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排序方式: 共有186条查询结果,搜索用时 15 毫秒
81.
G. E. Romero 《Astrophysics and Space Science》1995,234(1):49-55
Several versions of the shock-in-jet model of the blazars' variability have been proposed in recent years. This family of models resorts to the interaction between a relativistic shock and a feature in the parsec-scale jet associated to an AGN in order to explain the origin of the rapid variability recently observed in many extragalactic radio sources. The nature of the involved features varies from one particular model to another. Here I discuss a possible origin for this variety of subparsec-sized features in the frame of the two-fluid model of extragalactic jets. Macroscopic Kelvin-Helmholtz instabilities are shown to be capable of producing the required plurality of morphologies in the innermost part of the beams, in such a way that the different models have particular ranges of applicability according to the differencies in the parameters of the flows. 相似文献
82.
We report the detection of a weak and extended source in the radio continuum at 1.42 GHz around the position of the high energy
gamma ray pulsar PSR 1055-52. The source is also detectable at 408 MHz. It presents non-thermal spectral indices suggesting
a synchrotron nature. We discuss the possibility of a physical association with the pulsar.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
83.
Summary A set of mesoscale numerical simulations using the Emanuel and Kain-Fritsch deep convection schemes has been performed in order to determine the sensitivity of the forecast-especially, the rainfall-to the scheme used. The study is carried out for two cases of heavy precipitation in the coastal zone of the Western Mediterranean, where the topographic forcing is of primary influence. The first one, characterized by an almost stationary synoptic situation, is dominated by warm, moist advection at low levels; the second one, of frontal type, presents a much stronger dynamic forcing at upper levels. Although the comparison attempt is conditioned by the limited number of considered cases, the numerical results provide at least some preliminary conclusions. The inclusion of a convective scheme improves the forecast precipitation, through two actions: directly, producing more realistic rainfall patterns in areas of convection; indirectly, avoiding excessive precipitation in areas with orographic or dynamical upward forcing by drying and stabilizing the atmosphere upstream. In particular, the Kain-Fritsch scheme seems to be more sensitive to the orographic forcing, in agreement with observations.With 21 Figures 相似文献
84.
Ion trap mass spectrometry (ITMS) was used to obtain further qualitative information about the chemical composition of humic-like
substances (HULIS) in atmospheric particulate matter. Particles ≤10 μm (PM10) were collected on quartz fiber filters for 24 h in the region of Basel (Switzerland) and extracted with water. HULIS were
separated from inorganic salts by size exclusion chromatography (SEC) and detected by electrospray ionization in the negative
ion mode (ESI(−)). Series of consecutive fragment ion spectra (MSn) were recorded by ITMS. Full scan mass spectra of the extracts showed a mass distribution pattern characteristic for HULIS.
Different molecular ions were selected from this pattern for further fragmentations. Among them the molecular ion m/z 299 was considered as representative and intensively studied. Many MS2 and MS3 fragment spectra contained a fragment m/z 97 and a neutral loss of 80 u. Time-of-flight (TOF) MS and deuterium exchange experiments identified m/z 97 as hydrogen sulfate. MS2 and MS3 fragment spectra supported the existence of sulfate covalently bound to HULIS. The fragmentation behavior of sulfated HULIS
could be confirmed by model compounds. 相似文献
85.
Javier Romero Gérard Pergent Christine Pergent-Martini Miguel-Angel Mateo Cécile Regnier 《Marine Ecology》1992,13(1):69-83
Abstract. The ecosystem associated to the Mediterranean seagrass Posidonia oceanica shows a clear distinction in two subcompartments regarding turnover time: aboveground and belowground. Aboveground parts (leaves) are highly dynamic, and most of the leaf material is decomposed or exported in less than one year, representing a net loss of nutrients. In contrast, belowground biomass (roots and rhizomes) has a turnover time of the order of centuries, with a consequent accumulation of organic matter in the sediment. The accumulation rates for the single elements rank in the order C > N > P. This ecosystem may be considered as a sink for biogenic elements. 相似文献
86.
Oscar E. Romero Dierk Hebbeln Gerold Wefer 《Deep Sea Research Part I: Oceanographic Research Papers》2001,48(12):46
Flux of siliceous plankton and taxonomic composition of diatom and silicoflagellate assemblages were determined from sediment trap samples collected in coastal upwelling-influenced waters off northern Chile (30°S, CH site) under “normal” or non-El Niño (1993–94) and El Niño conditions (1997–98). In addition, concentration of biogenic opal and siliceous plankton, and diatom and silicoflagellate assemblages preserved in surface sediments are provided for a wide area between 27° and 43°S off Chile. Regardless of the year, winter upwelling determines the maximum production pattern of siliceous microorganisms, with diatoms numerically dominating the biogenic opal flux. During the El Niño year the export is markedly lower: on an annual basis, total mass flux diminished by 60%, and diatom and silicoflagellate export by 75%. Major components of the diatom flora maintain much of their regular seasonal cycle of flux maxima and minima during both sampling periods. Neritic resting spores (RS) of Chaetoceros dominate the diatom flux, mirroring the influence of coastal-upwelled waters at the CH trap site. Occurrence of pelagic diatoms species Fragilariopsis doliolus, members of the Rhizosoleniaceae, Azpeitia spp. and Nitzschia interruptestriata, secondary components of the assemblage, reflects the intermingling of warmer waters of the Subtropical Gyre. Dictyocha messanensis dominates the silicoflagellate association almost year-around, but Distephanus pulchra delivers ca. 60% of its annual production in less than three weeks during the winter peak. The siliceous thanatocoenosis is largely dominated by diatoms, whose assemblage shows significant qualitative and quantitative variations from north to south. Between 27° and 35°S, the dominance of RS Chaetoceros, Thalassionema nitzschioides var. nitzschioides and Skeletonema costatum reflects strong export production associated with occurrence of coastal upwelling. Both highest biogenic opal content and diatom concentration at 35° and 41°–43°S coincide with highest pigment concentrations along the Chilean coast. Predominance of the diatom species Thalassiosira pacifica and T. poro-irregulata, and higher relative contribution of the silicoflagellate Distephanus speculum at 41°–43°S suggest the influence of more nutrient-rich waters and low sea surface temperatures, probably associated with the Antarctic Circumpolar Water. 相似文献
87.
