Very Large Interstellar Grains as Evidenced by the Mid-Infrared Extinction

TL;DR

This study provides evidence for the presence of very large, micron-sized interstellar grains, particularly graphite, by analyzing mid-infrared extinction and thermal emission data, revealing their significant contribution to dust mass and IR emission.

Contribution

It introduces constraints on micron-sized interstellar grains using mid-IR extinction data, demonstrating their substantial presence and specific properties in the Galactic diffuse interstellar medium.

Findings

Micron-sized graphite grains account for ~14.6% of dust mass.

These grains have a mean size of ~1.2 microns.

They contribute ~2.5% to total IR emission.

Abstract

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.