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71.
Substantial stream–disc overflow found in three-dimensional SPH simulations of cataclysmic variables
Stefan Kunze Roland Speith Frederic V. Hessman 《Monthly notices of the Royal Astronomical Society》2001,322(3):499-514
We study numerically the interaction of the infalling gas stream and the rim of the accretion disc in cataclysmic variables. The simulations were performed with a smoothed particle hydrodynamics scheme with high spatial resolution. Parameters of the systems AM CVn, OY Car, DQ Her, U Gem and IP Peg were used for the simulations. The simulations cover a wide range of orbital periods, mass ratios and mass transfer rates, as well as different thermal states of the accretion disc. The main result of this study is that the accretion stream is not stopped at the impact region (the bright spot at the outer rim of the disc). In fact, after undergoing the shock interaction, most of the matter is deflected vertically and flows in a more or less diffuse stream to inner parts of the disc, hitting the disc surface close to the circularization radius at orbital phase 0.5. This is a common feature in all systems for all simulated parameters. This stream overflow can cause the X-ray absorption dips observed in cataclysmic variables (CVs) and low-mass X-ray binaries (LMXBs) around orbital phase 0.7, if the inclination is at least 65°. Under certain circumstances, namely a sudden increase of the mass transfer rate from the secondary or a rather small disc, parts of the overflowing stream bounce off the disc surface after hitting it at orbital phase ≈0.5. Another absorption region can be expected around orbital phase 0.2.
In our simulations most of the infalling matter reaches the inner disc very quickly. This must alter the evolution of the quiescent disc and the outburst behaviour considerably compared with purely viscous transport of the material through the disc from the outer rim, and therefore should be taken into account in dwarf nova outburst cycle calculations. To our knowledge, the consequences of such a massive stream overflow for the dwarf nova outburst cycle have not been considered yet. 相似文献
In our simulations most of the infalling matter reaches the inner disc very quickly. This must alter the evolution of the quiescent disc and the outburst behaviour considerably compared with purely viscous transport of the material through the disc from the outer rim, and therefore should be taken into account in dwarf nova outburst cycle calculations. To our knowledge, the consequences of such a massive stream overflow for the dwarf nova outburst cycle have not been considered yet. 相似文献
72.
We calculate the evolution of planets undergoing a strong tidal encounter using smoothed particle hydrodynamics (SPH), for a range of periastron separations. We find that outside the Roche limit, the evolution of the planet is well-described by the standard model of linear, non-radial, adiabatic oscillations. If the planet passes within the Roche limit at periastron, however, mass can be stripped from it, but in no case do we find enough energy transferred to the planet to lead to complete disruption. In light of the three new extrasolar planets discovered with periods shorter than two days, we argue that the shortest-period cases observed in the period-mass relation may be explained by a model whereby planets undergo strong tidal encounters with stars, after either being scattered by dynamical interactions into highly eccentric orbits, or tidally captured from nearly parabolic orbits. Although this scenario does provide a natural explanation for the edge found for planets at twice the Roche limit, it does not explain how such planets will survive the inevitable expansion that results from energy injection during tidal circularization. 相似文献
73.
Impulse waves generated by rapid subaerial landslides into water bodies may pose a threat to riparian settlements and infrastructure. Empirically derived prediction equations based on experiments at laboratory scale provide information on key wave characteristics for preliminary hazard assessment. This research discusses existing prediction methods for spatial wave propagation features and compares their results with own impulse wave height decay experiments. While some prediction methods are based on simplified approaches for wave generation such as rigid body slides, others take only limited sets of slide parameters into account, narrowing their range of applicability considerably. The prediction methods are intentionally applied outside their ranges of applicability with the aim to assess their characteristics on an extended parameter range. It is found that a combination of separate terms for wave generation and wave propagation from two different existing prediction methods provides the best representation of the experimental data. 相似文献
74.
The Las Colinas landslide, which was triggered by the 13 January 2001, Mw?=?7.7 El Salvador earthquake, was highly destructive. The local site is composed of pyroclastic flows, brown cinders, soft pyroclastic fall deposits and a thin palaeosol and is characterised by steep slopes. The extremely high ground motions recorded near the landslide location are assumed to be both produced by site effects and responsible for the landslide. To characterise the ground motion amplifications due to site effects in terms of the variation in geometrical and geological settings, parametric studies were conducted with a linear elastic slope model, which was vertically subjected to the scattering SV wave of the Gabor wavelet. The results show that a maximum amplification is obtained when the model slope angle is approximately 30° (similar to the actual slope angle), and the maximum amplification is located approximately 20 m behind the crest, where the actual movement was initiated. Additionally, a slope with a height of approximately 160 m enhances ground motion amplification. The subsurface geology is found to induce a greater effect on amplification than that of the slope topography. In particular, a soft pyroclastic fall deposit is observed to contribute most of the ground motion amplification. According to the numerical results, it can be concluded that the local site conditions induced extremely high ground motions that then contributed to the slope movement. Although the thin, buried layer of palaeosol did not cause any significant amplification, its weak cohesion enhanced movement. 相似文献
75.
