Piecewise-continuous boundaries using the MODFLOW FHB and MT3DMS HSS packages     

Tonkin M  Tajani Z 《Ground water》2012,50(2):296-300

It is often necessary to simulate the influx to a groundwater model of water containing dissolved contaminants. Until fairly recently, users of MODFLOW and MT3DMS were restricted to varying the flux of water and contaminants on a stress-period basis: when a time-varying loading pattern required simulation, the modeler's only recourse was to discretize the model into many stress periods. From a practical standpoint this is cumbersome, while from a technical standpoint it requires that the modeler define a priori an appropriate time discretization that can accurately reproduce time-varying flow and mass loading. This is particularly undesirable when attempting to infer a time-varying flow or mass loading using inverse methods. The advent of the Flow and Head Boundary (FHB) package in the late 1990s greatly mitigated these limitations from the flow perspective. The recent release of the Hydrocarbon Spill Source (HSS) package for MT3DMS has essentially removed the limitation from the contaminant mass perspective. This Methods Note verifies the FHB and HSS packages by comparison with more commonly used boundary packages and highlights some benefits of their combined use, with reference to the reconstruction of historic flow and mass fluxes through inverse modeling. (Note: The Flow and Head Boundary and Hydrocarbon Spill Source packages are referred to throughout as "FHB" and "HSS", respectively,--that is, omitting version number suffixes--as the discussion presented should apply to all releases of each package.).  相似文献   

On the linear stability of weakly ionized, magnetized planar shear flows     

Matthew W. Kunz 《Monthly notices of the Royal Astronomical Society》2008,385(3):1494-1510

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The effect of magnetic fields on star cluster formation     

Daniel J. Price  Matthew R. Bate 《Monthly notices of the Royal Astronomical Society》2008,385(4):1820-1834

We examine the effect of magnetic fields on star cluster formation by performing simulations following the self-gravitating collapse of a turbulent molecular cloud to form stars in ideal magnetohydrodynamics. The collapse of the cloud is computed for global mass-to-flux ratios of  ∞, 20, 10, 5  and 3, i.e. using both weak and strong magnetic fields. Whilst even at very low strengths the magnetic field is able to significantly influence the star formation process, for magnetic fields with plasma  β < 1  the results are substantially different to the hydrodynamic case. In these cases we find large-scale magnetically supported voids imprinted in the cloud structure; anisotropic turbulent motions and column density striations aligned with the magnetic field lines, both of which have recently been observed in the Taurus molecular cloud. We also find strongly suppressed accretion in the magnetized runs, leading to up to a 75 per cent reduction in the amount of mass converted into stars over the course of the calculations and a more quiescent mode of star formation. There is also some indication that the relative formation efficiency of brown dwarfs is lower in the strongly magnetized runs due to a reduction in the importance of protostellar ejections.  相似文献   

High-energy X-ray spectra of Seyferts and Unification schemes for active galactic nuclei     

Matthew Middleton  Chris Done  Nick Schurch 《Monthly notices of the Royal Astronomical Society》2008,383(4):1501-1505

The Unified Model of active galactic nuclei (AGN) predicts that the sole difference between type 1 and 2 Seyfert galaxies nuclei is the viewing angle with respect to an obscuring structure around the nucleus. High-energy photons above 20 keV are not affected by this absorption if the column is Compton thin, so their 30–100 keV spectra should be the same. However, the observed spectra at high energies appear to show a systematic difference, with type 1 Seyfert galaxies having Γ∼ 2.1 whereas type 2 Seyfert galaxies are harder with Γ∼ 1.9. We estimate the mass and the accretion rate of Seyferts detected in these high-energy samples, and show that they span a wide range in   L / L _Edd  . Both black hole binary systems and AGN show a correlation between spectral softness and Eddington fraction, so these samples are probably heterogeneous, spanning a range of intrinsic spectral indices which are hidden in individual objects by poor signal-to-noise ratio. However, the mean Eddington fraction for the type 1 Seyfert galaxies is higher than for the type 2 Seyfert galaxies, so the samples are consistent with this being the origin of the softer spectra seen in type 1 Seyfert galaxies. We stress that high-energy spectra alone are not necessarily a clean test of Unification schemes, but that the intrinsic nuclear properties should also change with   L / L _Edd  .  相似文献   

