VERY LARGE INTERSTELLAR GRAINS AS EVIDENCED BY THE MID-INFRARED EXTINCTION

Abstract

The sizes of interstellar grains are widely distributed, ranging from a few\nangstroms to a few micrometers. The ultraviolet (UV) and optical extinction\nconstrains the dust in the size range of a couple hundredth micrometers to\nseveral submicrometers. The near and mid infrared (IR) emission constrains the\nnanometer-sized grains and angstrom-sized very large molecules. However, the\nquantity and size distribution of micrometer-sized grains remain unknown as\nthey are gray in the UV/optical extinction and they are too cold and emit too\nlittle in the IR to be detected by IRAS, Spitzer, or Herschel. In this work, we\nemploy the ~3-8 micron mid-IR extinction which is flat in both diffuse and\ndense regions to constrain the quantity, size, and composition of the\nmicron-sized grain component. We find that, together with nano- and\nsubmicron-sized silicate and graphite (as well as PAHs), micron-sized graphite\ngrains with C/H=137 ppm and a mean size of ~1.2 micron closely fit the observed\ninterstellar extinction of the Galactic diffuse interstellar medium from the\nfar-UV to the mid-IR as well as the near-IR to millimeter thermal emission\nobtained by COBE/DIRBE, COBE/FIRAS, and Planck up to lambda < 1000 micron. The\nmicron-sized graphite component accounts for ~14.6% of the total dust mass and\n~2.5% of the total IR emission.\n