Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era

Abstract

Infrared (IR) emission spectra are calculated for dust composed of mixtures of amorphous silicate and graphitic grains, including varying amounts of polycyclic aromatic hydrocarbon (PAH) particles. The models are constrained to reproduce the average Milky Way extinction curve. The calculations include the effects of single-photon heating. Updated IR absorption properties for the PAHs are presented, that are consistent with observed emission spectra, including those newly obtained by Spitzer Space Telescope. We find a size distribution for the PAHs that results in emission band ratios consistent with observed spectra of the Milky Way and other galaxies. Emission spectra are presented for various intensities of the illuminating starlight. We calculate how the efficiency of emission into different IR bands depends on PAH size; the strong 7.7um emission feature is produced mainly by PAH particles containing <1000 C atoms. We show how the emission spectrum depends on U, the starlight intensity relative to the local interstellar radiation field. The submm and far-infrared emission is compared to the observed emission from the local interstellar medium. Using a simple distribution function, we calculate the emission spectrum for dust heated by a distribution of starlight intensities, such as occurs within galaxies. The models are parameterized by the PAH mass fraction qpah, the lower cutoff Umin, and the fraction gamma of the dust heated by starlight with U>Umin. We present graphical procedures using IRAC and MIPS photometry to estimate qpah, Umin, and gamma, the fraction f_PDR of the dust luminosity coming from photodissociation regions with U>100, and the total dust mass Mdust.

Figures (23)

• Figure 1: Observed 5--20$\mu$ m spectra for: (a) Reflection nebula NGC 7023 Werner+Uchida+Sellgren_etal_2004; (b) Orion Bar PDR Verstraete+Pech+Moutou_etal_2001; (c) M17 PDR Peeters+Tielens+Boogert_etal_2005; (d) Planetary nebula NGC 7027 vanDiedenhoven+Peeters+vanKerckhoven_etal_2004; (e) Seyfert Galaxy NGC 5194 Smith+Draine+Dale_etal_2006. Also shown (f) is the emission calculated for the present dust model with $q_{\rm PAH}=4.6\%$, illuminated by the local diffuse starlight with $U=1$ and $10^5$ (see Fig. \ref{['fig:variousU_spec']}).
• Figure 2: Solid curve: adopted absorption cross section per C from eq. (\ref{['eq:adopted Cabs']}) with C/H$\approx$3.2 (e.g., C$_{64}$H$_{20}$). For the neutrals, anions, and cations listed in the figure legends, the horizontal line segments indicate the average absorption over that frequency intervals, taken from theoretical calculations by Malloci+Joblin+Mulas_2006. The heavy solid line segment is the average for the 6 species shown.
• Figure 3: Absorption cross section per C atom for (a) neutral and ionized PAHs, and (b) for ionized carbonaceous grains, with properties of PAHs for $6\,{\rm \AA}<a<50\,{\rm \AA}$, and properties of graphite spheres for $a\gtrsim 100\,{\rm \AA}$. See §\ref{['sec:PAH Cross Sections']} for details.
• Figure 4: Temperature probability distribution $dP/d\ln T$ for selected carbonaceous grains heated by starlight with $U=1$ and $U=10^4$.
• Figure 5: Normalized time-averaged emission spectra for $U<10^4$ for (a) neutral and (b) ionized PAHs of various sizes (see text).