Silicon, Sulfur, and Iron in the Interstellar Medium: A High-resolution X-Ray Spectral Study of GX 340+0

High-resolution X-ray spectroscopy provides a powerful probe of the interstellar medium (ISM), giving direct access to the composition and physical state of dust grains and atomic species in dense environments. We present a study of the gas and dust along the line of sight to the bright low-mass X-ray binary GX 340+00, which samples higher-density gases in the ISM. Using deep Chandra/HETG spectra, we characterize X-ray absorption fine structure from dust, gas absorption lines, and the optical depths of the Si, S, and Fe K edges. By fitting these three edges simultaneously, we reduce degeneracies in the dust composition and find that amorphous olivine dominates the fractional contribution among the dust columns (∼65%), followed by metallic iron (∼19%), iron sulfides (pyrrhotite and troilite; ∼10%), and fayalite (∼5%), with the remaining species contributing only a few percent in total. From the inferred stoichiometry, we estimate that ∼74% of Fe is associated with silicates, ∼8% with sulfides, and ∼18% with metallic iron, suggesting that Fe is predominantly incorporated in iron-rich silicate grains along this sight line. We detect S ii absorption and infer a sulfur dust fraction of ∼35%. We also detect absorption structure near the Ca and Ar K edges, highlighting the need for improved atomic photoabsorption data. The Chandra/HETG spectral resolution remains essential to disentangle dust and gas contributions at the Si and S K edges, providing a benchmark for dust characterization in high-density regions in the ISM.