Modern Problems in High Resolution X-ray Absorption from the Cold Interstellar Medium (HEAD 2020 in Rosemont, IL)

The X-ray energy band is sensitive to absorption by all abundant metals in the interstellar medium (ISM) — carbon, oxygen, neon, silicon, magnesium, and iron — whether they are in gas or dust form. Photoelectric edges seen in high resolution X-ray spectra of Galactic X-ray point sources allow us to measure absolute abundances of gas and solid phase metals in the ISM, with fewer underlying assumptions. X-ray absorption fine structure (XAFS) in the photoelectric edge also reveals dust grain mineralogies and grain sizes. We review open problems in the field of astromineralogy, including problems in reconciling X-ray absorption spectroscopy with ISM observations at other wavelengths. In the future, high resolution imaging spectroscopy enabled by micro-calorimeters can probe the constituent elements of large dust grains, which are relatively invisible in absorption. Gratings spectroscopy is also necessary to obtain high resolution spectra at soft energies, required for probing the state of neutral carbon and oxygen — the two most abundant interstellar metals and the two most important actors in the field of astromineralogy.

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