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171.
Ca-Al rich refractory mineral inclusions (CAIs) found at 1-6% mass fraction in primitive chondrites appear to be 1-3 million years older than the dominant (chondrule) components which were accreted into the same parent bodies. A prevalent concern is that it is difficult to retain CAIs for this long against gas-drag-induced radial drift into the Sun. We reassess the situation in terms of a hot inner (turbulent) nebula context for CAI formation, using analytical models of nebula evolution and particle diffusion. We show that outward radial diffusion in a weakly turbulent nebula can overcome inward drift, and prevent significant numbers of CAI-size particles from being lost into the Sun for times on the order of 106 years. CAIs can form early, when the inner nebula was hot, and persist in sufficient abundance to be incorporated into primitive planetesimals at a much later time. Small (?0.1 mm diameter) CAIs persist for longer times than large (?5 mm diameter) ones. To obtain a quantitative match to the observed volume fractions of CAIs in chondrites, another process must be allowed for: a substantial enhancement of the inner hot nebula in silicate-forming material, which we suggest was caused by rapid inward drift of meter-sized objects. This early in nebula history, the drifting rubble would have a carbon content probably an order of magnitude larger than even the most primitive (CI) carbonaceous chondrites. Abundant carbon in the evaporating material would help keep the nebula oxygen fugacity low, plausibly solar, as inferred for the formation environment of CAIs. The associated production of a larger than canonical amount of CO2 might also play a role in mass-independent fractionation of oxygen isotopes, leaving the gas rich in 16O as inferred from CAIs and other high temperature condensates. 相似文献
172.
173.
Jeffrey A. Macdonald Manfred A. Biondi Rainer Johnsen 《Planetary and Space Science》1984,32(5):651-654
The dissociative recombination coefficients α for capture of electrons by H3+ and H5+ ions have been determined as a function of electron temperature Te using a microwave afterglow-mass spectrometer apparatus. At ion and neutral temperatures Tu+ = Tn = 240 K, the coefficient α (H3+) is found to vary slowly with Te at first, decreasing from 1.6 × 10?7 cm3/s at Te = 240 K to 1.2 × 10?7 cm3/s at Te = 500 K, thereafter falling as Te?1 over the range 500 K ? Te, ? 3000 K. These results, which have a ± 20% uncertainty, agree satisfactorily over the common energy range (0.03–0.36 eV) with the recombination cross sections determined in merged beam measurements by Auerbach et al. At T+ = Tn = 128 K, the coefficient α(H5+) is found to be (1.8 ± 0.3) × 10?6 [Te(K)/300]?0.69 cm3/s over the range 128 K ? Te ? 3000 K, with a more rapid decrease, as Te?1, between 3000 K and 5500 K. The implications of these results for modelling planetary atmospheres and interstellar clouds are briefly touched on. 相似文献
174.
A statistical analysis of possible clumping (not periodicity) of emission line redshifts of QSO's shows the available data to be compatible with random fluctuations of a smooth, nonclumped distribution. This result is demonstrated with Monte Carlo simulations as well as with the Kolmogorov-Smirnov test. It is in complete disagreement with the analysis by Varshni, which is shown to be incorrect. 相似文献
175.
Precipitation of decay products of atmospheric 222Rn in small containers and the effects of their precipitation on alpha-particle-track-measurements of radon activities were studied. From decay curves of decay product activities on surfaces exposed to radon-containing air the ratios 214Pb/218Po (=r) in the precipitates were determined. From r average ages of precipitating decay products were calculated. Average ages and r were low, indicating rapid precipitation of decay products, and decreased with container size. Activities on exposed surfaces were approximately those expected if decay products precipitate completely and uniformly on the internal surfaces of containers.Decay products precipitated on foils of correct thickness covering cellulose nitrate (CN) track detectors produce tracks. Therefore track densities produced by given radon activities are increased by such covers to extents predictable from the decay product activities on the overlays. Tracks formed under overlays may etch to larger, more vertical-sided, more easily counted forms. When suspended, uncovered, in air at distances from surfaces greater than the ranges of the alpha particles, three types of CN recorded tracks at the rate (ρ′) of 0.12 tracks cm?2 (pCi Rn)?1 1?1 hr?1 as predicted by a theoretical expression. In small containers (ρ′) varied with position and container size. 相似文献
176.
William D. Langer Leonard C. Rosen Jeffrey M. Cohen A. G. W. Cameron 《Astrophysics and Space Science》1969,5(3):259-271
An equation of state for cold matter above white dwarf densities is evaluated. The gas is considered to be a mixture of degenerate neutrons, protons and electrons combined with nuclei of one type (that is only oneA andZ value). We derive the equilibrium equations for the mixture and calculate the number densities as well as theA andZ of the nucleus. Finally we calculate an equation of state, which smoothly goes over to that of a neutron, proton electron gas mixture at a density of 5×1013 g/cm3. 相似文献
177.
On the relative importance of the driving forces of plate motion 总被引:4,自引:0,他引:4
178.
Jeffrey M. Cohen William D. Langer Leonard C. Rosen A. G. W. Cameron 《Astrophysics and Space Science》1970,6(2):228-239
Using an equation of state for cold degenerate matter which takes nuclear forces and nuclear clustering into account, neutron star models are constructed. Stable models were obtained in the mass range above 0.065M
and density range 1014.08 to 1015.4 gm/cm3. All of these models were found to be bound. The outer crystalline layer of the star was found to have a thickness of 200 m or more depending on the mass of the model. 相似文献
179.
Observations are presented of the Caii infrared triplet (8498 Å, 8542 Å, and 8662 Å) at three positions on the solar disk to make possible direct analyses of the lines and comparisons with theoretical computations. The source functions for the two strongest lines (8542 Å and 8662 Å) are equal at those heights corresponding to the wings of the lines (¦¦ > 0.4 Å) but not to those of the cores. We suggest that the apparent source function inequality in the cores is due to limb darkening caused by inhomogeneities in the chromosphere.Of the National Bureau of Standards and University of Colorado. 相似文献
180.
Jeffrey M. Moore 《Icarus》1984,59(2):205-220
The tectonic and volcanic modifications of Dione are described and interpreted. It is proposed that after the formation of a brittle outer shell, but before the end of heavy meteoritic bombardment, global expansion due to radionuclide heating (and perhaps a loss of oblateness due to tidal despinning and orbital recession) produced a global system of lineaments. An NH3 · H2O melt was produced and “erupted” on the surface to form plains units. Cooling of the interior (or a phase change) led to horizontal compression in the surface. Compression of a thick deposit of plains material, possibly overlying a décollement surface, is proposed to explain cratered-plains ridges developed as thrust or high-angle reverse faults. Following formation of ridges and smooth plains, the surface experienced light cratering. 相似文献