Photometric survey of binary near-Earth asteroids     

P. Pravec  P. Scheirich  L. Šarounová  G. Hahn  G. Esquerdo  Z. Krzeminski  B.D. Warner  M.C. Nolan  L.A.M. Benner  A. Galád  W. Holliday  Yu.N. Krugly  R. Whiteley  D.R. DeGraff  S. Larson  W.R. Cooney Jr.  J. Zhu  R. Dyvig  V. Reddy  Š. Gajdoš  G. Masi  D. Higgins  B. Knight  R. Behrend  G. Burki  C. Demeautis  N. Waelchli  A. Klotz  M. Rieugné  V. Cotrez  G. Kober 《Icarus》2006,181(1):63-93

Photometric data on 17 binary near-Earth asteroids (15 of them are certain detections, two are probables) were analysed and characteristic properties of the near-Earth asteroid (NEA) binary population were inferred. We have found that binary systems with a secondary-to-primary mean diameter ratio Ds/Dp?0.18 concentrate among NEAs smaller than 2 km in diameter; the abundance of such binaries decreases significantly among larger NEAs. Secondaries show an upper size limit of Ds=0.5-1 km. Systems with Ds/Dp?0.5 are abundant but larger satellites are significantly less common. Primaries have spheroidal shapes and they rotate rapidly, with periods concentrating between 2.2 to 2.8 h and with a tail of the distribution up to ∼4 h. The fast rotators are close to the critical spin for rubble piles with bulk densities about 2 g/cm³. Orbital periods show an apparent cut-off at Porb∼11 h; closer systems with shorter orbital periods have not been discovered, which is consistent with the Roche limit for strengthless bodies. Secondaries are more elongated on average than primaries. Most, but not all, of their rotations appear to be synchronized with the orbital motion; nonsynchronous secondary rotations may occur especially among wider systems with Porb>20 h. The specific total angular momentum of most of the binary systems is similar to within ±20% and close to the angular momentum of a sphere with the same total mass and density, rotating at the disruption limit; this suggests that the binaries were created by mechanism(s) related to rotation near the critical limit and that they neither gained nor lost significant amounts of angular momentum during or since formation. A comparison with six small asynchronous binaries detected in the main belt of asteroids suggests that the population extends beyond the region of terrestrial planets, but with characteristics shifted to larger sizes and longer periods. The estimated mean proportion of binaries with Ds/Dp?0.18 among NEAs larger than 0.3 km is 15±4%. Among fastest rotating NEAs larger than 0.3 km with periods between 2.2 and 2.8 h, the mean proportion of such binaries is (66⁺¹⁰₋₁₂)%.  相似文献   

Envisat and SARAL/AltiKa Observations of the Antarctic Ice Sheet: A Comparison Between the Ku-band and Ka-band     

Frédérique Rémy  Thomas Flament  Aurélie Michel  Denis Blumstein 《Marine Geodesy》2015,38(3):510-521

The AltiKa altimeter onboard SARAL is a joint CNES/ISRO mission launched in February 2013 that has the same 35 days repeat orbit of the previous European altimeters, Envisat, and ERS-1/2. SARAL/AltiKa is thus a unique opportunity to extend the repeat observations of this orbit that have been surveyed since 1991. However, the altimeter operates in Ka-band, which is higher than the previous frequencies, and offers new paths of investigation. The penetration depth is theoretically reduced from around 10 m in Ku-band to less than 1 m in Ka-band, such that the volume echo originates from the near subsurface. Second, the sharper antenna aperture leads to a narrower leading edge that reduces the impact of the ratio between surface and volume echoes of the height retrieval. Indeed, the spatial and temporal observations of AltiKa at cross-over points and along-track indicate that the impact of backscatter changes on the height decreasesfrom 0.3 m/dB for the Ku-band to only 0.05 m/dB for the Ka-band. Therefore, the height measurement is stable over time. Moreover, the volume echo in the Ka-band results from the near subsurface layer and is mostly controlled by ice grain size, unlike the Ku-band.  相似文献   

On type-I migration near opacity transitions. A generalized Lindblad torque formula for planetary population synthesis     

Fr��d��ric S. Masset 《Celestial Mechanics and Dynamical Astronomy》2011,111(1-2):131-160

