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1.
Turbulent plane‐shear flow is found to show same basic effects of mean‐fieldMHD as rotating turbulence. In particular, the mean electromotive force (EMF) includes highly anisotropic turbulent diffusion and alpha‐effect. Only magnetic diffusion remains for spatially‐uniform turbulence. The question is addressed whether in this case a self‐excitation of a magnetic field by so‐called sher‐current dynamo is possible and the quasilinear theory provides a negative answer. The streamaligned component of the EMF has the sign opposite to that required for dynamo. If, however, the turbulence is not uniform across the flow direction then a dynamo‐active α ‐effect emerges. The critical magnetic Reynolds number for the alpha‐shear dynamo is estimated to be slightly above ten. Possibilities for cross‐checking theoretical predictions with MHD experiments are discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

2.
A plane‐shear flow in a fluid with forced turbulence is considered. If the fluid is electrically‐conducting then a mean electromotive force (EMF) results even without basic rotation and the magnetic diffusivity becomes a highly anisotropic tensor. It is checked whether in this case self‐excitation of a large‐scale magnetic field is possible (so‐called W̄ × J̄ ‐dynamo) and the answer is NO. The calculations reveal the cross‐stream components of the EMF perpendicular to the mean current having the wrong signs, at least for small magnetic Prandtl numbers. After our results numerical simulations with magnetic Prandtl number of about unity have only a restricted meaning as the Prandtl number dependence of the diffusivity tensor is rather strong. If, on the other hand, the turbulence field is strati.ed in the vertical direction then a dynamo‐active α ‐effect is produced. The critical magnetic Reynolds number for such a self‐excitation in a simple shear flow is slightly above 10 like for the other – but much more complicated – flow patterns used in existing dynamo experiments with liquid sodium or gallium. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

3.
Nova V5116 Sgr 2005 No. 2, discovered on 2005 July 4, was observed with XMM‐Newton in March 2007, 20 months after the optical outburst. The X‐ray spectrum showed that the nova had evolved to a pure supersoft X‐ray source, indicative of residual H‐burning on top of the white dwarf. The X‐ray light‐curve shows abrupt decreases and increases of the flux by a factor 8 with a periodicity of 2.97 h, consistent with the possible orbital period of the system. The EPIC spectra are well fit with an ONe white dwarf atmosphere model, with the same temperature both in the low and the high flux periods. This rules out an intrinsic variation of the X‐ray source as the origin of the flux changes, and points to a possible partial eclipse as the origin of the variable light curve. The RGS high resolution spectra support this scenario showing a number of emission features in the low flux state, which either disappear or change into absorption features in the high flux state. A new XMM‐Newton observation in March 2009 shows the SSS had turned off and V51 16 Sgr had evolved into a weaker and harder X‐ray source (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

4.
Abstract– We studied three large (2–4 mm diameter) igneous‐textured inclusions in the Buzzard Coulee (H4) chondrite using microanalytical techniques (OLM, SEM, EMPA, SIMS) to better elucidate the origins of igneous inclusions in ordinary chondrites. The inclusions are clasts that come in two varieties (1) white inclusions Bz‐1 and Bz‐2 represent a nearly holocrystalline assemblage of low‐Ca and high‐Ca pyroxene (63–66 area%) and cristobalite (33–36%) and (2) tan inclusion Bz‐3 is glass‐rich (approximately 60%) with low‐Ca and high‐Ca pyroxene phenocrysts. The bulk compositions of the inclusions determined by modal reconstruction are all SiO2‐rich (approximately 67 wt% for Bz‐1 and Bz‐2, approximately 62% for Bz‐3), but Bz‐3 is enriched in incompatible elements (e.g., REE approximately 4–5 × CI abundances), whereas Bz‐2 and Bz‐1 are depleted in those elements that are most incompatible in pyroxene (e.g., La‐Ho approximately 0.15–0.4 × CI abundances). These bulk compositions do not resemble what one would expect for partial or complete shock melting of a chondritic precursor, and show no evidence for overall volatility control. We infer that the inclusions originated through igneous differentiation and FeO reduction, with Bz‐3 forming as an “andesitic” partial melt (approximately 30–40% partial melting of an H chondrite precursor), and Bz‐1 and Bz‐2 forming as pyroxene‐cristobalite cumulates from an Si‐rich melt. We suggest that both types of melts experienced a period of transit through a cold, low‐pressure space environment in which cooling, FeO reduction, and interaction with a vapor occurred. Melts may have been lofted into space by excavation or splashing during collisions, or by pyroclastic volcanism. Our results indicate intriguing similarities between the inclusions in Buzzard Coulee and the silicates in some iron (IIE‐type) and stony iron (IVA‐type) meteorites, suggesting a genetic relationship.  相似文献   

