Evolution and ecology of influenza A viruses.

Cosmochemistry of the rare earth elements: meteorite studies.

▪ Abstract At luminosities above 1011 , infrared galaxies become the dominant population of extragalactic objects in the local Universe (z ≲ 0.3), being more numerous than optically selected starburst and Seyfert galaxies and quasi-stellar objects at comparable bolometric luminosity. The trigger for the intense infrared emission appears to be the strong interaction/merger of molecular gas-rich spirals, and the bulk of the infrared luminosity for all but the most luminous objects is due to dust heating from an intense starburst within giant molecular clouds. At the highest luminosities (Lir > 1012 ), nearly all objects appear to be advanced mergers powered by a mixture of circumnuclear starburst and active galactic nucleus energy sources, both of which are fueled by an enormous concentration of molecular gas that has been funneled into the merger nucleus. These ultraluminous infrared galaxies may represent an important stage in the formation of quasi-stellar objects and powerful radio galaxies. They may al...

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Luminous infrared galaxies

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

▪ 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

We report for the first time in situ observations of 5-50μm spherical carbonaceous structures in the Tissint Martian meteorite comprising of pyrite (FeS 2 ) cores and carbonaceous outer coatings. The structures are characterized as smooth immiscible spheres with curved boundaries occasionally following the contours of the pyrite inclusion. The structures bear striking resemblance to similar-sized immiscible carbonaceous spheres found in hydrothermal calcite vein deposits in the Mullaghwornia Quarry in central Ireland. Similar structures have been reported in Proterozoic and Ordovician sandstones from Canada as well as in a variety of astronomical sources including carbonaceous chondrites, chondritic IDPs and primitive chondritic meteorites. SEM and X-Ray elemental mapping confirmed the presence of organic carbon filling the crack and cleavage space in the pyroxene substrate, with further evidence of pyrite acting as an attractive substrate for the collection of organic matter. The detection of precipitated carbon collecting around pyrite grains is at variance with an igneous origin as proposed for the reduced organic component in Tissint, and is more consistent with a biogenetic origin.

Carbonaceous structures in the Tissint Martian Meteorite: evidence of a biogenetic origin

We discuss the possibility that a variable atmospheric load of porous 01 Μm-sized dielectric particles of extraterrestrial origin could lead to a fluctuating baseline for the atmospheric temperature near ground level Global warming has to be reckoned relative to this baseline

Extraterrestrial particles and the greenhouse effect

Graphite-Silicate Grain Mixtures and the Diffuse Galactic Light

/pdf/iron-at-the-galactic-centre-5bmz7c3fu0.pdf

Iron at the Galactic Centre

THE strengths of ultraviolet absorption lines of interstellar hydrogen have been measured in the spectra of several reddened early-type stars1–3. Interstellar H2 molecules are observed mainly in the two lowest rotational levels J = 0 (para-H2) and J = 1 (ortho-H2) with a ratio of populations corresponding to an average temperature ∼ 80 K. This temperature is consistent with the mean HI kinetic temperature determined in 21-cm absorption studies4,5, whereas higher rotational levels up to J = 5 or 6 are populated corresponding to significantly higher excitation temperatures3,6 in the range 150–200K. These observations indicate that a process other than thermal gas collisions is responsible for populating the higher rotational levels. The precise nature of the excitation mechanism is as yet unknown. A process which could contribute to non-thermal excitation of rotational levels involves encounters of charged dust grains with interstellar hydrogen. We have examined this mechanism and find that it could be of comparable, if not greater, importance in relation to other processes which have been proposed.

N. C. Wickramasinghe

Papers

Carbonaceous structures in the Tissint Martian Meteorite: evidence of a biogenetic origin

Extraterrestrial particles and the greenhouse effect

Graphite-Silicate Grain Mixtures and the Diffuse Galactic Light

Iron at the Galactic Centre

Charged dust grains and excitation of rotational levels of interstellar molecular hydrogen