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

The extended period of mass extinctions around the K/T boundary correlating with extraterrestrial amino acids in the sediment record constitutes strong evidence of a cometary cause. The input of extraterrestrial matter over 105 yr supports the hypothesis of a giant comet, fragmented into subcomets on close encounter with Jupiter, and subsequently perturbed into Earth-crossing orbits. Copious amounts of dust were emitted via this and possibly successive fragmenting encounters, and via normal cometary evaporation. The dynamics of dust from the disintegrating comet fragments favours retention in Earth-crossing orbits of the sub-micron fraction of organic composition. The shroud of dust accreted in the Earth’s upper atmosphere varied with time and imposed climatic stresses that caused species extinctions over 105 yr. While the iridium peak in the sediments coincides with the Chicxulub crater impactor, other iridium detail suggests that some of the impactor material was reinjected into space and in part re-accreted by Earth from the interplanetary orbits.

A jupiter fragmented comet: cause of the k/t boundary record

The observed high abundance of boron in type I carbonaceous chondrites may be due to the presence in the primitive solar nebula of graphite grains which have been irradiated by high energy nucleons at some stage of their history. The boron atoms thus produced by spallation reactions are stably locked within interstellar graphite grains and could make a significant contribution to the boron abundance of C1 chondrites.

The mystery of the cosmic boron abundance

Visible comets like Halley's differ from the great majority of comets in that they exist in adjacent pieces in gentle motion with respect to each other. From time to time the pieces rub together, causing a dusty surface which otherwise would be very dark and difficult to observe to be temporarily swept clean. The cleansed patches are then subject to evaporation, so making such multinuclear comets visible, unlike the majority of comets which have single nuclei and which stay dark. The existence of a large number of dark comets could have an interesting relation to the past history of the Earth.

Some Predictions on The Nature of Comet Halley

DULEY' second1 criticism of our hypothesis of graphite-iron-silicate grain mixtures2,3 contains further errors and misrepresentations of our statements. We wish to refer here to a few crucial facts which seem to have been overlooked by Duley1,4 and which invalidate his contention that the interstellar extinction curve does not provide additional evidence to support either graphite or silicate grains. The suggestion that a fit to the observed extinction curve on the basis of graphite, iron and silicate grains is confined to the adoption of the special set of values 0.06, 0.02 and 0.15 µm for their respective mean radii is incorrect. In our original paper2 we stated clearly that r (graphite)=0.045–0.07 µm, r (iron)≤0.02 µm and r (silicate)=0.15–0.18 µm are the requirements necessary to obtain good agreement with the observations. A wide range of relative proportions of the three grain species and several different types of size distributions2,3,5,6 have been shown to provide satisfactory fits to the available astronomical data. It should be stressed that the optical constants of graphite are such that the observed optical extinction curve and the ultraviolet hump at λ2200 A could be reproduced for a wide range of particle sizes. Iron and silicates in our model play a less important part at optical wavelengths so that constraints on their sizes and proportions relative to graphite are less rigorously specified by the optical extinction data.

/pdf/interstellar-dust-reply-to-duley-s-criticisms-1o61mkv234.pdf

Interstellar Dust—Reply to Duley's Criticisms

Theories of panspermia are rapidly coming into vogue, with the possibility of the transfer of viable bacterial cells from one planetary abode to another being generally accepted as inevitable. The panspermia models of Hoyle and Wickramasinghe require the transfer of viable bacterial cells from interstellar dust to comets and back into interplanetary and interstellar space. In such a cycle a viable fraction of as little as 10 −18 at the inception of a newly formed comet/planet system suffices for cometary panspermia to dominate over competing processes for the origin and transfer of life. The well-attested survival attributes of microbes under extreme conditions, which have recently been discovered, gives credence to the panspermia hypothesis. The prediction of the theory that comets bring microbes onto the Earth at the present time is testable if aseptic collections of stratospheric air above the tropopause can be obtained. We describe a recent collection of this kind and report microbiological analysis that shows the existence of viable cells at 41 km, falling to Earth at the rate of a few tonnes per day over the entire globe. Some of these cells have been cultured in the laboratory and found to include microorganisms that are not too different from related species on the Earth. This is in fact what the Hoyle-Wickramasinghe theory predicts. The weight of evidence goes against the more conservative explanation that organisms are being lofted to the high atmosphere from the ground.

N. C. Wickramasinghe

Papers

A jupiter fragmented comet: cause of the k/t boundary record

The mystery of the cosmic boron abundance

Some Predictions on The Nature of Comet Halley

Interstellar Dust—Reply to Duley's Criticisms

Progress Towards the Vindication of Panspermia