5.
With the increasingly precise measurement of the cosmic microwave background (CMB), cosmology has entered an era where a model's predictions become testable to percent‐level accuracy. In particular, the CMB spectrum has so far provided impressive support for the scenario of inflation, first invented to solve outstanding problems of standard cosmology. While current data (COBE, WMAP etc.) have already constrained cosmological parameters like Ω0 to high precision, next generation instruments such as the PLANCK satellite should give access to specific characteristics of the inflationary mechanism itself. Another tantalizing idea has been discussed in this context: Given the enormous expansion of the Universe during the phase of inflation, could it be that even Planck scale physics has been stretched to observable distances and is therefore within grasp in the CMB observations? In this contribution, I discuss the possibility of carrying through the calculation of the perturbation spectrum from an ansatz for short distance physics right to its imprint in the CMB. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

6.
Abstract— Queen Alexandra Range (QUE) 93148 is a small (1.1 g) olivine‐rich achondrite (mg 86) that contains variable amounts of orthopyroxene (mg 87) and kamacite (6.7 wt% Ni), with minor augite. Olivine in QUE 93148 contains an unusual suite of inclusions: (1) 5 × 100 μm sized lamellae with a CaO‐ and Cr2O3‐rich (～10 and 22 wt%, respectively) composition that may represent a submicrometer‐scale intergrowth of chromite and pyroxene(s); (2) 75 × 500 μm sized lamellar symplectites composed of chromite and two pyroxenes, with minor metal; (3) 15–20 μm sized, irregularly‐shaped symplectites composed of chromite and pyroxene(s); (4) 100–150 μm sized, elliptical inclusions composed of chromite, two pyroxenes, metal, troilite, and rare whitlockite. Type 1, 2, and 3 inclusions probably formed by exsolution from the host olivine during slow cooling, whereas type 4 more likely resulted from early entrapment of silicate and metallic melts followed by closed‐system oxidation. Queen Alexandra Range 93148 can be distinguished from most other olivine‐rich achondrites (ureilites, winonaites, lodranites, acapulcoites, brachinites, Eagle‐Station‐type pallasites, and pyroxene pallasites), as well as from mesosiderites, by some or all of the following properties: O‐isotopic composition, Fe‐Mn‐Mg relations of olivine, CaO and Cr2O3 contents of olivine, orthopyroxene compositions, molar Cr/(Cr + Al) ratios of chromite, metal composition, texture, and the presence of the inclusions. In terms of many of these properties, it shows an affinity to main‐group pallasites. Nevertheless, it cannot be identified as belonging to this group. Meteorite QUE 93148 appears to be a unique achondrite. Possibly it should be considered to be a pyroxene pallasite that is genetically related to main‐group pallasites. Alternatively, it may be derived from the mantle of the pallasite (howardite‐eucrite‐diogenite?) parent body.  相似文献   

7.
1E 1207.4–5209 is an X‐ray source located at the centre of the supernova remnant (SNR) PKS 1209–52 (G296.5+10.0) and is thought to be an isolated neutron star (INS) associated with the SNR. Its optical spectrum shows several absorption lines and the X‐ray spectrum exhibits three variable absorption features at what appears to be harmonically related wavelengths, the latter being interpreted as due to cyclotron resonance. However, there are several serious problems, uncertainties and difficulties in the above association (SNR/INS) and in the interpretation of the spectra. In view of these, we suggest an alternative explanation of the observed spectra in terms of blueshifts. This implies that the optical and X‐ray absorption spectra are due to the central object of the SNR in association with two separate absorption clouds ejected at successively increasing speeds. The clouds have their origins in jets resulting from the merger of two very massive compact stars. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

8.
Observations of hundreds of supersoft X‐ray sources (SSSs) in external galaxies have shed light on the diversity of the class and on the natures of the sources. SSSs are linked to the physics of Type Ia supernovae and accretion‐induced collapse, ultraluminous X‐ray sources and black holes, the ionization of the interstellar medium, and tidal disruption by supermassive black holes. The class of SSSs has an extension to higher luminosities: ultraluminous SSSs have luminosities above 1039 erg s–1. There is also an extension to higher energies: quasisoft X‐ray sources (QSSs) emit photons with energies above 1 keV, but few or none with energies above 2 keV. Finally, a significant fraction of the SSSs found in external galaxies switch states between observations, becoming either quasisoft or hard. For many systems “supersoft” refers to a temporary state; SSSs are sources, possibly including a variety of fundamentally different system types, that pass through such a state. We review those results derived from extragalactic data and related theoretical work that are most surprising and that suggest directions for future research (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

