Galactic halo cusp–core: tidal compression in mergers     

Avishai Dekel  Jonathan Devor  Guy Hetzroni 《Monthly notices of the Royal Astronomical Society》2003,341(1):326-342

We explain in simple terms how the build-up of dark haloes by merging compact satellites, as in the cold dark matter (CDM) cosmology, inevitably leads to an inner cusp of density profile  ρ∝ r ^−α  with  α≳ 1  , as seen in cosmological N -body simulations. A flatter halo core with  α < 1  exerts on the satellites tidal compression in all directions, which prevents the deposit of stripped satellite material in the core region. This makes the satellite orbits decay from the radius where  α∼ 1  to the halo centre with no local tidal mass transfer, and thus causes a rapid steepening of the inner profile to  α > 1  . These tidal effects, the resultant steepening of the profile to a cusp, and the stability of this cusp to tandem mergers with compact satellites are demonstrated using N -body simulations. The transition at  α∼ 1  is then addressed using toy models in the limiting cases of impulse and adiabatic approximations and using tidal radii for satellites on radial and circular orbits. In an associated paper, we address the subsequent slow convergence from either side to an asymptotic stable cusp with  α≳ 1  . Our analysis thus implies that an inner cusp is enforced when small haloes are typically more compact than larger haloes, as in the CDM scenario, such that enough satellite material makes it intact into the inner halo and is deposited there. We conclude that a necessary condition for maintaining a flat core, as indicated by observations, is that the inner regions of the CDM satellite haloes be puffed up by about 50 per cent such that when they merge into a larger halo they would be disrupted outside the halo core. This puffing up could be due to baryonic feedback processes in small haloes, which may be stimulated by the tidal compression in the halo cores.  相似文献   

Jets from Young Stars and Compact Objects Environments     

Guy Pelletier 《Astrophysics and Space Science》2001,277(1-2):225-234

Several classes of cosmic objects, such as Young Stellar Objects, Active Galactic Nuclei, Micro-Quasars, Pulsars and probably Gamma Ray Bursts, display powerful winds and jets; for some of them the flow is even ultrarelativistic. For all these classes of objects, the magnetic field is supposed to play a major role in launching and collimating the flow, together with the angular momentum transfer. It probably plays an important role for the turbulent transport in accretion disks also. Regarding the high energy radiation of relativistic jets and the cosmic ray generation, the magnetic field is of course the acceleration agent and could produce the Ultra High Energy Cosmic Rays in some extragalactic objects. The main growth points of these topics are presented, mostly in the case of black hole environments; the case of Young Stellar Objects is more complicated because of the interaction of the stellar magnetosphere with the accretion disk, and the models for this interaction are not yet founded on a reliable theory.  相似文献   

Silica‐rich igneous rims around magnesian chondrules in CR carbonaceous chondrites: Evidence for condensation origin from fractionated nebular gas     

Alexander N. KROT  Guy LIBOUREL  Cyrena A. GOODRICH  Michail I. PETAEV 《Meteoritics & planetary science》2004,39(12):1931-1955

Abstract— The outer portions of many type I chondrules (Fa and Fs <5 mol%) in CR chondrites (except Renazzo and Al Rais) consist of silica‐rich igneous rims (SIRs). The host chondrules are often layered and have a porphyritic core surrounded by a coarse‐grained igneous rim rich in low‐Ca pyroxene. The SIRs are sulfide‐free and consist of igneously‐zoned low‐Ca and high‐Ca pyroxenes, glassy mesostasis, Fe, Ni‐metal nodules, and a nearly pure SiO₂ phase. The high‐Ca pyroxenes in these rims are enriched in Cr (up to 3.5 wt% Cr₂O₃) and Mn (up to 4.4 wt% MnO) and depleted in Al and Ti relative to those in the host chondrules, and contain detectable Na (up to 0.2 wt% Na₂O). Mesostases show systematic compositional variations: Si, Na, K, and Mn contents increase, whereas Ca, Mg, Al, and Cr contents decrease from chondrule core, through pyroxene‐rich igneous rim (PIR), and to SIR; FeO content remains nearly constant. Glass melt inclusions in olivine phenocrysts in the chondrule cores have high Ca and Al, and low Si, with Na, K, and Mn contents that are below electron microprobe detection limits. Fe, Ni‐metal grains in SIRs are depleted in Ni and Co relative to those in the host chondrules. The presence of sulfide‐free, SIRs around sulfide‐free type I chondrules in CR chondrites may indicate that these chondrules formed at high (>800 K) ambient nebular temperatures and escaped remelting at lower ambient temperatures. We suggest that these rims formed either by gas‐solid condensation of silica‐normative materials onto chondrule surfaces and subsequent incomplete melting, or by direct SiO_(gas) condensation into chondrule melts. In either case, the condensation occurred from a fractionated, nebular gas enriched in Si, Na, K, Mn, and Cr relative to Mg. The fractionation of these lithophile elements could be due to isolation (in the chondrules) of the higher temperature condensates from reaction with the nebular gas or to evaporation‐recondensation of these elements during chondrule formation. These mechanisms and the observed increase in pyroxene/olivine ratio toward the peripheries of most type I chondrules in CR, CV, and ordinary chondrites may explain the origin of olivine‐rich and pyroxene‐rich chondrules in general.  相似文献   

