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981.
982.
D. J. Huisman G. T. Klaver A. Veldkamp B. J. H. van Os 《International Journal of Earth Sciences》2000,89(1):154-169
2 O contents, which can be attributed to the Alpine source supplying fresh, sodic plagioclase-rich material instead of the local,
strongly weathered sediments. Increasing K2O/Al2O3 can be attributed to a similar decrease in degree of weathering. However, this trend is disturbed by the loss of K from clay
minerals during weathering in organic-rich layers. Local high TiO2 anomalies, caused by preferential sorting and concentration, are found in most Pliocene sections, but they are absent in
the Upper Pliocene and Lower Pleistocene Alpine-derived deposits. This change is probably due to a change in the energy of
the fluvial system. Finally, (pyrite-) S contents drop (siderite-) Fe contents rise. Micromorphological observations indicate
that the Pliocene pyrite was formed when freshwater deposits were flooded with seawater during short-term events. The decrease
in S, and the increase in siderite-Fe, can be attributed to decreasing marine influence, as a result of the marine regression
at the Pliocene–Pleistocene transition.
Received: 28 August 1999 / Accepted: 2 November 1999 相似文献
983.
984.
985.
R. A. Stathakis B. J. Boyle D. H. Jones M. S. Bessell T. J. Galama Lisa M. Germany M. Hartley D. M. James C. Kouveliotou I. J. Lewis Q. A. Parker K. S. Russell E. M. Sadler C. G. Tinney J. van Paradijs P. M. Vreeswijk 《Monthly notices of the Royal Astronomical Society》2000,314(4):807-814
Supernova 1998bw holds the record for the most energetic Type Ic explosion, one of the brightest radio supernovae and probably the first supernova associated with a γ -ray burst. In this paper we present spectral observations of SN 1998bw observed in a cooperative monitoring campaign using the Anglo-Australian Telescope, the UK Schmidt Telescope and the Siding Springs Observatories 2.3-m telescope. We investigate the evolution of the spectrum between 7 and 94 d after V -band maximum in comparison with well-studied examples of Type Ic SNe in order to quantify the unusual properties of this supernova event. Though the early spectra differ greatly from observations of classical Ic supernovae (SNe), we find that the evolution from the photospheric to nebular phases is slow but otherwise typical. The spectra differ predominantly in the extensive line blending and blanketing which has been attributed to the high velocity of the ejecta. We find that by day 19, the absorption line minima blueshifts are 10–50 per cent higher than other SNe and on day 94 emission lines are 45 per cent broader, as expected if the progenitor had a massive envelope. However, it is difficult to explain the extent of line blanketing entirely by line broadening, and we argue that an additional contribution from other species is present, indicating unusual relative abundances or physical conditions in the envelope. 相似文献
986.
Arno P. Schoenmakers † A. G. de Bruyn H. J. A. Röttgering H. van der Laan C. R. Kaiser 《Monthly notices of the Royal Astronomical Society》2000,315(2):371-380
We present four Mpc-sized radio galaxies which consist of a pair of double-lobed radio sources, aligned along the same axis, and with a coinciding radio core. We call these peculiar radio sources 'double-double' radio galaxies (DDRGs) and propose a general definition of such sources: a 'double-double' radio galaxy consists of a pair of double radio sources with a common centre. Furthermore, the two lobes of the inner radio source must have a clearly extended, edge-brightened radio morphology. Adopting this definition, we find several other candidate DDRGs in the literature. We find that in all sources the smaller (inner) pair of radio lobes is less luminous than the larger (outer) pair, and that the ratio of 1.4-GHz flux density of these two pairs appears to be anticorrelated with the projected linear size of the inner source. Also, the outer radio structures are large, exceeding 700 kpc. We discuss possible formation scenarios of the DDRGs, and we conclude that an interruption of the jet-forming central activity is the most likely mechanism. For one of our sources (B 1834+620) we have been able observationally to constrain the length of time of the interruption to a few Myr. We discuss several scenarios for the cause of the interruption, and suggest multiple encounters between interacting galaxies as a possibility. Finally, we discuss whether such interruptions help the formation of extremely large radio sources. 相似文献
987.
S. Goedhart † D. J. van der Walt A. J. Schutte ‡ 《Monthly notices of the Royal Astronomical Society》2000,315(2):316-324
Near-infrared photometry was performed on 56 southern 6.7-GHz methanol maser sources. A simple spherically symmetric model of the radiative transfer through a dust shell was developed and used to study the conditions in the dust cloud in which the masers are produced. The parameters investigated were the size of the cloud, the spectral type of the embedded star, the optical depth of the dust cloud and the dust density distribution. It was found that the infrared colours of the models have a complex dependence on the parameters and that no unique combination of parameter values explains the spectral energy distribution of any particular source. The model effectively reproduces the far-infrared ( IRAS ) colours but cannot simultaneously explain the near-infrared colours for any of the observed sources. 相似文献
988.
Mariano Méndez Michiel van der Klis 《Monthly notices of the Royal Astronomical Society》2000,318(3):938-942
We use data from the Rossi X-ray Timing Explorer to search for harmonics and sidebands of the two simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in Sco X-1. We do not detect any of these harmonics or sidebands, with 95 per cent confidence upper limits to their power between ∼1 and ∼10 per cent of the power of the upper kHz QPO. The oscillations produced at these frequencies may be attenuated in a scattering corona around the neutron star. We find that upper limits to the unattenuated power of some of the strongest theoretically predicted harmonics and sidebands are as low as ∼2 per cent of the unattenuated power of the high-frequency QPO in Sco X-1. 相似文献
989.
David A. H. Buckley Mark Cropper Kurt van der Heyden S. B. Potter Dayal T. Wickramasinghe 《Monthly notices of the Royal Astronomical Society》2000,318(1):187-194
We present polarimetric and spectroscopic observations of the ROSAT source RX J1141.3−6410, recently identified as a polar. The detection of circular polarization variations, with an amplitude of 10 per cent, over a 3.16-h period confirms that the system is a polar (AM Herculis star). Supporting evidence comes from the nature of the emission lines and their radial velocity variability. In addition, we observe continuum slope changes in the far-red spectral region (∼6000–8200 Å), indicative of phase dependent cyclotron emission. Polarimetric modelling at two wavelengths establishes RX J1141.3−6410 as a single-pole system, with i ∼ β ∼70°. The accretion region is extended in magnetic longitude, and is totally self-occulted for ∼25 per cent of the orbit. The radial velocity curves derived from the emission lines show a phasing with maximum blueshift occurring with Δ φ ∼0.05 of maximum intensity and circular polarisation. In addition, the broader component of the lines exhibit a substantial radial velocity phase shift with respect to the narrower component, in the sense that the broad component preceeds the narrow. This can be readily understood if the narrower component is principally a result of orbital motion of the stream material and the broad component mainly a result of streaming motion near the coupling region. The phasing of the Ca ii near-infrared line radial velocities also supports this general picture. 相似文献
990.
Summary This paper discusses the causes of the current wave of renewal and relocation of football stadiums in the Netherlands and tries to shed some light on the changing form and location of Dutch football stadiums. Comparison with developments in the United Kingdom shows not only remarkable resemblances and mutual influences, but also some significant differences. In an inextricable upward spiral of causes and consequences such as recent successes of clubs and the national team, rising attendances, increasingly positive media attention, rising interests of commercial investors, new international and national requirements on stadium construction and an increasingly positive attitude of municipalities, a new wave of stadium redevelopment and relocation has emerged in the Netherlands. In this paper it will become clear that every stadium tells its own story; some of these stories are highlighted. 相似文献