The composition of solid phase Fe in the interstellar medium

Understanding the composition of interstellar Fe, suspected to condense into solid dust grains within the diffuse interstellar medium (ISM), remains a challenge. We use the XMM Reflection Grating Spectrometer to conduct a high-resolution spectral analysis of Fe L shell absorption in the ISM toward the X-ray binaries Cygnus X-1 and GX 339-4. Meteoritic studies of suspected interstellar grain analogs and computational simulations hint that metallic Fe nanoparticles or Fe-rich sulfides could dominate. Using cross-sections from Lee et al. (2009) and Kortright & Kim (2000), we model the absorption and scattering profiles of metallic Fe, fayalite, ferrous sulfate, hematite, and lepidocrocite using a power-law distribution of grain sizes. We also include gas-phase Fe oxidation states from neutral to Fe³+. We find that gas-phase Fe accounts for a small fraction of total interstellar Fe in these sight lines. The observed dust absorption in the Fe L spectral features are offset from the laboratory cross-sections we examined, indicating discrepancies in the energy scale calibration. To address this, we search for the best fitting extinction cross-sections together with shifts in their energy scale. Based on our analysis, we suggest iron oxides in the Fe³+ oxidation state as the prime candidates for Fe-bearing dust in these sight lines. To confirm this, further laboratory assessments of energy calibration for suspected interstellar compounds is required.