We give an expression for the Lindblad torque acting on a low-mass planet embedded in a protoplanetary disk that is valid even at locations where the surface density or temperature profile cannot be approximated by a power law, such as an opacity transition. At such locations, the Lindblad torque is known to suffer strong deviation from its standard value, with potentially important implications for type I migration, but the full treatment of the tidal interaction is cumbersome and not well suited to models of planetary population synthesis. The expression that we propose retains the simplicity of the standard Lindblad torque formula and gives results that accurately reproduce those of numerical simulations, even at locations where the disk temperature undergoes abrupt changes. Our study is conducted by means of customized numerical simulations in the low-mass regime, in locally isothermal disks, and compared to linear torque estimates obtained by summing fully analytic torque estimates at each Lindblad resonance. The functional dependence of our modified Lindblad torque expression is suggested by an estimate of the shift of the Lindblad resonances that mostly contribute to the torque, in a disk with sharp gradients of temperature or surface density, while the numerical coefficients of the new terms are adjusted to seek agreement with numerics. As side results, we find that the vortensity related corotation torque undergoes a boost at an opacity transition that can counteract migration, and we find evidence from numerical simulations that the linear corotation torque has a non-negligible dependency upon the temperature gradient, in a locally isothermal disk.  相似文献   

Reddening law and interstellar dust properties along Magellanic sight-lines     

Frédéric Zagury 《Astrophysics and Space Science》2007,312(1-2):113-125

This study establishes that SMC, LMC and Milky Way extinction curves obey the same extinction law which depends on the 2200 Å bump size and one parameter, and generalizes the Cardelli, Clayton and Mathis (Cardelli et al., Astrophys. J. 345, 245, 1989) relationship. This suggests that extinction in all three galaxies is of the same nature. The role of linear reddening laws over all the visible/UV wavelength range, particularly important in the SMC but also present in the LMC and in the Milky Way, is also highlighted and discussed.  相似文献   

Artificial groundwater recharge to a semi-arid basin: case study of Mujib aquifer,Jordan   总被引：1，自引：1，他引：0  

Tamer A. Al-Assa’d  Fayez Ahmad Abdulla 《Environmental Earth Sciences》2010,60(4):845-859

Mujib watershed is an important groundwater basin which is considered a major source for drinking and irrigation water in Jordan. Increased dependence on groundwater needs improved aquifer management with respect to understanding deeply recharge and discharge issues, planning rates withdrawal, and facing water quality problems arising from industrial and agricultural contamination. The efficient management of this source depends on reliable estimates of the recharge to groundwater and is needed in order to protect Mujib basin from depletion. Artificial groundwater recharge was investigated in this study as one of the important options to face water scarcity and to improve groundwater storage in the aquifer. A groundwater model based on the MODFLOW program, calibrated under both steady- and unsteady-state conditions, was used to investigate different groundwater management scenarios that aim at protecting the Mujib basin. The scenarios include variations of abstraction levels combined with different artificial groundwater recharge quantities. The possibilities of artificial groundwater recharge from existing and proposed dams as well as reclaimed municipal wastewater were investigated. Artificial recharge options considered in this study are mainly through injecting water directly to the aquifer and through infiltration from reservoir. Three scenarios were performed to predict the aquifer system response under different artificial recharge options (low, moderate, and high) which then compared with no action (recharge) scenario. The best scenario that provides a good recovery for the groundwater table and that can be feasible is founded to be by reducing current abstraction rates by 20% and implementing the moderate artificial recharge rates of 26 million(M)m³/year. The model constructed in this study helps decision makers and planners in selecting optimum management schemes suitable for such arid and semi-arid regions.  相似文献   

Sampling and Characterizing Rare Earth Elements in Groundwater in Deep-Lying Fractures in Granitoids Under In Situ High-Pressure and Low-Redox Conditions     

Linda J. Alakangas  Frédéric A. Mathurin  Mikko Faarinen  Bill Wallin  Mats E. Åström 《Aquatic Geochemistry》2014,20(4):405-418

Several countries are preparing to dispose of radioactive nuclear waste deep underground in crystalline rock. This type of bedrock is commonly extensively fractured and consequently carries groundwater that serves as a medium for transporting metals and radionuclides. A group of metals of particular interest in this context is the rare earth elements (REEs), because they are analogues of actinides contained within radioactive waste and are tracers of hydrological pathways and geochemical processes. Concentrations of REEs are commonly low in these groundwaters, leading to values below detection limits of standard monitoring methods, particularly for the heavy REEs. We present a new technical set-up for monitoring REEs (and other trace metals) in groundwater in fractured crystalline rock. The technique consists of passing the fracture groundwater, commonly under high pressure and containing reduced chemical species, through a device that maintains the physicochemical character of the groundwater. Within the device, diffusive gradient in thin-film (DGT) discs are installed in triplicate. With this set-up, we studied REEs in groundwater in fractures at depths of approximately ?144, ?280, and ?450 m in granitoids in the Äspö Hard Rock Laboratory in southern Sweden. The entire REE suite was detected (concentrations down to 0.1 ng L^?1) and was differently fractionated among the groundwaters. The shallowest groundwater, composed of dilute modern Baltic Sea water, was enriched in the heavy REEs, whereas the deeper groundwaters, dominated by old saline water, were depleted in the heavy REEs. Deployment periods varying from 1 to 4 weeks delivered similar REE concentrations, indicating stability and reproducibility of the experimental set-up. The study finds that 1 week of deployment may be enough. However, if the overall setting and construction allow for longer deployment times, 2–3 weeks will be optimal in terms of reaching reliable REE concentrations well above the detection limit while maintaining the performance of the DGT samplers.  相似文献   

