Mineralogy of interstellar dust in the X-ray regime

Interstellar dust is an important ingredient of the interstellar medium (ISM) of galaxies. Dust appears in every stage of stellar evolution, from evolved stars and supernovae to protoplanetary disks. Knowing the dust grain chemistry, size and morphology of dust we can understand its production and destruction mechanisms and reveal the evolution history of our Galaxy. Silicate dust in particular is an important component of the life cycle of matter in the ISM and it mainly consists of the abundant astrophysical elements Si, Mg, Fe and O. In the last decades, high-resolution X-ray spectroscopy has proven to be a powerful tool for studying interstellar dust mineralogy. X-ray absorption fine structures (XAFS) are oscillatory modulations observed near the photoelectric absorption edges of solid material (dust), and their shape is the ultimate footprint of the dust chemical composition, size and crystallinity (i.e. lattice structure). In this presentation I will demonstrate the newest laboratory measurements of XAFS from astrophysical dust templates in the O K and Fe L photoabsorption edges (Psaradaki et al. 2020), and their application to high-resolution X-ray spectra of a sample of bright Galactic background sources. I will present recent results (Psaradaki et al. 2022) on dust mineralogy in the diffuse regions in our Galaxy using Chandra and XMM-Newton observations. We found that Mg-rich silicates represent the bulk of dust mass in the diffuse regions of the ISM. The new laboratory measurements also enabled the investigation of the composition of Fe bearing dust grains, along with the depletion of this element into solids. We found that iron is heavily depleted from the gas phase into solids; more than 90% of iron is in dust, and the bulk of solid phase iron is included in silicates, while a smaller amount is in metallic Fe. Finally, in this talk I will discuss the prospects of studying the silicate content of dust grains and their size distribution through the XRISM performance verification phase (PV) targets.