Ambipolar diffusion in the solar atmosphere     

S. M. Chitre  V. Krishan 《Monthly notices of the Royal Astronomical Society》2001,323(2):L23-L25

The process of non-linear ambipolar diffusion in the region overlying the solar surface can be an effective mechanism for producing sharp magnetic structures and current sheets. These may be the sites responsible for the occurrence of connectivity of magnetic field lines, and the subsequent explosive input of energy for heating of some of the features in the atmosphere of the Sun..  相似文献   

Exposure history of the Torino meteorite     

R. Wieler  Th. Graf  P. Signer  S. Vogt  G. F. Herzog  C. Tuniz  D. Fink  L. K. Fifield  J. Klein  R. Middleton  A. J. T. Jull  P. Pellas  J. Masarik  G. Dreibus 《Meteoritics & planetary science》1996,31(2):265-272

Abstract— We determined He, Ne, Ar, ¹⁰Be, ²⁶Al, ³⁶Cl, and ¹⁴C concentrations, as well as cosmic-ray track densities and halogen concentrations in different specimens of the H6 chondrite Torino, in order to constrain its exposure history to cosmic radiation. The Torino meteoroid had a radius of ～20 cm and travelled in interplanetary space for 2.5–10 Ma. Earlier, Torino was part of a larger body. The smallest possible precursor had a radius of 55 cm and a journey through space longer than ～65 Ma. If the first-stage exposure took place in a body with a radius of >3 m or in the parent asteroid, then it lasted nearly 300 Ma. The example of Torino shows that it is easy to underestimate first-stage exposure ages when constructing two-stage histories.  相似文献   

Stellar models and Hyades: the Hipparcos test     

V. Castellani  S. Degl'Innocenti  P. G. Prada Moroni 《Monthly notices of the Royal Astronomical Society》2001,320(1):66-72

We compare theoretical stellar models for main sequence (MS) stars with the Hipparcos data base for the Hyades cluster to give a warning against the uncritical use of available theoretical scenarios and to show how formal MS fittings can be fortuitous if not fictitious. Moreover, we find that none of the current theoretical scenarios appears able to account for an observed mismatch between theoretical predictions and observations of the coolest Hyades MS stars. Finally, we show that current theoretical models probably give too faint He burning luminosities unlike the case of less massive He burning models, with degenerate progenitors, which have been suggested to suffer the opposite discrepancy.  相似文献   

Simulations of deep pencil-beam redshift surveys     

N. Yoshida  J. Colberg  S.D.M. White  A.E. Evrard  T.J. MacFarland †  H.M.P. Couchman  A. Jenkins  C.S. Frenk  F.R. Pearce  G. Efstathiou  J.A. Peacock  P.A. Thomas 《Monthly notices of the Royal Astronomical Society》2001,325(2):803-816

We create mock pencil-beam redshift surveys from very large cosmological N -body simulations of two cold dark matter (CDM) cosmogonies, an Einstein–de Sitter model ( τ CDM) and a flat model with Ω₀=0.3 and a cosmological constant (ΛCDM). We use these to assess the significance of the apparent periodicity discovered by Broadhurst et al. Simulation particles are tagged as 'galaxies' so as to reproduce observed present-day correlations. They are then identified along the past light-cones of hypothetical observers to create mock catalogues with the geometry and the distance distribution of the Broadhurst et al. data. We produce 1936 (2625) quasi-independent catalogues from our τ CDM (ΛCDM) simulation. A couple of large clumps in a catalogue can produce a high peak at low wavenumbers in the corresponding one-dimensional power spectrum, without any apparent large-scale periodicity in the original redshift histogram. Although the simulated redshift histograms frequently display regularly spaced clumps, the spacing of these clumps varies between catalogues and there is no 'preferred' period over our many realizations. We find only a 0.72 (0.49) per cent chance that the highest peak in the power spectrum of a τ CDM (ΛCDM) catalogue has a peak-to-noise ratio higher than that in the Broadhurst et al. data. None of the simulated catalogues with such high peaks shows coherently spaced clumps with a significance as high as that of the real data. We conclude that in CDM universes, the regularity on a scale of ∼130  h ⁻¹ Mpc observed by Broadhurst et al. has a priori probability well below 10⁻³.  相似文献   

Large-scale structure in the ELAIS S1 Survey     

E. A. Gonzalez-Solares  S. Oliver  C. Gruppioni  F. Pozzi  C. Lari  M. Rowan-Robinson  S. Serjeant  F. La Franca  M. Vaccari 《Monthly notices of the Royal Astronomical Society》2004,352(1):44-48

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UBV RI CCD photometric study of the open clusters Basel 4 and NGC 7067     

R. K. S. Yadav  Ram Sagar 《Monthly notices of the Royal Astronomical Society》2004,349(4):1481-1492

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X-ray source populations in the Galactic plane     

A. D. P. Hands  R. S. Warwick  M. G. Watson  D. J. Helfand 《Monthly notices of the Royal Astronomical Society》2004,351(1):31-56

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Dynamical and compositional assessment of near‐Earth object mission targets     

Richard P. BINZEL  Ettore PEROZZI  Andrew S. RIVKIN  Alessandro ROSSI  Alan W. HARRIS  Schelte J. BUS  Giovanni B. VALSECCHI  Stephen M. SLIVAN 《Meteoritics & planetary science》2004,39(3):351-366

Abstract— Using an H‐plot analysis, we identify 234 currently known near‐Earth objects that are accessible for rendezvous with a “best case” delta‐V of less than 7 km/s. We provide a preliminary compositional interpretation and assessment of these potential targets by summarizing the taxonomic properties for 44 objects. Results for one‐half (22) of this sample are based on new spectroscopic measurements presented here. Our approach provides an easy‐to‐update method for giving guidelines to both observers and mission analysts for focusing on objects for which actual mission opportunities are most likely to be found. Observing prospects are presented for categorizing the taxonomic properties of the most accessible targets that are not yet measured.  相似文献   

Depositional and post‐depositional history of warm stage deposits at Knocknacran,Co. Monaghan,Ireland: implications for preservation of Irish last interglacial deposits     

A. P. M. Vaughan  L. A. Dowling  F. J. G. Mitchell  S.‐E. Lauritzen  A. M. McCabe  P. Coxon 《第四纪科学杂志》2004,19(6):577-590

Organic‐rich deposits, uncovered during overburden removal from mantled gypsum karst at Knocknacran opencast gypsum mine, Co. Monaghan, are the best candidate to date for a last interglacial record in Ireland. The two till and organic‐rich deposits (preserved at different quarry elevations) were emplaced on to a Tertiary dolerite surface during high‐energy flood events and subsequently folded and faulted by movement towards sinkholes in underlying gypsum. Uranium–thorium disequilibrium dating suggests that the organic‐rich deposits in the upper section were hydrologically isolated at ca. 41 ka and those in the lower section at ca. 86 ka. Interpretation of the pollen content, although tentative because of the depositional and post‐depositional history of the material, suggests that the organic material originated in a warm stage possibly warmer than the post‐Eemian interstadials. The unusual setting of preservation may indicate that in situ, last interglacial deposits have generally been removed by erosion in Ireland. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Mapping of crustal heterogeneity in the North Sea basin via the propagation of L_g-waves     

B. L. N. Kennett  S. Gregersen  S. Mykkeltveit  R. Newmark 《Geophysical Journal International》1985,83(1):299-306

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