Infrared Emission from Interstellar Dust. II. The Diffuse Interstellar Medium

Abstract

We present a quantitative model for the infrared emission from dust in the diffuse interstellar medium. The model consists of a mixture of amorphous silicate grains and carbonaceous grains, each with a wide size distribution ranging from molecules containing tens of atoms to large grains ≳1 μm in diameter. We assume that the carbonaceous grains have properties like polycyclic aromatic hydrocarbons (PAHs) at very small sizes and graphitic properties for radii a ≳ 50 Å. On the basis of recent laboratory studies and guided by astronomical observations, we propose "astronomical" absorption cross sections for use in modeling neutral and ionized PAHs from the far-ultraviolet to the far-infrared. We also propose modifications to the far-infrared emissivity of "astronomical silicate." We calculate energy distribution functions for small grains undergoing "temperature spikes" caused by stochastic absorption of starlight photons using realistic heat capacities and optical properties. Using a grain-size distribution consistent with the observed interstellar extinction, we are able to reproduce the near-IR to submillimeter emission spectrum of the diffuse interstellar medium, including the PAH emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 μm. The model is compared with the observed emission at high Galactic latitudes as well as in the Galactic plane, as measured by the COBE /DIRBE, COBE /FIRAS, IRTS/MIRS, and IRTS/NIRS instruments. The model has 60 × 10 -6 of C (relative to H) locked up in PAHs, with 45 × 10 -6 of C in a component peaking at ~6 Å ( N C ≈ 100 carbon atoms) to account for the PAH emission features and with 15 × 10 -6 of C in a component peaking at ~50 Å to account for the 60 μm flux. The total infrared emission is in excellent agreement with COBE /DIRBE observations at high Galactic latitudes, just as the albedo for our grain model is in accord with observations of the diffuse Galactic light. The aromatic absorption features at 3.3 and 6.2 μm predicted by our dust model are consistent with observations. We calculate infrared emission spectra for our dust model heated by a range of starlight intensities, from 0.3 to 10 4 times the local interstellar radiation field, and we tabulate the intensities integrated over the SIRTF /IRAC and MIPS bands. We also provide dust opacities tabulated from the extreme-ultraviolet to submillimeter wavelengths.