicate sulfur sequestration in PAHs.

Abstract

Despite its biogeneic and astrochemical importance, sulfur (S), the 10th most abundant element in the interstellar medium (ISM) with a total abundance of S/H~2.2E-5, largely remains undetected in molecular clouds. Even in the diffuse ISM where S was previously often believed to be fully in the gas phase, in recent years observational evidence has suggested that S may also be appreciably depleted from the gas. What might be the dominant S reservoir in the ISM remains unknown. Solid sulfides like MgS, FeS and SiS_2 are excluded as a major S reservoir due to the undetection of their expected infrared spectral bands in the ISM. In this work, we explore the potential role of sulfurated polycyclic aromatic hydrocarbon (PAH) molecules -- PAHs with sulfur heterocycles (PASHs) -- as a sink for the missing S. Utilizing density function theory, we compute the vibrational spectra of 18 representative PASH molecules. It is found that these molecules exhibit a prominent, C--S stretching band at ~10 micron and two relatively weak, C--S deformation bands at 15 and 25 micron that are not mixed with the nominal PAH bands at 6.2, 7.7, 8.6, 11.3 and 12.7 micron If several parts per million of S (relative to H) are locked up in PAHs, the 10 micron C--S band would be detectable by Spitzer and JWST. To quantitatively explore the amount of S/H depleted in PASHs, detailed comparison of the infrared emission spectra of PASHs with the Spitzer and JWST observations is needed.