We construct size distributions for carbonaceous and silicate grain populations in different regions of the Milky Way, LMC, and SMC. The size distributions include sufficient very small carbonaceous grains (including polycyclic aromatic hydrocarbon molecules) to account for the observed infrared and microwave emission from the diffuse interstellar medium. Our distributions reproduce the observed extinction of starlight, which varies depending on the interstellar environment through which the light travels. As shown by Cardelli, Clayton, and Mathis in 1989, these variations can be roughly parameterized by the ratio of visual extinction to reddening, RV. We adopt a fairly simple functional form for the size distribution, characterized by several parameters. We tabulate these parameters for various combinations of values for RV and bC, the C abundance in very small grains. We also find size distributions for the line of sight to HD 210121 and for sight lines in the LMC and SMC. For several size distributions, we evaluate the albedo and scattering asymmetry parameter and present model extinction curves extending beyond the Lyman limit.

https://iopscience.iop.org/article/10.1086/318651/pdf

Dust Grain-Size Distributions and Extinction in the Milky Way, Large Magellanic Cloud, and Small Magellanic Cloud

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 μm emission feature is produced mainly by PAH particles containing Umin. We present graphical procedures using Spitzer IRAC and MIPS photometry to estimate the parameters qPAH, Umin, and γ, the fraction fPDR of the dust luminosity coming from photodissociation regions with U > 100, and the total dust mass Mdust.

pdf/infrared-emission-from-interstellar-dust-iv-the-silicate-13zj6eieay.pdf

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

▪ Abstract This review surveys the observed properties of interstellar dust grains: the wavelength-dependent extinction of starlight, including absorption features, from UV to infrared; optical lum...

https://www.ifa.hawaii.edu/users/reipurth/reviews/draine.pdf

Interstellar dust grains

Endospores of Bacillus spp., especially Bacillus subtilis, have served as experimental models for exploring the molecular mechanisms underlying the incredible longevity of spores and their resistance to environmental insults. In this review we summarize the molecular laboratory model of spore resistance mechanisms and attempt to use the model as a basis for exploration of the resistance of spores to environmental extremes both on Earth and during postulated interplanetary transfer through space as a result of natural impact processes.

/pdf/resistance-of-bacillus-endospores-to-extreme-terrestrial-and-2qp3zxqicl.pdf

Resistance of Bacillus endospores to extreme terrestrial and extraterrestrial environments.

▪ Abstract The Universe is in transition. At early times, galactic evolution was dominated by hierarchical clustering and merging, processes that are violent and rapid. In the far future, evolution will mostly be secular—the slow rearrangement of energy and mass that results from interactions involving collective phenomena such as bars, oval disks, spiral structure, and triaxial dark halos. Both processes are important now. This review discusses internal secular evolution, concentrating on one important consequence, the buildup of dense central components in disk galaxies that look like classical, merger-built bulges but that were made slowly out of disk gas. We call these pseudobulges. We begin with an “existence proof”—a review of how bars rearrange disk gas into outer rings, inner rings, and stuff dumped onto the center. The results of numerical simulations correspond closely to the morphology of barred galaxies. In the simulations, gas is transported to small radii, where it reaches high densities and...

Secular Evolution and the Formation of Pseudobulges in Disk Galaxies

The 3-20 μm IR thermal emission spectrum of comet Hale-Bopp (C/1995 O1) at a heliocentric distance of rh=1.15 AU has been calculated using fluffy aggregates of silicate core-organic refractory mantle interstellar dust as the comet dust model. The principal dust properties, similar to those obtained for other comets, provide good fits to the observation. Our results are significantly different from the general opinion that Hale-Bopp is rich in submicron grains. The mean grain masses derived here are 2-3 orders of magnitude higher than those estimated from the empirical superheat method or from the IR emission modeling, both of which use compact silicate/carbon particles. It has been shown that the IR emission alone cannot give a reliable dust production rate. We have also modeled the crystalline silicate emission spectrum.

From Interstellar Dust to Comets: Infrared Emission from Comet Hale-Bopp (C/1995 O1)

The ubiquitous presence of dust within the disks of spiral galaxies is well established. Dust is present in large quantities over enormous extents in the Universe; for example, Burton1 concludes that, in our Galaxy, dust is distributed very similarly to the H I distribution, with an exponential radius in the plane of 8 kpc and an exponential scale height perpendicular to the plane of 120 pc. Simple dynamical arguments about the motion of interstellar grains out of the galactic disk show the possibility of confining a significant quantity of dust in a potential well high above the galactic plane. Although the possibility of dust being repelled away from the galaxy by the radiation pressure of the stars in the disk has earlier been considered2, we have noted that the functional difference between the gravitational and radiation forces, as the dust recedes, may lead to the existence of stable regions outside the galaxy reminiscent of the libration points for interplanetary dust produced by competing gravitational forces alone (refs 3, 4 ; F.F., J.M .G., B.B. and S.A., in preparation). Here we predict that the presence of dust in these regions may be revealed in bright edge-on galaxies, especially by using the polarization of the scattered light from the symmetric lanes. The detection of scattered light above the galactic plane may be an indicator that the parent galaxy has not suffered close encounters with other galaxies at least within the timescale required to establish the dust layers.

Is there dust in galactic haloes

The 6.86-micron absorption in W33A has been investigated using the results of laboratory experiments simulating the photochemistry of interstellar grain mantles. The 5-8-micron IR spectrum, and particularly the 6.86-micron absorption, of W33A can be reproduced with ultraviolet irradiated water-rich ices. The 5-8-micron spectrum of W33A exhibits effects which are interpreted as due to dust material heated to different temperatures along the line of sight. Photolyzed dust mantles with a temperature less than 100 K provide the 6.0-micron absorption, while dust mantles heated to more than 100 K are the main contributors to the 6.86-micron absorption feature. Different photolysis and heating scenarios, as well as line of sight effects, are required to account for the 5-8-micron spectra of other protostellar objects.

/pdf/ions-in-grain-mantles-a-new-explanation-for-the-6-86-micron-2moi4zzszl.pdf

Ions in grain mantles - A new explanation for the 6.86 micron absorption in W33A

Examination of the spatial distribution of CO intensity of Comet Halley indicates that a large fraction of CO originates from the refractory organic component in the coma, rather than directly from the volatiles in the nucleus. Based on the fluffy aggregate interstellar dust comet model, we have estimated the upper limits of the total amount of CO provided by coma dust. The implications from the comparison of the predicted results with the observed value have been discussed.

The Distributed CO in Comet Halley

Interstellar grain evolution and temperatures in spiral galaxies.

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