Frederic H. Nichols 《Estuaries and Coasts》1985,8(2):136-144
Long-term studies were used to examine (1) contrasting time scales and mechanisms of structural variations within two benthic communities and (2) the usefulness of long data sets for evaluating human impact. A 10-year study of a San Francisco Bay mudflat, the details of which are reported elsewhere, has revealed large short-term (on the order of months) variations in species abundances within a community composed predominantly of opportunistic species. The study site, located in a highly urbanized estuary, is subject to the influence of a nearby sewage-treatment facility. However, rapid changes in population size of the common species, in part due to periodic natural habitat disturbance, impedes the detection of anthropogenic influences on community composition. Only a very long-term data set may provide evidence of progressive change. Data collected for a 20-year period on the benthic community at 200 m depth in the main basin of Puget Sound, an environment subject to little apparent habitat disturbance show that numerical abundance of the common species can also change markedly. Here, however, numerical dominance shifts from one species to another at irregular, multiyear intervals. Recent increases in two heretofore rare species, and a significant increase in total numbers of individuals suggest that long-term changes may be occurring in this community. These two long-term data sets demonstrate the importance of measuring both the amplitude and the periodicity of fluctuations in population size of aquatic species as well as long-term fluctuations and patterns in environmental factors before attempting to demonstrate the effect of anthropogenic influences on aquatic communities. The results of these studies also demonstrate the usefulness of long-term data sets for revealing the potential importance of interactions among species in determining abundance patterns in the soft-bottom benthos. 相似文献
76.
An impounded area (Cabbagehead Bayou) in Upper Tampa Bay, Florida, was studied for 13 mo prior to and 22 mo after berm removal. Sampling for small fish and macrobenthos was conducted both at Cabbagehead Bayou and at a nearby control site, Double Branch Bay. Before and After Control and Impact (BACI) analyses, as well as similarity indices, were used to evaluate faunal characteristics in control versus impacted areas. Berm removal drastically changed periodicity of water cover in the impacted site, with the previously impounded site becoming almost entirely exposed during low tide. Fish abundances, biomass, and mean monthly number of species decreased in Cabbagehead Bayou after tidal flow was established as compared to when it was impounded. Responses of macrofauna varied among taxa, but total amphipod and polychaete abundance displayed a relative increase in the impacted site after berm removal. BACI analyses indicated that the relationship between mean values for almost all polychaete and amphipod taxa in the impacted and control sites changed significantly after berm removal. Additional differences were apparent in BACI analyses, which showed a significant change in the relationship of fish abundance and biomass at both sites for prebreach vs post-breach samples. The changes in relationships were generally not indicative of convergence of control and impacted sites, however. Community similarity of fish, amphipod, and polychaete assemblages increased during the last year of the post-breach sampling but remained generally very low. These findings suggest that distinct faunal responses to alteration of tidal periodicity can be detected within less than 2 yr after reestablishment of tidal flow. However, attainment of close correspondence to control conditions for certain fauna may require more than 22 mo. Therefore, future studies attempting to document recovery of restored wetlands may benefit from extended duration. 相似文献
77.
A note on 10Be‐derived mean erosion rates in catchments with heterogeneous lithology: examples from the western Central Andes
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Sebastien Carretier Vincent Regard Riccardo Vassallo Joseph Martinod Frederic Christophoul Eric Gayer Laurence Audin Christelle Lagane 《地球表面变化过程与地形》2015,40(13):1719-1729
Millennial catchment–mean erosion rates derived from terrestrial cosmogenic nuclides are generally based on the assumption that the lithologies of the parent rock each contain the same proportion of quartz. This is not always true for large catchments, in particular at the edge of mountainous plateaus where quartz‐rich basement rocks may adjoin sedimentary or volcano‐sedimentary rocks with low quartz content. The western Central Andes is an example of this type of situation. Different quartz contents may be taken into account by weighting the TCN production rates in the catchment. We recall the underlying theory and show that weighting the TCN production rate may also lead to bias in the case of a spatial correlation between erosion rate and lithology. We illustrate the difference between weighted and unweighted erosion rates for seven catchments (16 samples) in southern Peru and northern Chile and show variations up to a factor of 2 between both approaches. In this dataset, calculated erosion rates considering only granitoid outcrops are better correlated with catchment mean slopes than those obtained without taking into account the geological heterogeneity of the drained watershed. This dataset analysis demonstrates that weighting erosion rates by relative proportions of quartz is necessary to evaluate the uncertainties for calculated catchment–mean erosion rates and may reveal the correlation with geomorphic parameters. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
78.