Norman Silverberg Aida Martínez Sergio Aguíiga Jos D. Carriquiry Nancy Romero Evgeni Shumilin Soledad Cota 《Estuarine, Coastal and Shelf Science》2004,59(4):575-587
The vertical flux of particulate matter at 330 m depth in San Lázaro Basin off southern Baja California ranged from 63 to 587 mg m−2 d−1 between August and November 1996. Organic carbon contents were between 5.6 and 14.8%, yielding flux rates of 9–40 mgC m−2 d−1. In December 1997 and January 1998, at the height of the strong El Niño event, the respective fluxes (47–202 mg m−2 d−1 and 3–8 mgC m−2 d−1) were comparable. The February–June 1998 records, however, revealed sharply reduced mass (1–6 mg m−2 d−1) and organic carbon (0.2–0.8 mgC m−2 d−1) fluxes. The organics collected in 1996 were predominantly autochthonous (δ13C=−22‰; C/N=8). The variations in δ15N (8.3–11.0‰) suggest an alternation of new and regenerated production, possibly associated with fluctuations in the intensity of deep mixing during that autumn. The relatively high organic matter fluxes in December 1997 appear to be associated with regenerated production. The average composition from February to June 1998 (δ13C=−23.6‰; 15N=11.7‰; C/N=10.5) indicates degraded material of marine origin. The maximum δ15N value found (14‰) suggests that deeper, denitrified waters were brought to the surface and possibly advected laterally. Regime changes in the waters of the basin occur at 6–10 week intervals, evidenced by concurrent shifts in most of the measured parameters, including fecal pellet types and metal chemistry. The marine snow-dominated detritus collected showed a shift from a mixed diatom-rich-radiolarian-coccolith assemblage in late 1996 to a coccolith-dominated assemblage, including the contents of fecal pellets, during the 1997–1998 El-Niño period. T–S profiles, plankton analysis and chlorophyll contents of the upper water column indicated that the strong phytoplankton bloom, normally associated with seasonal upwelling along the Pacific coast of Baja, did not occur during the spring of 1998. The persistence of oligotrophic conditions during the 1997–1998 El Niño event favored the dominance of nanoplankton and reduced the vertical flux of particles. 相似文献
88.
The paper reports the results of suction-controlled triaxial tests performed on compacted samples of two well-graded granular materials in the range of coarse sand–medium gravel particle sizes: a quartzitic slate and a hard limestone. The evolution of grain size distributions is discussed. Dilatancy rules were investigated. Dilatancy could be described in terms of stress ratio, plastic work input and average confining stress. The shape of the yield locus in a triaxial plane was established by different experimental techniques. Yielding loci in both types of lithology is well represented by approximate elliptic shapes whose major axis follows approximately the K0 line. Relative humidity was found to affect in a significant way the evolution of grain size distribution, the deviatoric stress–strain response and the dilatancy rules. 相似文献
89.
I. A. Grenier M. M. Kaufman Bernadó G. E. Romero 《Astrophysics and Space Science》2005,297(1-4):109-118
A subset of the unidentified EGRET γ-ray sources with no active galactic nucleus or other conspicuous counterpart appears
to be concentrated at medium latitudes. Their long-term variability and their spatial distribution indicate that they are
distinct from the more persistent sources associated with the nearby Gould Belt. They exhibit a large scale height of 1.3
± 0.6 kpc above the Galactic plane. Potential counterparts for these sources include microquasars accreting from a low-mass
star and spewing a continuous jet. Detailed calculations have been performed of the jet inverse Compton emission in the radiation
fields from the star, the accretion disc, and a hot corona. Different jet Lorentz factors, powers, and aspect angles have
been explored. The up-scattered emission from the corona predominates below 100 MeV whereas the disc and stellar contributions
are preponderant at higher energies for moderate (∼15∘) and small (∼1∘) aspect angles, respectively. Yet, unlike in the high-mass, brighter versions of these systems, the external Compton emission
largely fails to produce the luminosities required for 5 to 10 kpc distant EGRET sources. Synchrotron-self-Compton emission
appears as a promising alternative. 相似文献
90.