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排序方式: 共有379条查询结果,搜索用时 93 毫秒
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Biver Nicolas Bockelée-Morvan Dominique Colom Pierre Crovisier Jacques Henry Florence Lellouch Emmanuel Winnberg Anders Johansson Lars E.B. Gunnarsson Marcus Rickman Hans Rantakyrö Fredrik Davies John K. Dent William R.F. Paubert Gabriel Moreno Raphaël Wink Jörn Despois Didier Benford Dominic J. Gardner Matt Lis Dariusz C. Mehringer David Phillips Thomas G. Rauer Heike 《Earth, Moon, and Planets》2002,90(1-4):5-14
The bright comet Hale–Bopp provided the first opportunity to follow the outgassing rates of a number of molecular species over a large range of heliocentric distances. We present the results of our observing campaign at radio wavelengths which began in August 1995 and ended in January 2002. The observations were carried out with the telescopes of Nançay, IRAM, JCMT, CSO and, since September 1997, SEST. The lines of nine molecules (OH, CO, HCN, CH3OH, H2CO, H2S, CS, CH3CN and HNC) were monitored. CS, H2S, H2CO, CH3CN were detected up to rh= 3–4 AU from the Sun, while HCN and CH3OH were detected up to 6 AU. CO, which is the main driver of cometary activity at heliocentric distances larger than 3–4 AU, was last detected in August 2001, at rh= 14 AU. The gas production rates obtained from this programme contain important information on the nature of cometary ices, their thermal properties and sublimation mechanisms.Line shapes allow to measure gas expansion velocities, which, at large heliocentric distances, might be directly connected to the temperature of the nucleus surface. Inferred expansion velocity of the gas varied as rh -0.4 within 7 AU from the Sun, but remained close to 0.4 km s-1 further away. The CO spectra obtained at large rhare strongly blueshifted and indicative of an important day-to-night asymmetry in outgassing and expansion velocity. The kinetic temperature of the coma, estimated from the relative intensities of the CH3OH and CO lines, increased with decreasing rh, from about 10 K at 7 AU to 110 K around perihelion. 相似文献
323.
Biver Nicolas Bockelée-Morvan Dominique Crovisier Jacques Colom Pierre Henry Florence Moreno Raphaël Paubert Gabriel Despois Didier Lis Dariusz C. 《Earth, Moon, and Planets》2002,90(1-4):323-333
We present a comparative study on molecular abundances in comets basedon millimetre/submillimetre observations made with the
IRAM 30-m,JCMT, CSO and SEST telescopes. This study concerns a sample of 24comets (6 Jupiter-family, 3 Halley-family, 15 long-period)
observedfrom 1986 to 2001 and 8 molecular species (HCN, HNC, CH3CN,CH3OH, H2CO, CO, CS, H2S). HCN was detected in all comets,while at least 2 molecules were detected in 19 comets.
From the sub-sample of comets for which contemporary H2O productionrates are available, we infer that the HCN abundance relative to water variesfrom 0.08% to 0.25%. With respect
to other species, HCN is the moleculewhich exhibits the lowest abundance variation from comet to comet. Therefore,production
rates relative to that of HCN can be used for a comparative study ofmolecular abundances in the 19 comets. It is found that:
CH3OH/HCN varies from ≤ 9 to 64; CO/HCN varies from ≤ 24 to 180; H2CO/HCN varies between 1.6 and 10; and H2S/HCN varies between 1.5 and 7.6.
This study does not show any clear correlation between the relative abundancesand the dynamical origins of the comets, or
their dust-to-gas ratios. 相似文献
324.
Gabriel M. Guerra Souleymane Zio Jose J. Camata Jonas Dias Renato N. Elias Marta Mattoso Paulo L. B. Paraizo Alvaro L. G. A. Coutinho Fernando A. Rochinha 《Computational Geosciences》2016,20(1):265-281
Numerical models can help to push forward the knowledge about complex dynamic physical systems. Modern approaches employ detailed mathematical models, taking into consideration inherent uncertainties on input parameters (phenomenological parameters or boundary and initial conditions, among others). Particle-laden flows are complex physical systems found in nature, generated due to the (possible small) spatial variation on the fluid density promoted by the carried particles. They are one of the main mechanisms responsible for the deposition of sediments on the seabed. A detailed understanding of particle-laden flows, often referred to as turbidity currents, helps geologists to understand the mechanisms that give rise to reservoirs, strategic in oil exploration. Uncertainty quantification (UQ) provides a rational framework to assist in this task, by combining sophisticated computational models with a probabilistic perspective in order to deepen the knowledge about the physics of the problem and to access the reliability of the results obtained with numerical simulations. This work presents a stochastic analysis of sediment deposition resulting from a turbidity current considering uncertainties on the initial sediment concentrations and particles settling velocities. The statistical moments of the deposition mapping, like other important features of the currents, are approximated by a Sparse Grid Stochastic Collocation method that employ a parallel flow solver for the solution of the deterministic problems associated to the grid points. The whole procedure is supported and steered by a scientific workflow management engine designed for high performance computer applications. 相似文献
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Landslides - Long-runout landslides can be extremely hazardous and unpredictable. The prediction of long-runout landslides is challenging because it is unclear what factors control their mobility,... 相似文献
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Gabriel N. Genzano Diego Giberto Laura Schejter Claudia Bremec & Pablo Meretta 《Marine Ecology》2009,30(1):33-46
This paper provides updated taxonomic knowledge about hydrozoan species and provides ecological information including geographical and bathymetric distributions and biological substrata for the various hydroid assemblages from the Sub-Antarctic Biogeographical Region, mainly Buenos Aires and the Uruguayan coasts. Five of the 41 species found are new records for the study region. Thirty-one species (75.6%), all found at depths of less than 80 m, have cosmopolitan distributions. Biodiversity decreased markedly below 80 m depth, where nine species (≈22%) distributed in the Southern hemisphere and one endemic species (2.4%) were found. Most species were non-specific epizoites, occurring on diverse substrata. A non-parametric multivariate similarity analysis revealed nine species groups that were correlated with large-scale and local oceanographic patterns and with the availability of suitable substrata. The main hydroid substrata for eight of the groups were cnidarians, molluscs (mainly blue mussels), quartzite rocks and sponges. In a single group found in Patagonian scallop beds, the main biological substrata were polychaete tubes, other hydroids and scallops. 相似文献