Interpersonal communication about climate change: how messages change when communicated through simulated online social networks     

Paul Connor  Emily Harris  Sophie Guy  Julian Fernando  Daniel B. Shank  Tim Kurz  Paul G. Bain  Yoshihisa Kashima 《Climatic change》2016,136(3-4):463-476

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Habitat change on Horn Island,Mississippi, 1940-2010, determined from textural features in panchromatic vertical aerial imagery     

Guy W. Jeter Jr. 《国际地球制图》2016,31(9):985-994

Habitat-type land cover on Horn Island, Mississippi, northern Gulf of Mexico, was estimated for the years 1940 and 2010 using a combination of panchromatic imagery and 2010 ground survey data. A grey-level co-occurrence matrix was applied to compute reflectance coefficient of variation (CV) and texture indices. The relationships of 2010 CV ranges with known habitat types defined training regions of interest in the 1940 imagery as a substitute for 1940 ground data. Texture indices contrast, correlation, energy and entropy then served as input bands for maximum likelihood classifications which produced 1940 and 2010 habitat maps. Analysis determined that wetter habitats on Horn expanded linearly over the seven-decade period. This is attributed to constraints on sediment supply and the impacts of severe storms which led to decreases in soil depth to the water table. If this trend continues, marsh habitat will cover 31% of Horn Island’s land area by 2050.  相似文献   

Effects of short flexible seagrass Zostera noltei on flow,erosion and deposition processes determined using flume experiments          下载免费PDF全文

Florian Ganthy  Laura Soissons  Pierre‐Guy Sauriau  Romaric Verney  Aldo Sottolichio 《Sedimentology》2015,62(4):997-1023

Innovative flume experiments were conducted in a recirculating straight flume. Zostera noltei meadows were sampled in their natural bed sediments in the field at contrasting stages of their seasonal growth. The aims of this study were: (i) to quantify the combined effects of leaf flexibility and development characteristics of Zostera noltei canopies on their interaction with hydrodynamics; and (ii) to quantify the role of Zostera noltei meadows in suspended sediment trapping and bed sediment resuspension related with changes in hydrodynamic forcing caused by the seasonal development of seagrasses. Velocity within the canopy was significantly damped. The attenuation in velocity ranged from 34 to 87% compared with bare sediments and was associated with a density threshold resulting from the flow‐induced canopy reconfiguration. The reduction in flow was higher in dense canopies at higher velocities than in less dense canopies, in which the reduction in flow was greater at low velocities. These contrasted results can be explained by competition between a rough‐wall boundary layer caused by the bed and a shear layer caused by the canopy. The velocity attenuation was associated with a two to three‐fold increase in bottom shear stress compared with unvegetated sediment. Despite the increase in near‐bed turbulence, protection of the sediment against erosion increased under a fully developed meadow, while sediment properties were found to be the main factor controlling erosion in a less developed meadow. Deposition fluxes were higher on the vegetated bed than on bare sediments, and these fluxes increased with leaf density. Fewer freshly deposited sediments were resuspended in vegetated beds, resulting in an increase in net sediment deposition with meadow growth. However, in the case of a very high leaf area index, sediment was mostly deposited on leaves, which facilitated subsequent resuspension and resulted in less efficient sediment trapping than in the less developed meadow.  相似文献   

Corrigendum to “Can the spread of non-native oysters (Crassostrea gigas) at the early stages of population expansion be managed?” [Marine Pollution Bulletin 60 (2010) 1059–1064]     

Claire Guy  Dai Roberts 《Marine pollution bulletin》2010,60(12):2359

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Palaeogeographic,climatic and tectonic change in southeastern Australia: the Late Neogene evolution of the Murray Basin     

Sandra McLaren  Malcolm W. Wallace  Stephen J. Gallagher  John A. Miranda  Guy R. Holdgate  Laura J. Gow  Ian Snowball  Per Sandgren 《Quaternary Science Reviews》2011,30(9-10):1086-1111