79.
Anastassia Yu. Borisova Rémi Freydier Mireille Polvé Stefano Salvi Frederic Candaudap Thierry Aigouy 《Geostandards and Geoanalytical Research》2008,32(2):209-229
Microscopic melt inclusions found in magmatic minerals are undoubtedly one of the most important sources of information on the chemical composition of melts. This paper reports on the successful application of near-infrared (NIR) femtosecond laser ablation (LA) - inductively coupled plasma-mass spectrometry to in situ determination of incompatible trace elements (Li, Rb, Sr, Y, Zr, Nb, Cs, Ba, REE, Ta, Th, U) and ore metals (As, Mo, Pb) in individual melt inclusions hosted in quartz from the Mount Pinatubo dacites, Philippines. The determined elements cover a concentration range of five orders of magnitude. Femtosecond LA-ICP-MS analyses of twenty-eight individual melt inclusions demonstrate the efficiency of the microanalytical technique and suggests a spectacular homogeneity of the entrapped melt, at least with respect to the following incompatible trace elements: Rb, Sr, Nb, Cs, Ba, La, Ce, Pr, Nd, Pb, Th. The analytical precision (1s) for Na, Ca, Rb, Sr, Y, Nb, Ba and LREE ranged from 3 to 20%. Comparison of trace element concentrations in Mt. Pinatubo melt inclusions determined by femtosecond LA-ICP-MS with those of melt inclusions previously analysed by secondary ion mass spectrometry analysis (SIMS) and those of matrix glasses previously determined by nanosecond LA-ICP-MS showed an agreement typically within 30–40%. The homogeneity of trace element concentrations of the Mt. Pinatubo melt inclusions and the matrix glasses is consistent with the melt inclusion origin as homogeneous rhyolitic melt that was trapped in quartz phenocrysts at the final crystallisation stages of the host adakite (dacite) magma. 相似文献
80.
Djim M. L. Diongue Christine Stumpp Olivier Roupsard Didier Orange Frederic C. Do Serigne Faye 《水文研究》2023,37(1):e14787
Sustainable water management in semi-arid agriculture practices requires quantitative knowledge of water fluxes within the soil-vegetation-atmosphere system. Therefore, we used stable-isotope approaches to evaluate evaporation (Ea), transpiration (Ta), and groundwater recharge (R) at sites in Senegal's Groundnut basin and Ferlo Valley pasture region during the pre-monsoon, monsoon, and post-monsoon seasons of 2021. The approaches were based upon (i) the isothermal evaporation model (for quantifying Ea); (ii) water and isotope mass balances (to partition Ea and Ta for groundnut and pasture); and (iii) the piston displacement method (for estimating R). Ea losses derived from the isothermal evaporation model corresponded primarily to Stage II evaporation, and ranged from 0.02 to 0.09 mm d−1 in the Groundnut basin, versus 0.02–0.11 mm d−1 in Ferlo. At the groundnut site, Ea rates ranged from 0.01 to 0.69 mm d−1; Ta was in the range 0.55–2.29 mm d−1; and the Ta/ETa ratio was 74%–90%. At the pasture site, the ranges were 0.02–0.39 mm d−1 for Ea; 0.9–1.69 mm d−1 for Ta; and 62–90% for Ta/ETa. The ETa value derived for the groundnut site via the isotope approach was similar to those from eddy covariance measurements, and also to the results from the previous validated HYDRUS-1D model. However, the HYDRUS-1D model gave a lower Ta/ETa ratio (23.2%). The computed groundwater recharge for the groundnut site amounted to less than 2% of the local annual precipitation. Recommendations are made regarding protocols for preventing changes to isotopic compositions of water in samples that are collected in remote arid regions, but must be analysed days later. The article ends with suggestions for studies to follow up on evidence that local aquifers are being recharged via preferential pathways. 相似文献