Populating the Galaxy with pulsars – I. Stellar and binary evolution     

Paul D. Kiel  Jarrod R. Hurley  Matthew Bailes  James R. Murray 《Monthly notices of the Royal Astronomical Society》2008,388(1):393-415

The computation of theoretical pulsar populations has been a major component of pulsar studies since the 1970s. However, the majority of pulsar population synthesis has only regarded isolated pulsar evolution. Those that have examined pulsar evolution within binary systems tend to either treat binary evolution poorly or evolve the pulsar population in an ad hoc manner. Thus, no complete and direct comparison with observations of the pulsar population within the Galactic disc has been possible to date. Described here is the first component of what will be a complete synthetic pulsar population survey code. This component is used to evolve both isolated and binary pulsars. Synthetic observational surveys can then be performed on this population for a variety of radio telescopes. The final tool used for completing this work will be a code comprised of three components: stellar/binary evolution, Galactic kinematics and survey selection effects. Results provided here support the need for further (apparent) pulsar magnetic field decay during accretion, while they conversely suggest the need for a re-evaluation of the assumed typical millisecond pulsar formation process. Results also focus on reproducing the observed     diagram for Galactic pulsars and how this precludes short time-scales for standard pulsar exponential magnetic field decay. Finally, comparisons of bulk pulsar population characteristics are made to observations displaying the predictive power of this code, while we also show that under standard binary evolutionary assumption binary pulsars may accrete much mass.  相似文献   

Probing the neutral fraction of the IGM with GRBs during the epoch of reionization     

Matthew McQuinn  Adam Lidz  Matias Zaldarriaga  Lars Hernquist  Suvendra Dutta 《Monthly notices of the Royal Astronomical Society》2008,388(3):1101-1110

We show that near-infrared observations of the red side of the Lyα line from a single gamma-ray burst (GRB) afterglow cannot be used to constrain the global neutral fraction of the intergalactic medium (IGM),     , at the GRB's redshift to better than     . Some GRB sightlines will encounter more neutral hydrogen than others at fixed     owing to the patchiness of reionization. GRBs during the epoch of reionization will often bear no discernible signature of a neutral IGM in their afterglow spectra. We discuss the constraints on     from the   z = 6.3  burst, GRB050904, and quantify the probability of detecting a neutral IGM using future spectroscopic observations of high-redshift, near-infrared GRB afterglows. Assuming an observation with signal-to-noise ratio similar to the Subaru FOCAS spectrum of GRB050904 and that the column density distribution of damped Lyα absorbers is the same as measured at lower redshifts, a GRB from an epoch when     can be used to detect a partly neutral IGM at 97 per cent confidence level ≈10 per cent of the time (and, for an observation with three times the sensitivity, ≈30 per cent of the time).  相似文献   

Ecological and morphological response of brackish tidal marshland to the next century of sea level rise: Westham Island, British Columbia   总被引：2，自引：0，他引：2  

Matthew L. Kirwan  A. Brad Murray   《Global and Planetary Change》2008,60(3-4):471-486

In response to climatic warming, eustatic sea level has been predicted to rise by about 50 cm in the next century. While feedbacks between vegetation growth and sediment deposition tend to allow marshes to maintain their morphology under a constant rate of sea level rise, recent observations of marsh deterioration suggest that changes in the rate of sea level rise may induce loss of economically and ecologically important marshland. We have developed a three dimensional model of tidal marsh evolution that couples vegetation growth and sediment transport processes including bed accretion and wave erosion. We use the model to simulate the response of marshes and tidal flats along the Fraser River Delta, British Columbia to 100 yr forecasts of sea level change. Under low sea level-rise scenarios, the delta and its marshes prograde slightly, consistent with historical measurements. While accretionary processes greatly mediate the response to increased rates of sea level rise, vegetation zones transgress landward under median and high sea level rise rate scenarios. In these scenarios, low marsh erosion and constriction of high marsh vegetation against a dyke at its landward edge result in a 15–35% loss of marshland in the next century. Several important behavioral changes take place after 2050, suggesting that predictions based on field observations and short term model experiments may not adequately characterize (and sometimes underestimate) long-term change. In particular, the replacement of highly productive high marsh vegetation by less productive low marsh vegetation results in continued reduction of the system's total biomass productivity, even as the rate of loss of vegetated area begins to decline.  相似文献   

