Shu Wang^1,2, Aigen Li², and B. W. Jiang^1
1Department of Astronomy, Beijing Normal University, Beijing 100875, China
²Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA

The sizes of interstellar grains are widely distributed, ranging from a few angstroms to a few micrometers. The ultraviolet (UV) and optical extinction constrains the dust in the size range of a couple hundredths of micrometers to several submicrometers. The near and mid infrared (IR) emission constrains the nanometer-sized grains and angstrom-sized very large molecules. However, the quantity and size distribution of micrometer-sized grains remain unknown because they are gray in the UV/optical extinction and they are too cold and emit too little in the IR to be detected by IRAS, Spitzer, or Herschel. In this work, we employ the ~3–8 μm mid-IR extinction, which is flat in both diffuse and dense regions to constrain the quantity, size, and composition of the μm-sized grain component. We find that, together with nano- and submicron-sized silicate and graphite (as well as polycyclic aromatic hydrocarbons), μm-sized graphite grains with C/H ≈ 137 ppm and a mean size of ~1.2 μm closely fit the observed interstellar extinction of the Galactic diffuse interstellar medium from the far-UV to the mid-IR, as well as the near-IR to millimeter thermal emission obtained by COBE/DIRBE, COBE/FIRAS, and Planck up to λ lesssim 1000 μm. The μm-sized graphite component accounts for ~14.6% of the total dust mass and ~2.5% of the total IR emission.

Reference
Wang S, Li A and Jiang BW (2015) Very large interstellar grains as evidenced by the mid-infrared extinction. Astrophysical Journal 811:38.
Link to Article [doi:10.1088/0004-637X/811/1/38]