The Murray Basin is a low-lying but extensive intracratonic depocentre in southeastern Australia, preserving an extraordinary record of Late Neogene sedimentation. New stratigraphic and sedimentologic data allow the long-term evolution of the basin to be re-evaluated and suggest a significant role for: (1) tectonism in controlling basin evolution, and (2) progressive and step-wise climatic change beginning in the early Pleistocene. Tectonic change is associated with regional uplift, occurring at approximately the same rate from the early Pliocene until the present day, and possibly associated with changing mantle circulation patterns or plate boundary processes. This uplift led to the defeat and re-routing of the Murray River, Australia’s major continental drainage system. Key to our interpretation is recognition of timing relationships between four prominent palaeogeographic features – the Loxton-Parilla Sands strandplain, the Gambier coastal plain, palaeo megalake Bungunnia and the Kanawinka Escarpment. Geomorphic and stratigraphic evidence suggest that during the Early Pliocene the ancestral Murray River was located in western Victoria, flowing south along the Douglas Depression. Relatively small amounts of regional uplift (<200 m) defeated this drainage system, dramatically changing the palaeogeography of southeastern Australia and forming Plio-Pleistocene megalake Bungunnia. At its maximum extent Lake Bungunnia covered more than 50,000 km², making it one of the largest known palaeo- or modern-lakes in an intracontinental setting. Magnetostratigraphic constraints suggest lake formation c. 2.4 Ma. The formation of Lake Bungunnia influenced the Pliocene coastal dynamics, depriving the coastline of a sediment source and changing the coastal system from a prograding strandline system to an erosional one. Erosion during this period formed the Kanawinka Escarpment, a palaeo sea-cliff and one of the most prominent and laterally extensive geomorphic features in southeastern Australia. Marine sediments c. 800 ka to c. 1.16 Ma represent the time of re-establishment of depositional coastal dynamics and of a permanent outlet for the Murray River. This age range is consistent with our best estimate of the age of the youngest Lake Bungunnia sediments and points towards an early Pleistocene age for the demise of the lake system. The youngest Lake Bungunnia sediment, present on a number of distinct terraces, suggests that progressive, step-wise climatic change played a role in the demise of the lake. However, in order for the ancestral Murray River system to have been able to breach the pre-existing tectonic dam, it is likely that tectonic change and/or temporarily enhanced discharge was also significant. This scenario indicates that the modern Murray River has only been in existence for at most 700 ka.  相似文献   

Near real time satellite imagery to support and verify timely flood modelling     

Giuliano Di Baldassarre  Guy Schumann  Paul Bates 《水文研究》2009,23(5):799-803

The study investigates the capability of coarse resolution synthetic aperture radar (SAR) imagery to support flood inundation models. A hydraulic model of a 98‐km reach of the River Po (Northern Italy) was calibrated on the October 2000 high‐magnitude flood event with extensive and high‐quality field data. During the June 2008, low‐magnitude flood event a SAR image was acquired and processed in near real time (NRT) in order to provide adequate data for quick verification and recalibration of the hydraulic model. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Downscaling the hydrological cycle in the Mackenzie basin with the Canadian regional climate model     

Murray D. MacKay  Ronald E. Stewart  Guy Bergeron 《大气与海洋》2013,51(3):179-211

Abstract

The Canadian Regional Climate Model (CRCM) has been nested within the Canadian Centre for Climate Modelling and Analysis ‘ second generation General Circulation Model (GCM), for a single month simulation over the Mackenzie River Basin and environs. The purpose of the study is to assess the ability of the higher resolution CRCM to downscale the hydrological cycle of the nesting GCM. A second 1‐month experiment, in which the CRCM was nested within analyzed fields of a global data assimilation system, was also performed to examine the sensitivity of the basin moisture budget to atmospheric lateral boundary forcing.

We have found that the CRCM can produce realistic lee cyclogenesis, preferentially in the Liard sub‐basin, along with associated circulation and precipitation patterns, as well as an improved rainshadow in the lee of the Rocky Mountains compared to the GCM. While these features do quantitatively affect the monthly average climate statistics, the basin scale moisture budgets of the models were remarkably similar, though some of this agreement is due to compensating errors in the GCM. Both models produced excessive precipitation compared to a recent climatology for the region, the cause of which is traced to lateral boundary forcing. A second experiment, identical to the first except that the CRCM was forced with analyzed fields at the lateral boundaries, produced a qualitatively different basin moisture budget, including a much more realistic precipitation field. Errors in the moisture budget of the first experiment appear to be associated with the poor representation of the Aleutian Low in the GCM, and do not appear to be strongly connected to (local) surface processes within the models. This suggests that an effective strategy for modelling the hydrological cycle of the Mackenzie Basin on the fast climate timescale ‐ a major requirement of the Mackenzie GEWEX Study ‐ will involve nesting the CRCM within analyzed (or re‐analyzed) atmospheric fields.  相似文献