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 interstellar dust model of comets is numerically worked out to satisfy several basic constraints provided by observations of comet Halley and to derive the porosity of coma dust. The observational constraints are: (1) the strengths of the 3.4 μm and 9.7 μm emission bands; (2) the relative amount of silicates to organic materials; (3) the mass distribution of the dust. The results indicate that coma dust has a porosity in the range 0.93 < P < 0.975. Preliminary calculations concerning the observed linear polarization of comet Halley are presented.

/pdf/the-dust-in-the-coma-of-comet-halley-fify8593mq.pdf

The Dust in the Coma of Comet Halley

The dust in the coma of comet Halley.

Interstellar dust and related topics : Symposium no. 52, held at the State University of New York at Albany, Albany, N.Y., U.S.A

Solar system bodies such as comets and meteorites are examined for interstellar signatures which may ultimately indicate what the protoplanetary nebula conditions were at the time the bodies formed. This involves retracing the sequence starting with a dense interstellar cloud, through the accretion disk phase of the protoplanetary nebula to the various steps in the aggregation process and finally to the physical and chemical metamorphic processes in the final body during the subsequent 4.55 billion years. Models of interstellar dust and comets which aid in such analysis are presented.

/pdf/the-interstellar-connection-to-solar-system-bodies-958rqh8go0.pdf

The interstellar connection to solar system bodies

For the first time a laboratory simulation of the effect of the interstellar environment has been used to provide quantitative estimates of bacterial spore survival in the space between the stars. In the diffuse regions between clouds ten percent survival is limited to at most hundreds of years although one in ten thousand may survive for several thousand years. Within common dense clouds the ten percent life expectancy is extended to tens of millions of years because of the severely reduced ultraviolet within these clouds as well as because of the accretion of ultraviolet absorbing mantles on the spores. The random motion of molecular clouds is shown to provide a possible vehicle for transport of spores from one solar system to another. The most hazardous times in such a journey are at the start and finish and, although the requirements for survival during these periods are quantified here, the possibility or probability of their being satisfied remains pure conjecture.

Panspermia — a Modern Astrophysical and Biological Approach

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