Very Large Interstellar Grains as Evidenced by the Mid-Infrared
  Extinction
  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 hundredth 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 as
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 micron mid-IR extinction which is flat in both diffuse and
dense regions to constrain the quantity, size, and composition of the
micron-sized grain component. We find that, together with nano- and
submicron-sized silicate and graphite (as well as PAHs), micron-sized graphite
grains with C/H=137 ppm and a mean size of ~1.2 micron 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 lambda < 1000 micron. The
micron-sized graphite component accounts for ~14.6% of the total dust mass and
~2.5% of the total IR emission.
