The ultraviolet extinction by hollow spherical particles of graphite

Using the discrete dipole approximation, we have calculated the extinction efficiency of hollow spherical particles of graphite as a possible constituent of interstellar grains. The particles had a shell structure with the basal plane perpendicular to the radius. The calculations were made on the particles having the outer radiusR ₀=10 and 5nm in the wave number region from 0.8 to 8.0 m^–1 using the anisotropic optical constants. It was found that the hollow particles with the inner radiusR ₁0.65R ₀ yield an extinction feature at 4.6 m^–1, which fits fairly well to one observed in the interstellar extinction.     

Similarity between the structure and stability of isothermal and polytropic gas spheres

As shown by Chiéze, polytropic gas spheres imbedded in an ambient medium with a given pressure exhibit a relation between the mass and radius similar to isothermal gas spheres, providedn(polytropic index)>3 or <–1. Here, further investigation is made of the polytropic spheres. Not only the relation between radius and external pressure, but the relation between the central density ( _c ) and the mass (M) is similar to isothermal spheres and neutron star models. Dynamical calculation shows that the stability of polytropic spheres is lost at the configuration which corresponds to the first peak in the _c —M relation, as is the case for neutron star models in general relativity.     

On the Possible Hazard on the Major Cities Caused by Asteroid Impact in the Pacific Ocean – II

Earlier, it was pointed out that asteroids with diameters close to 200 m that hit the Pacific Ocean once in 10⁴ y create large tsunami waves which could cause serious hazards to the cities and countries lying along the periphery. Here, a more detailed investigation is carried out in relation to tsunami hazards created by earthquakes. It is shown that the energy involved in the asteroid induced tsunami wave is some 300 times greater than large tsunami waves and 100 times greater than the Chile tsunami of 1960, the largest this century. Second, if the impact should take place in the Pacific south of Japan, so that the tsunami wave enters Tokyo Bay, the damage could cost 800 billion dollars, which is 6 times greater than the damage due to the Hanshin (Kōbe-Osaka) earthquake of 1995. This revised version was published online in July 2006 with corrections to the Cover Date.     

Are cratering and probably related geological records periodic?

The controversial topic of periodicity in geological records in relation to astronomical modeling is reviewed. Although impact cratering, frequency distribution of geo-magnetic reversals, and mass extinction of fauna yield periods when certain tests are applied, none of them can be regarded significant in the sense of mathematical statistics. The first two records yield periods of 30 Myr, while the faunal-extinction record yields a period of ~ 26 – 27 Myr. It seems that although catastrophes in the form of large impacts trigger mass extinctions, certain geophysical or geological conditions need be satisfied for mass extinctions to be realized. One should not expect to find an indisputable periodicity in cratering record because random impacts by asteroids are dominant. Thus, the earth-crossing cometary flux modulated by the galactic tidal force appears consistent with the weak detected periodicity.