Investigating the interstellar dust through the Fe K-edge^⋆

¹ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
e-mail: d.rogantini@sron.nl
² Anton Pannekoek Astronomical Institute, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Netherlands Organisation for Scientific Research (NWO) DUBBLE@ESRF, CS 4022, 38043 Grenoble Cedex 9, France
⁵ Debye Institute for Nanomaterials Science, Utrecht University, Universiteitweg 99, 3584 CG Utrecht, The Netherlands
⁶ Astrophysikalisches Institut und Universitäts-Sternwarte (AIU), Schillergäßchen 2-3, 07745 Jena, Germany

Received: 27 July 2017
Accepted: 4 September 2017

Abstract

Context. The chemical and physical properties of interstellar dust in the densest regions of the Galaxy are still not well understood. X-rays provide a powerful probe since they can penetrate gas and dust over a wide range of column densities (up to 10²⁴ cm^-2). The interaction (scattering and absorption) with the medium imprints spectral signatures that reflect the individual atoms which constitute the gas, molecule, or solid.

Aims. In this work we investigate the ability of high resolution X-ray spectroscopy to probe the properties of cosmic grains containing iron. Although iron is heavily depleted into interstellar dust, the nature of the Fe-bearing grains is still largely uncertain.

Methods. In our analysis we use iron K-edge synchrotron data of minerals likely present in the ISM dust taken at the European Synchrotron Radiation Facility. We explore the prospects of determining the chemical composition and the size of astrophysical dust in the Galactic centre and in molecular clouds with future X-ray missions. The energy resolution and the effective area of the present X-ray telescopes are not sufficient to detect and study the Fe K-edge, even for bright X-ray sources.

Results. From the analysis of the extinction cross sections of our dust models implemented in the spectral fitting program SPEX, the Fe K-edge is promising for investigating both the chemistry and the size distribution of the interstellar dust. We find that the chemical composition regulates the X-ray absorption fine structures in the post edge region, whereas the scattering feature in the pre-edge is sensitive to the mean grain size. Finally, we note that the Fe K-edge is insensitive to other dust properties, such as the porosity and the geometry of the dust.