INTERSTELLAR DUST GRAINS Draine, B.T
Annual review of astronomy and astrophysics,
01/2003, Volume:
41, Issue:
1
Journal Article
Peer reviewed
Open access
This review surveys the observed properties of interstellar dust grains: the
wavelength-dependent extinction of starlight, including absorption features,
from UV to infrared; optical luminescence; ...infrared emission; microwave
emission; optical, UV, and X-ray scattering by dust; and polarization of
starlight and of infrared emission. The relationship between presolar grains in
meteorites and the interstellar grain population is discussed. Candidate grain
materials and abundance constraints are considered. A dust model consisting of
amorphous silicate grains, graphite grains, and polycyclic aromatic
hydrocarbons is compared with observed emission and scattering. Some issues
concerning evolution of interstellar dust are discussed.
We present a synthesis of the astronomical observations constraining the wavelength-dependent extinction, emission, and polarization from interstellar dust from UV to microwave wavelengths on diffuse ...Galactic sight lines. Representative solid-phase abundances for those sight lines are also derived. Given the sensitive new observations of polarized dust emission provided by the Planck satellite, we place particular emphasis on dust polarimetry, including continuum polarized extinction, polarization in the carbonaceous and silicate spectroscopic features, the wavelength-dependent polarization fraction of the dust emission, and the connection between optical polarized extinction and far-infrared polarized emission. Together, these constitute a set of constraints that should be reproduced by models of dust in the diffuse interstellar medium.
Radiation pressure acting on gas and dust causes H II regions to have central densities that are lower than the density near the ionized boundary. H II regions in static equilibrium comprise a family ...of similarity solutions with three parameters: Delta *b, Delta *g, and the product Q 0 n rms; Delta *b characterizes the stellar spectrum, Delta *g characterizes the dust/gas ratio, Q 0 is the stellar ionizing output (photons/s), and n rms is the rms density within the ionized region. Adopting standard values for Delta *b and Delta *g, varying Q 0 n rms generates a one-parameter family of density profiles, ranging from nearly uniform density (small Q 0 n rms) to shell-like (large Q 0 n rms). When Q 0 n rms 1052 cm--3 s--1, dusty H II regions have conspicuous central cavities, even if no stellar wind is present. For given Delta *b, Delta *g, and Q 0 n rms, a fourth quantity, which can be Q 0, determines the overall size and density of the H II region. Examples of density and emissivity profiles are given. We show how quantities of interest--such as the peak-to-central emission measure ratio, the rms-to-mean density ratio, the edge-to-rms density ratio, and the fraction of the ionizing photons absorbed by the gas--depend on Delta *b, Delta *g, and Q 0 n rms. For dusty H II regions, compression of the gas and dust into an ionized shell results in a substantial increase in the fraction of the stellar photons that actually ionize H (relative to a uniform-density H II region with the same dust/gas ratio and density n = n rms). We discuss the extent to which radial drift of dust grains in H II regions can alter the dust-to-gas ratio. The applicability of these solutions to real H II regions is discussed.
Abstract
Aligned interstellar grains produce polarized extinction (observed at wavelengths from the far-ultraviolet to the mid-infrared) and polarized thermal emission (observed at far-infrared and ...submm wavelengths). The grains must be quite nonspherical, but the actual shapes are unknown. The
relative
efficacy for aligned grains to produce polarization at optical versus infrared wavelengths depends on particle shape. The discrete dipole approximation is used to calculate polarization cross sections for 20 different convex shapes, for wavelengths from 0.1 to 100
μ
m, and grain sizes
a
eff
from 0.05 to 0.3
μ
m. Spheroids, cylinders, square prisms, and triaxial ellipsoids are considered. Minimum aspect ratios required by the observed starlight polarization are determined. Some shapes can also be ruled out because they provide too little or too much polarization at far-infrared and submm wavelengths. The ratio of 10
μ
m polarization to integrated optical polarization is almost independent of grain shape, varying by only ±8% among the viable convex shapes; thus, at least for convex grains, uncertainties in grain shape cannot account for the discrepancy between predicted and observed 10
μ
m polarization toward Cyg OB2-12.
IR emission spectra are calculated for dust heated by starlight, for mixtures of amorphous silicate and graphitic grains, including varying amounts of 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. We find a size distribution for the PAHs giving emission band ratios consistent with the observed spectra of the Milky Way and other galaxies. Emission spectra are presented for a wide range of starlight intensities. We calculate how the efficiency of emission into different IR bands depends on PAH size; the strong 7.7 km emission feature is produced mainly by PAH particles containing <10 super(3) C atoms. We also calculate how the emission spectrum depends on U, the starlight intensity relative to the local interstellar radiation field. The submillimeter 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 q sub(PAH), the lower cutoff U sub(min) of the starlight intensity distribution, and the fraction g of the dust heated by starlight with U> U sub(min). We present graphical procedures using Spitzer IRAC and MIPS photometry to estimate the parameters q sub(PAH), U sub(min), and g, the fraction f sub(PDR) of the dust luminosity coming from photodissociation regions with U> 100, and the total dust mass M sub(dust).
