Modeling the Infrared Extinction toward the Galactic Center
  We model the ~1--19$\mum$ infrared (IR) extinction curve toward the Galactic
Center (GC) in terms of the standard silicate-graphite interstellar dust model.
The grains are taken to have a power law size distribution with an exponential
decay above some size. The best-fit model for the GC IR extinction constrains
the visual extinction to be Av~38--42 mag. The limitation of the model, i.e.,
its difficulty in simultaneously reproducing both the steep ~1--3$\mum$ near-IR
extinction and the flat ~3--8$\mum$ mid-IR extinction is discussed. We argue
that this difficulty could be alleviated by attributing the extinction toward
the GC to a combination of dust in different environments: dust in diffuse
regions (characterized by small Rv and steep near-IR extinction), and dust in
dense regions (characterized by large Rv and flat UV extinction).