印度阶-奥伦尼克阶界线层型候选剖面--安徽巢湖平顶山西剖面地层序列   总被引：12，自引：3，他引：9  

童金南  Hans J.Hansen  赵来时  左景勋 《地层学杂志》2005,29(2):205-214

位于安徽省中部巢湖市郊的平顶山西剖面被推荐为印度阶-奥伦尼克阶界线的全球层型候选剖面。系统记述了该剖面的地层序列,包括岩石地层、生物地层、磁性地层、碳氧同位素地层和旋回地层等方面的最新研究成果。该剖面包括3个岩石地层单元、7个牙形石带、5个菊石带、3个双壳类带;在剖面下段地层中识别出2个正向磁性带和2个反向磁极性带;与地球轨道天文周期相对应的Milankovitch气候沉积旋回也可以明确地辨认;碳氧同位素δ1 3Ccarb演变呈现两次大的旋回。以牙形石N eospathodus waageni首现定义的印度阶-奥伦尼克阶界线在该剖面上位于三叠系第二个主要的正向极性带近顶部,也接近三叠纪碳同位素δ1 3Ccarb首次正向漂移峰值处  相似文献   

Comparative Study of Sand Porosity and a Technique for Determining Porosity of Undisturbed Marine Sediment     

Conrad W. Curry  Richard H. Bennett  Matthew H. Hulbert  Kenneth J. Curry  Richard W. Faas 《Marine Georesources & Geotechnology》2013,31(4):231-252

Porosity is a fundamental property of marine sediment from which wet bulk density can be easily determined and used in a variety of geoacoustic, geotechnical, and sedimentological studies, analyses, and models. However, methods of sampling marine sands suffer from the common problem of core disturbance making the in situ porosity difficult to obtain. Embedding the sediment within an epoxy resin matrix will minimize the disturbance to the microfabric and preserve the in situ sedimentary structure for subsequent study. Image analysis can then be used on thin sections to study the microfabric and porometry. A comprehensive review and analysis of published data on the porosity of predominantly clean sands has been completed and a simple, accurate, and nondestructive technique is described for preparing and measuring the porosity of marine sediment (siliciclastic sand) that has been infiltrated aboard ship immediately upon sample collection and chemically fixed and infiltrated with epoxy shortly thereafter. The average porosity of 36 samples of marine sand collected offshore Fort Walton Beach, Florida, and embedded with resin was determined to be 41.30%. From the review of published data the average porosity of sand was determined to be 37.7%, 42.3%, and 46.3% for packed, natural (in situ), and loose packing conditions, respectively, for a range of sorting coefficients and grain sizes.  相似文献   

Feeding habits of the ommastrephid squid Nototodarus sloanii on the Chatham Rise,New Zealand     

Matthew R. Dunn 《新西兰海洋与淡水研究杂志》2013,47(5):1103-1113

The diet of the squid Nototodarus sloanii was determined from examination of stomach contents of 388 specimens of 14.3–41.0 cm dorsal mantle length sampled at depths of 211–760 m on the Chatham Rise, New Zealand. Prey items were predominantly mesopelagic fishes, with some crustaceans and cephalopods. The most important prey species identified was Maurolicus australis, followed by Lampanyctodes hectoris, and unidentified squids (Teuthoidea). Multivariate analyses using distance‐based linear models, non‐parametric multi‐dimensional scaling, analysis of similarities, and similarity percentages, indicated crustaceans were more important in the diet of smaller squid (83–480 g), the fish component of the diet was dominated by L. hectoris on the east Chatham Rise and M. australis on the west Chatham Rise, and there may be a difference in diet with sex, with crustaceans and cephalopods more important for females. The results indicated that N. sloanii, similar to other ommastrephid squids, foraged primarily in the mesopelagic layers.  相似文献