The presence of ferromagnetic or ferrimagnetic nanoparticles in the interstellar medium would give rise to magnetic dipole radiation at microwave and submillimeter frequencies. Such grains may ...account for the strong millimeter-wavelength emission observed from a number of low-metallicity galaxies, including the Small Magellanic Cloud. We calculate the absorption and scattering cross sections for such grains, with particular attention to metallic Fe, magnetite Fe sub(3)O sub(4), and maghemite gamma -Fe sub(2)O sub(3), all potentially present in the interstellar medium. The rate of Davis-Greenstein alignment by magnetic dissipation is also estimated. We determine the temperature of free-flying magnetic grains heated by starlight and calculate the polarization of the magnetic dipole emission from both free-fliers and inclusions. For inclusions, the magnetic dipole emission is expected to be polarized orthogonally relative to the normal electric dipole radiation. Magnetic dipole radiation will contribute significantly to the 20-40 GHz anomalous microwave emission only if a large fraction of the Fe is in metallic Fe iron nanoparticles with extreme elongations. Finally, we present self-consistent dielectric functions for metallic Fe, magnetite Fe sub(3)O sub(4), and maghemite gamma -Fe sub(2)O sub(3), enabling calculation of absorption and scattering cross sections from microwave to X-ray wavelengths.
Multiwavelength study of extended astronomical objects requires combining images from instruments with differing point-spread functions (PSFs). We describe the construction of convolution kernels ...that allow one to generate (multiwavelength) images with a common PSF, thus preserving the colors of the astronomical sources. We generate convolution kernels for the cameras ofSpitzer,Herschel Space Observatory,Galaxy Evolution Explorer(GALEX), Wide-field Infrared Survey Explorer (WISE), ground-based optical telescopes (Moffat functions and sum of Gaussians), and Gaussian PSFs. Kernels for other telescopes includingIRAS,AKARI, andPlanck, are currently being constructed. These kernels allow the study of the spectral energy distribution (SED) of extended objects, preserving the characteristic SED in each pixel. The convolution kernels and the IDL packages used to construct and use them are made publicly available.
Abstract A previous study (Paper I) investigated the polarization properties of a variety of simple convex grain shapes, some of which were found to be consistent with the observed polarization ...properties of interstellar dust from far-ultraviolet to far-infrared. Here, we study the optical properties of 45 nonconvex shapes, all aggregates of N equal-sized spheres. We consider N = 2, N = 3, and N = 256 random aggregates obtained from three different aggregation schemes. We also consider “trimmed” N = 256 aggregates obtained by systematically trimming initially random aggregates to increase either flattening or elongation. The “macroporosities” of the studied aggregates range from P macro = 0.18 (for the N = 2 bisphere) to P macro ≈ 0.85 (for the N = 256 “BA” aggregates). The only aggregates consistent with observations of starlight polarization and polarized thermal emission are shapes that have been trimmed to increase their asymmetry. If interstellar grains are high-porosity aggregates, there must be processes causing extreme elongation or flattening; if not, interstellar grains must be dominated by fairly compact structures, with at most moderate porosities. The ratio of polarization in the 10 μ m silicate feature to starlight polarization in the optical is shown to be insensitive to porosity and shape. X-ray scattering may be the best tool to determine the porosity of interstellar grains. We propose that modest porosities of interstellar grains could be the result of “photolytic densification.” High polarization fractions observed in some Class 0 cores require processes to reduce porosities and/or increase asymmetries of aggregates in dense regions.
The Small Magellanic Cloud (SMC) has surprisingly strong submillimeter- and millimeter-wavelength emission that is inconsistent with standard dust models, including those with emission from spinning ...dust. Here, we show that the emission from the SMC may be understood if the interstellar dust mixture includes magnetic nanoparticles, emittingmagnetic dipole radiation resulting from thermal fluctuations in themagnetization. Themagnetic grains can be metallic iron, magnetite Fe sub(3)O sub(4), or maghemite gamma-Fe sub(2)O sub(3). The required mass of iron is consistent with elemental abundance constraints. Themagnetic dipole emission is predicted to be polarized orthogonally to the normal electric dipole radiation if the nanoparticles are inclusions in larger grains.We speculate that other low-metallicity galaxies may also have a large fraction of the interstellar Fe in magnetic materials.
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