Oxygen and iron in interstellar dust: an X-ray view

Understanding the chemistry of the interstellar medium (ISM) is fundamental for the comprehension of the Galaxy evolution. Some of the refractory interstellar elements, such as Si, Mg, O and Fe, are known to be locked up into dust grains. Iron is known to be highly depleted from the gas phase into solids, but the exact composition of the iron-bearing grains is not yet well understood. Oxygen is one of the most abundant elements in the Galaxy and important for life on Earth. Nevertheless, the exact reservoirs of oxygen in different interstellar environments still remain an open question. The X-ray energy band includes a plethora of transitions from atomic and solid species of elements from carbon to nickel. X-ray radiation from astronomical X-ray sources can be absorbed by atoms and solids in the ISM, producing distinct absorption features. In particular, the so-called X-ray absorption fine structures (XAFS) are features of dust observed near the corresponding photoelectric absorption edges. By studying these features, we can investigate the dust chemical composition, crystallinity and grain size in different density environments of the ISM. To study the XAFS we need up-to-date dust models. In this talk, I will discuss a group effort to build a global X-ray dust extinction model based on laboratory experiments. Here, I will focus on the atomic and solid phase features of the ISM observed near the photoelectric edges of oxygen and iron. I will present recent results on dust mineralogy using our newly calculated dust models and their application to the high-resolution X-ray absorption spectra of bright background sources along the galactic plane. In this study we use the XMM-Newton and Chandra satellites but I will also discuss the prospects of studying the dust with future X-ray missions, such as XRISM. Finally, I will give emphasis on the importance of an up-to-date X-ray atomic database for this study.