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61.
We discuss the difficulties encountered when the Heisenberg-Kolmogoroff model for turbulence is applied to the large-scale turbulence in: (A) molecular clouds (specifically the velocity vs size relationship) and (B) stars (specifically, the estimate of convective fluxes).A new model for large-scale turbulence is, therefore, needed. 相似文献
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66.
S. W. Allen R. W. Schmidt A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2002,334(2):L11-L15
We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parametrizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White, we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r 2500 , where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe, , and measure a positive cosmological constant, . Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies. 相似文献
67.
Long-range sidescan sonar can be used to map sediment distributions over wide expanses of deep ocean floor. Seven acoustic facies that arise from differing sediment or rock types have been mapped over the low-relief Saharan continental rise and Madeira abyssal plain. These have been calibrated with sampling, profiling and camera studies and the facies can be traced confidently on a regional scale using the sidescan data. The mapping of the sediment distribution shows that a complex interplay of turbidity current and debris flow processes can occur at a continental rise/abysaal plain transition over 1000 km from the nearest continental slope. 相似文献
68.
F. Goodarzi L.R. Snowdon P.R. Gunther W.A.M. Jenkins 《Marine and Petroleum Geology》1985,2(3):254-259
Samples of lower Palaeozoic bedrock from the Grand Banks of Newfoundland were examined using reflected light microscopy and Rock-Eval pyrolysis. These samples contained organic material which included bitumen (structureless organic material), acritarchs, chitinozoa, scolecodonts and graptolites. The reflectance of the organic material increased from acritarchs towards graptolites with bitumen showing a wide range of reflectance. Multiple phases of oil migration through one sample were inferred by examination of the bitumens: the lowest reflecting being more recent than the highest reflecting bitumen. Combined reflectance and Rock-Eval analysis indicate that these samples are mature to overmature. The two oldest samples (Arenig-Llanvirn, Ordovician) are assessed as mature and as having potential for generation of liquid hydrocarbons, but must be considered as mainly gas prone. 相似文献
69.
Gert Finger Reinhold J. Dorn Alan W. Hoffman Hamid Mehrgan Manfred Meyer Alan F. M. Moorwood Joerg Stegmeier 《Experimental Astronomy》2002,14(2):117-127
Three different methods are presented to subtract thermal drifts and low-frequency noise from the signal of infrared array.
The first is dead pixels with open Indium bumps, the second is reference output as implemented on the Hawaii2 multiplexer,
and the third is dark pixels to emulate reference cells having a capacity connected to the gate of the unit cell field-effect
transistor (FET). The third method is the most effective and yields a reduction in readout noise from15.4–9.4 erms. A novel
method will be described to extend this readout technique to the Aladdin 1 K × 1 K InSb array.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
70.
Ralph D. Lorenz A. J. Timothy Jull Timothy D. Swindle Jonathan I. Lunine 《Meteoritics & planetary science》2002,37(6):867-874
Abstract— We explore the likely production and fate of 14C in the thick nitrogen atmosphere of Saturn's moon Titan and investigate the constraints that measurements of 14C might place on Titan's photochemical, atmospheric transport and surface‐atmosphere interaction processes. Titan's atmosphere is thick enough that cosmic‐ray flux limits the production of 14C: absence of a strong magnetic field and the increased distance from the Sun suggest production rates of ?9 atom/cm2/s, ?4x higher than Earth. The fate and detectability of 14C depends on the chemical species into which it is incorporated: as methane it would be hopelessly diluted even in only the atmosphere. However, in the more likely case that the 14C attaches to the haze that rains out onto the surface (as tholin, HCN or acetylene and their polymers), haze in the atmosphere or recently deposited on the surface would be quite radioactive. Such radioactivity may lead to a significant enhancement in the electrical conductivity of the atmosphere which will be measured by the Huygens probe. Measurements with simple detectors on future missions could place useful constraints on the mass deposition rates of photochemical material on the surface and identify locations where surface deposits of such material are “freshest”. 相似文献