Interstellar Dust

The compositional make-up of interstellar (ISM) dust and the relative abundances of chemical elements in astrophysical environments are not well understood. Since dust is a primary repository of the ISM, and is responsible for the chemical evolution of stars, planets, and life itself, it has a profound effect on many areas of astrophysical research from cosmology to star and planet formation. Powerful X-ray satellites (ChandraXMM-NewtonSuzaku) pointed through dust at bright black hole (BH) and neutron star (NS) systems can be used to study ISM dust in unique ways:

  • X-ray dust scattering halos (Fig. 1) can be found surrounding many bright BH and NS systems with dusty lines-of-sight. Using knowledge that X-ray halo properties tightly depend on the size distribution of the dust grains and the relative distance of the scattering by the dust grains, we can use geometry arguments (Fig. 2) to study halo profiles and distinguish the location of dust clouds close to the X-ray system (sharp, narrow halos) against those close to us (flat, wide halos), and determine their distribution (Fig. 3). For some X-ray binary systems, we can measure scattering delay times to determine distance to the BH/NS system itself. 

  • We have begun a campaign to directly probe ISM dust composition by combining space based high spectral resolution Chandra HETGS X-ray measurements with laboratory synchrotron experiments at the ALS of X-ray absorption fine structure (XAFS, Fig. 4). Combined with infared Spitzer studies, we can begin to address such questions as:
    • what is the mineralogy of ISM dust -- is it crystalline or amorphous?
    • is there thermally emitting dust in/near X-ray emitting systems -- what of their properties?


Julia C. Lee, Jingen Xiang


Fig 1: Chandra High Energy Transmission Grating (HETGS) observations of Black Hole and Neutron Star systems often show an extended halo surrounding the X-ray emitting system (central black dot). Extending from the central regions in the E-W direction are spectra dispersed by the HETGS gratings.

Fig 2: Geometry of X-ray scattering depicting how we can combine scattering halo and X-ray source properties to determine location and spatial distribution of ISM dust.

Fig. 3: Based on our X-ray scattering halo study (Xiang, Lee, & Nowak 2007) of the neutron star system 4U 1624-490, we were able to determine both the dust distribution along the line-of-sight (black solid lines), as well as distance to this X-ray binary (red dot).
Click image for a larger version

Fig. 4: The absorption Cross Section of fayalite near the FeK photoelectric edge as measured at the ALS (black) compared with the same data "blurred" to mimic calorimeter resolutions for space based missions (in this case, the Suzaku XRS, but there is also relevance to the planned Constellation-X calorimeters). Figure taken from Lee & Ravel 2005.