9.
A spectral variability study of the two Narrow Line Seyfert 1 galaxies NGC 4051 and IRAS13224–3809 is presented. Both sources show a high degree of flux and spectral variability. The nuclear emission, lightly absorbed by warm material, has been decomposed into a direct power law emission and an ionized disc reflection plus constant emission from distant material. The ionized disc reflection component does not follow the variations of the primary component. Its flux is linearly correlated with the one of the power law component only at low fluxes, while it is almost constant at medium high‐flux. This behavior is expected when the light bending effect is important. If so, most of the primary emission comes from only a few gravitational radii from the black hole. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

10.
The Gao‐Guenie H5 chondrite that fell on Burkina Faso (March 1960) has portions that were impact‐melted on an H chondrite asteroid at ~300 Ma and, through later impact events in space, sent into an Earth‐crossing orbit. This article presents a petrographic and electron microprobe analysis of a representative sample of the Gao‐Guenie impact melt breccia consisting of a chondritic clast domain, quenched melt in contact with chondritic clasts, and an igneous‐textured impact melt domain. Olivine is predominantly Fo80–82. The clast domain contains low‐Ca pyroxene. Impact melt‐grown pyroxene is commonly zoned from low‐Ca pyroxene in cores to pigeonite and augite in rims. Metal–troilite orbs in the impact melt domain measure up to ~2 mm across. The cores of metal orbs in the impact melt domain contain ~7.9 wt% of Ni and are typically surrounded by taenite and Ni‐rich troilite. The metallography of metal–troilite droplets suggest a stage I cooling rate of order 10 °C s?1 for the superheated impact melt. The subsolidus stage II cooling rate for the impact melt breccia could not be determined directly, but was presumably fast. An analogy between the Ni rim gradients in metal of the Gao‐Guenie impact melt breccia and the impact‐melted H6 chondrite Orvinio suggests similar cooling rates, probably on the order of ~5000–40,000 °C yr?1. A simple model of conductive heat transfer shows that the Gao‐Guenie impact melt breccia may have formed in a melt injection dike ~0.5–5 m in width, generated during a sizeable impact event on the H chondrite parent asteroid.  相似文献   

11.
We have obtained new constraints on the cosmological parameters Ω m and σ 8 from the peculiar velocities of flat edge-on spiral galaxies from the RFGC catalog. Based on these results presented graphically, we have found the quantitative condition (Ω m /0.3)0.37 σ 8 = 0.92 ± 0.05. The estimates of Ω m and σ 8, along with their combinations Ω m α σ 8 for various α, are compared with the estimates by other authors.  相似文献   

12.
Abstract– Seventy‐four macrochondrules with sizes >3 mm were studied. Considering the extraordinary size of the chondrules (occasionally achieving a mass of 1000 times (and more) the mass of a normal‐sized chondrule), the conditions in the formation process must have been somewhat different compared with the conditions for the formation of the common chondrules. Macrochondrules are typically rich in olivine and texturally similar to specific chondrule types (barred, radial, porphyritic, and cryptocrystalline) of normal‐sized chondrules. However, our studies show that most of the macrochondrules are fine‐grained or have elongated crystals (mostly BO, RP, and C), which lead to the assumption that they were once totally molten and cooled quite rapidly. Porphyritic chondrules belong to the least abundant types of macrochondrules. This distribution of chondrule types is highly unusual and just a reverse of the distribution of chondrule types among the typical‐sized chondrules in most chondrite groups except for the CH and CB chondrites. New chondrule subtypes (like radial‐olivine [RO] or multi‐radial [MR] chondrules) are defined to better describe the textures of certain large chondrules. Macrochondrules may have formed due to melting of huge precursor dust aggregates or due to rapid collisions of superheated melt droplets, which led to the growth of large molten spherules in regions with high dust densities and high electrostatic attraction.  相似文献   

