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Mineralogy of Interstellar Dust in the X-ray Regime

Presentation #300.04 in the session ISM/Galaxies.

Published onMay 03, 2024
Mineralogy of Interstellar Dust in the X-ray Regime

Cosmic dust is a small (1-2%) but important component of the Galactic interstellar medium (ISM) as it appears in every stage of stellar evolution, from evolved stars and supernovae to protoplanetary disks. Dust plays a major role in the evolution of our Galaxy; it cools down interstellar clouds enabling star formation, it provides the surface for chemical reactions, and is thought to be the primary building block of new planets. However, the exact dust chemical composition in the ISM, along with its size and crystallinity (i.e. lattice structure) is still an active area of study. High-resolution X-ray spectroscopy is a powerful way to study the interstellar dust properties. The X-ray band is sensitive to absorption from n=1 (K edge) and n=2 (L edge) electrons of the most abundant interstellar metals: C, N, O, Ne, Si, Mg, and Fe. In particular, X-ray absorption fine structures (XAFS) are spectroscopic features observed near the photoelectric absorption edges of solid material (dust), and their shape is the ultimate footprint of the dust chemical composition, size, morphology and lattice structure. To study the XAFS we need up-to-date dust models. In this talk, I will discuss our collaborative effort to build a global X-ray dust extinction model based on laboratory experiments. I will demonstrate the newest laboratory experiment of XAFS from astrophysical dust samples in the O K and Fe L photoabsorption edges (Psaradaki et al. 2020,2021). I will further present recent results (Psaradaki et al. 2023) on dust mineralogy in the diffuse regions in our Galaxy using the new dust models and high-resolution X-ray spectra from Chandra and XMM-Newton satellites. By fitting together the O K and Fe L edges for a sample of background sources along the Galactic plane, we discovered that the Mg-rich amorphous pyroxene dust composition (Mg0.75Fe0.25SiO3), and metallic iron represents the bulk of the dust chemistry in the diffuse ISM. This gives the most comprehensive view of the dust mineralogy in the diffuse regions of ISM, through the X-ray energy band so far. Finally, I will focus on dust studies with XRISM, and discuss the prospects of studying the dust grain chemistry and size distribution in denser regions of the ISM.

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