Modeling the Infrared Interstellar Extinction
  How dust scatters and absorbs starlight in the interstellar medium (ISM)
contains important clues about the size and composition of interstellar dust.
While the ultraviolet (UV) and visible interstellar extinction is well studied
and can be closely fitted in terms of various dust mixtures (e.g., the
silicategraphite mixture), the infrared (IR) extinction is not well understood,
particularly, the mid-IR extinction in the 3-8 micron wavelength range is
rather flat (or "gray") and is inconsistent with the standard Mathis, Rumpl, &
Nordsieck (MRN) silicate-graphite dust model. We attempt to reproduce the flat
IR extinction by exploring various dust sizes and species, including amorphous
silicate, graphite, amorphous carbon and iron. We find that the flat IR
extinction is best explained in terms of micrometer-sized amorphous carbon dust
which consumes ~ 60 carbon atoms per million hydrogen atoms (i.e., C/H ~ 60
ppm). To account for the observed UV/visible and near-IR extinction, the
silicate-graphite model requires Si/H ~ 34 ppm and C/H ~ 292 ppm. We conclude
that the extinction from the UV to the mid-IR could be closely reproduced by a
mixture of submicrometer-sized amorphous silicate dust, submicrometer-sized
graphitic dust, and micrometer-sized amorphous carbon dust, at the expense of
excess C available in the ISM (i.e., this model requires a solid-phase C
abundance of C/H ~ 352 ppm, considerably exceeding what could be available in
the ISM).
