Local Stability Criterion for a Self-Gravitating Gaseous Disk of Spiral Galaxies

An improved linear stability theory of small-amplitude oscillations of a self-gravitating, infinitesimally thin gaseous disk of spiral galaxies has been developed by Bertin, Lau, Lin, Mark, Morozov, Polyachenko, and others in the approximation of moderately tightly wound gravity perturbations. In this regime, the generalized Lin–Shu type dispersion relation was also found by including higher order terms in the small parameter 1/kr for wavenumber k and radius r. It was shown that in the differentially rotating disks for nonaxisymmetric (spiral)perturbations Toomre's modified critical Q-parameter is larger than the standard one: the fact that the spiral perturbations in the nonuniformly rotating system are more unstable than the axisymmetric ones is taken into account in this modified local stability criterion. We use hydrodynamical simulations to test the validity of the modified local criterion. This revised version was published online in July 2006 with corrections to the Cover Date.     

Y/Ho Ratios in the Late Cenozoic Basalts from the Eastern Tuva, Russia: An ICP-MS Study with Enhanced Data Quality

A set of forty seven Late Cenozoic basaltic rocks from the Eastern Tuva (southern Siberia, Russia) have been studied by ICP-MS using In as an internal standard. Yttrium and Ho concentrations, determined along with other trace elements, varied systematically from one analytical run to another and covered the entire charge-and-radius-controlled field within the range 24 < Y/Ho < 34, so obscuring any geochemical interpretation. A correction procedure was therefore developed in which the Y/Ho ratios were recalculated using concentrations of these elements determined in international and in-house reference materials (BHVO-1, AGV-1, BIR-1 and U-94-5). Statistical analysis of the recalculated data set revealed two groups of samples not related by their genesis but rather by their respective analytical runs. Fourteen samples originally analysed in five different runs were then re-measured by the same instrument in peak-hopping mode using only specific mass peaks (m/z = 89, 115 and 165). All of these samples yielded Y/Ho ratios in a narrow range between 29 and 31, thus illustrating how analytical problems in the determination of Y and Ho by routine ICP-MS procedures can be overcome. Finally, a mean value of Y/Ho = 30.3 ± 1.1 (1s) was derived for the basaltic rocks of the Eastern Tuva. This is close to the value of 28.8 accepted for chondrites and for ocean island basalts. Despite the uniform Y/Ho ratio, the element concentrations in basaltic rocks from the Eastern Tuva show systematic enrichment in heavy rare earth elements and yttrium from the Miocene to the Quaternary.     

New Taxa of Chrysomelidae (Insecta:Coleoptera)from Rovno Amber,Late Eocene

<正>Leaf beetles Chrysomelidae of Rovno amber,from the Late Eocene,are recorded and described. Chrysomelidae of Rovno amber are represented by three subfamilies:Galerucinae(Alticini),Chrysomelinae,and Eumolpinae.Two new genera and three new species of Alticini:Manobiomorpha Nadein,gen.nov.(type species Manobiomorpha eocenica Nadein,sp.nov.),Psyllototus Nadein,gen.nov.(type species Psyllototus progenitor Nadein,sp.nov.),and Crepidodera decolorata Nadein et Perkovsky,sp.nov.are described.A new chrysomeline genus and species Paleophaedon minutus Nadein gen.nov.et sp.nov.is described.Probable trophic association of Crepidodera decolorata sp.nov.,the taxonomic positions of Manobiomorpha gen.nov.and Psyllototus gen.nov., and the composition of leaf beetle faunas of Middle and Late Eocene of Europe are discussed.     

Determination of a shallow velocity–depth model from seismic refraction data by coherence inversion1

Seismic refractions have different applications in seismic prospecting. The travel- times of refracted waves can be observed as first breaks on shot records and used for field static calculation. A new method for constructing a near-surface model from refraction events is described. It does not require event picking on prestack records and is not based on any approximation of arrival times. It consists of the maximization of the semblance coherence measure computed using shot gathers in a time window along refraction traveltimes. Time curves are generated by ray tracing through the model. The initial model for the inversion was constructed by the intercept-time method. Apparent velocities and intercept times were taken from a refraction stacked section. Such a section can be obtained by appling linea moveout corrections to common-shot records. The technique is tested successfully on synthetic and real data. An important application of the proposed method for solving the statics problem is demonstrated.     

Synchrotron emission in the fast cooling regime: which spectra can be explained?

We consider the synchrotron emission from relativistic shocks assuming that the radiating electrons cool rapidly (either through synchrotron or any other radiation mechanism). It is shown that the theory of synchrotron emission in the fast cooling regime can account for a wide range of spectral shapes. In particular, the magnetic field, which decays behind the shock front, brings enough flexibility to the theory to explain the majority of gamma-ray burst spectra even in the parameter-free fast cooling regime. Also, we discuss whether location of the peak in observed spectral energy distributions of gamma-ray bursts and active galactic nuclei can be made consistent with predictions of diffusive shock acceleration theory, and find that the answer is negative. This result is a strong indication that a particle injection mechanism, other than the standard shock acceleration, works in relativistic shocks.     

Radiochemical Determination of Gold in Rocks

A method developed for radiochemical determination of gold in rock samples has been applied to six Bulgarian, four ZGI (GDR) and one USGS geostandards. The method is based on acid dissolution of the sample, followed by extraction of gold with copper diethyldithiocarbamate solution in CHCl₃. The chemical yield was determined by using ¹⁹⁹Au tracer. The present data are compared with published values for the ZGI and USGS samples.