13.
Based on recent evidence that oxide grains condensed from a plasma will contain oxygen that is mass‐independently fractionated compared to the initial composition of the vapor, we present a first attempt to evaluate the potential magnitude of this effect on dust in the primitive solar nebula. This assessment relies on previous studies of nebular lightning to provide reasonable ranges of physical parameters to form a very simple model to evaluate the plausibility that lightning could affect a significant fraction of nebular dust and that such effects could cause a significant change in the oxygen isotopic composition of solids in the solar nebula over time. If only a small fraction of the accretion energy is dissipated as lightning over the volume of the inner solar nebula, then a large fraction of nebular dust will be exposed to lightning. If the temperature of such bolts is a few percent of the temperatures measured in terrestrial discharges, then dust will vaporize and recondense in an ionized environment. Finally, if only a small average decrease is assumed in the 16O content of freshly condensed dust, then over the last 5 Myr of nebular accretion the average Δ17O of the dust could increase by more than 30 per mil. We conclude that it is possible that the measured “slope 1” oxygen isotope line measured in meteorites and their components represents a time‐evolution sequence of nebular dust over the last several million years of nebular evolution where 16O‐rich materials formed first, then escaped further processing as the average isotopic composition of the dust gradually became increasingly depleted in 16O.  相似文献   

14.
Zircon in five samples of variably comminuted, melted, and hydrothermally altered orthogneiss from the Maniitsoq structure of southern West Greenland yield a weighted mean 207Pb/206Pb age of 3000.9 ± 1.9 Ma (ion probe data, n = 37). The age data constitute a rare example of pervasive and nearly complete isotopic resetting of zircon during a regional hydrothermal event. Many zircon grains are homogeneous or display weak flame‐like patterns in backscattered electron images. Other grains show complex internal textures, where homogeneous, high‐U fronts commonly cut across relict igneous‐type oscillatory zonation. Inclusions of quartz, plagioclase, mica, and other Al ± Na ± Ca ± Fe‐bearing silicates are very common. In two samples, selective replacement of zircon with baddeleyite occurs along concentric zones with relict igneous zonation, and as specks a few microns large within recrystallized, high‐U areas. We interpret the 3000.9 ± 1.9 Ma date as the minimum age of the recently proposed impact structure at Maniitsoq. The great geographical extent and intensity of the hydrothermal event suggest massive invasion of water into the currently exposed crust, implying that the age of the hydrothermal alteration would closely approximate the age of the proposed impact at Maniitsoq. At the western margin of the Taserssuaq tonalite complex, which postdates the Maniitsoq event, a 207Pb/206Pb mean age of 2994.6 ± 3.4 Ma obtained from zircon has mostly retained igneous‐type oscillatory zonation. A subsequent thermal event at approximately 2975 Ma is recorded in several samples by zircon with baddeleyite replacement textures.  相似文献   

15.
Glass‐bearing inclusions hosted by different mineral phases in SNC meteorites provide important information on the conditions that prevailed during formation of early phases and/or on the composition of the primary trapped liquids/melts of these rocks. Although extensive previous work has been reported on such inclusions, several questions are still unresolved. We performed a chemical and petrographic study of the constituents (glasses and mineral assemblage) of glassy and multiphase inclusions in Shergotty and Chassigny. We focused on obtaining accurate trace element contents of glasses and co‐existing minerals and discussing their highly variable REE contents. Our results reveal an unusual geochemistry of trace element contents that appear to be independent of their major element compositions. Chemical equilibrium between phases inside inclusions as well as between glasses and host minerals could not be established. The LREE contents of glasses in glass inclusions can vary by up to two orders of magnitude. The depletion in trace element abundances shown by glasses seem to be inconsistent with these phases being residual melts. The light lithophile element contents of glasses are highly variable with enrichment in incompatible elements (e.g., Be, Sr, Ba, and LREE) indicating some processes involving percolation of fluids. All of these features are incompatible with glass‐bearing inclusions in the host minerals acting as closed systems preserving unmodified primary liquids/melts. Glass‐bearing inclusions in Shergotty and Chassigny appear to have been altered (as was the rock itself) by different postformational processes (e.g., shock, metamorphism, metasomatic [?] fluids) that affected these meteorites with different degree of intensity. Our results indicate that these inclusions could not preserve a reliable sample of the primary trapped melt.  相似文献   