Dispersion modeling of air pollutants in the atmosphere: a review     

Ádám Leelőssy  Ferenc Molnár Jr.  Ferenc Izsák  Ágnes Havasi  István Lagzi  Róbert Mészáros 《Central European Journal of Geosciences》2014,6(3):257-278

Modeling of dispersion of air pollutants in the atmosphere is one of the most important and challenging scientific problems. There are several natural and anthropogenic events where passive or chemically active compounds are emitted into the atmosphere. The effect of these chemical species can have serious impacts on our environment and human health. Modeling the dispersion of air pollutants can predict this effect. Therefore, development of various model strategies is a key element for the governmental and scientific communities. We provide here a brief review on the mathematical modeling of the dispersion of air pollutants in the atmosphere. We discuss the advantages and drawbacks of several model tools and strategies, namely Gaussian, Lagrangian, Eulerian and CFD models. We especially focus on several recent advances in this multidisciplinary research field, like parallel computing using graphical processing units, or adaptive mesh refinement.  相似文献   

Lessons learned from two case histories of seismic microzonation in Italy     

Filippo Santucci de Magistris  Anna d’Onofrio  Augusto Penna  Rodolfo Puglia  Francesco Silvestri 《Natural Hazards》2014,74(3):2005-2035

The prediction of the variability of the seismic ground motion in a given built-up area is considered an effective tool to plan appropriate urban development, to undertake actions on seismic risk mitigation and to understand the damage pattern caused by a strong-motion event. The procedures for studying the seismic response and the seismic microzonation of an urban area are well established; nevertheless, some controversial points still exists and are discussed here. In this paper, the selection of a reference input motion, the construction of a subsoil model and the seismic response analysis procedures are discussed in detail, based on the authors’ experience in two Italian case histories: the seismic microzonation of the city of Benevento, which was a predictive study, and the simulation of seismic response and damage distribution in the village of San Giuliano di Puglia, which was a retrospective analysis.  相似文献   

2015 Nier Prize for Pierre Beck          下载免费PDF全文

Frédéric Moynier 《Meteoritics & planetary science》2015,50(8):1493-1494

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Evidence for delayed poleward expansion of North Atlantic surface waters during the last interglacial (MIS 5e)     

Nicolas Van Nieuwenhove  Henning A. Bauch  Frédérique Eynaud  Evguenia Kandiano  Elsa Cortijo  Jean-Louis Turon 《Quaternary Science Reviews》2011,30(7-8):934-946

The Last Interglacial (Marine Isotopic Stage or MIS 5e) surface ocean heat flux from the Rockall Basin (NE Atlantic) towards the Arctic Ocean was reconstructed by analysing dinoflagellate cyst (dinocyst) assemblages in four sediment cores. Together with records of stable isotopes and ice-rafted detritus, the assemblage data reflect the northward retreat of ice(berg)-laden waters and the gradual development towards interglacial conditions at the transition from the Saalian deglaciation (Termination II) into MIS 5e. At the Rockall Basin, this onset of the Last Interglacial is soon followed by the appearance of the thermophilic dinocyst species Spiniferites mirabilis, with relative abundances higher than those observed at present in the area. North of the Iceland-Scotland Ridge, however, S. mirabilis only appears in significant numbers during late MIS 5e, between ～118 and 116.5 ka. Hence, fully marine Last Interglacial conditions with most intense Atlantic surface water influence occurred during late MIS 5e in the Nordic seas, and consequently also farther north in the Arctic Ocean, and at times when northern hemisphere summer insolation was already significantly decreased. The stratigraphic position of this Late Interglacial optimum is supported by planktic foraminifers and contrasts with the timing of the early Holocene climatic optimum in this area. We interpret the delayed northward expansion of Atlantic waters towards the polar latitudes as a result of the Saalian ice sheet deglaciation and its specific impact on the subsequent water mass evolution in this region.  相似文献