16.
Abstract– We investigate the hypothesis that many chondrules are frozen droplets of spray from impact plumes launched when thin‐shelled, largely molten planetesimals collided at low speed during accretion. This scenario, here dubbed “splashing,” stems from evidence that such planetesimals, intensely heated by 26Al, were abundant in the protoplanetary disk when chondrules were being formed approximately 2 Myr after calcium‐aluminum‐rich inclusions (CAIs), and that chondrites, far from sampling the earliest planetesimals, are made from material that accreted later, when 26Al could no longer induce melting. We show how “splashing” is reconcilable with many features of chondrules, including their ages, chemistry, peak temperatures, abundances, sizes, cooling rates, indented shapes, “relict” grains, igneous rims, and metal blebs, and is also reconcilable with features that challenge the conventional view that chondrules are flash‐melted dust‐clumps, particularly the high concentrations of Na and FeO in chondrules, but also including chondrule diversity, large phenocrysts, macrochondrules, scarcity of dust‐clumps, and heating. We speculate that type I (FeO‐poor) chondrules come from planetesimals that accreted early in the reduced, partially condensed, hot inner nebula, and that type II (FeO‐rich) chondrules come from planetesimals that accreted in a later, or more distal, cool nebular setting where incorporation of water‐ice with high Δ17O aided oxidation during heating. We propose that multiple collisions and repeated re‐accretion of chondrules and other debris within restricted annular zones gave each chondrite group its distinctive properties, and led to so‐called “complementarity” and metal depletion in chondrites. We suggest that differentiated meteorites are numerically rare compared with chondrites because their initially plentiful molten parent bodies were mostly destroyed during chondrule formation.  相似文献   

17.
We present further considerations regarding the strong 14C variation in AD 774/5. For its cause, either a solar super‐flare or a short gamma‐ray burst were suggested. We show that all kinds of stellar or neutron star flares would be too weak for the observed energy input at Earth in AD 774/5. Even though Maehara et al. (2012) present two super‐flares with ∼1035 erg of presumably solar‐type stars, we would like to caution: These two stars are poorly studied and may well be close binaries, and/or having a M‐type dwarf companion, and/or may be much younger and/or much more magnetic than the Sun – in any such case, they might not be true solar analog stars. From the frequency of large stellar flares averaged over all stellar activity phases (maybe obtained only during grand activity maxima), one can derive (a limit of) the probability for a large solar flare at a random time of normal activity: We find the probability for one flare within 3000 years to be possibly as low as 0.3 to 0.008 considering the full 1σ error range. Given the energy estimate in Miyake et al. (2012) for the AD 774/5 event, it would need to be ∼2000 stronger than the Carrington event as solar super‐flare. If the AD 774/5 event as solar flare would be beamed (to an angle of only ∼24°), 100 times lower energy would be needed. A new AD 774/5 energy estimate by Usoskin et al. (2013) with a different carbon cycle model, yielding 4 ot 6 time lower 14C production, predicts 4–6 times less energy. If both reductions are applied, the AD 774/5 event would need to be only ∼4 times stronger than the Carrington event in 1859 (if both had similar spectra). However, neither 14C nor 10Be peaks were found around AD 1859. Hence, the AD 774/5 event (as solar flare) either was not beamed that strongly, and/or it would have been much more than 4‐6 times stronger than Carrington, and/or the lower energy estimate (Usoskin et al. 2013) is not correct, and/or such solar flares cannot form (enough) 14C and 10Be. The 1956 solar energetic particle event was followed by a small decrease in directly observed cosmic rays. We conclude that large solar super‐flares remain very unlikely as the cause for the 14C increase in AD 774/5. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

18.
We measured the radial velocity of the star θ1 Ori D from IUE spectra and used published observations. Based on these data, we determined the period of its radial-velocity variations, P=20.2675±0.0010 days, constructed the phase radial-velocity curve, and solved it by least squares. The spectroscopic orbital elements were found to be the following: the epoch of periastron passage Ep=JD 2430826.6±0.1, the system's center-of-mass velocity /Gg=32.4±1.0 km s?1, K=14.3±1.5 km s?1, Ω=3.3±0.1 rad, e=0.68±0.09, a1 sin i = 3 × 1010 km, and f1 = 0.0025M⊙. Twice the period, P=40.528±0.002 days, is also consistent with the observations.  相似文献   

19.
We study the radial-velocity and light curves of the two eclipsing binaries EE Aqr and Z Vul. Using the latest version of the Wilson and Van Hamme (Computing Binary Star Observables, 2003) model, absolute parameters for the systems are determined. We find that EE Aqr and Z Vul are near-contact and semi-detached systems, respectively. The primary component of EE Aqr fills about 96% of its ‘Roche lobe’, while its secondary one appears close to completely filling this limiting volume. In a similar way, we find fill-out proportions of about 72 and 100% of these volumes for the primary and secondary components of Z Vul respectively. We compare our results with those of previous authors.  相似文献   

20.
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