γ-Rays from Her X-1     

Jeffrey M. Cohen  Errol Mustafa 《Astrophysics and Space Science》1986,128(2):355-361

Recent observations have detected bursts of extremely high energy pulsed (1.24 s)-rays (10¹² eV to 5×10¹⁴ eV) from Her X-1. We propose a model for producing these-rays.  相似文献   

A modern view of the dwarf nova Z Chamaeleontis     

R. A. Wade 《Astrophysics and Space Science》1987,131(1-2):507-509

Some recent photometric and spectroscopic results for this ultrashort period system are summarized, and several straightforward consequences of these results for our understanding of the evolution of cataclysmic binaries are pointed out: The space motion of Z Cha is characteristic of the old disk population. The white dwarf cannot be composed primarily of He, unless it grew by accretion by at least 20%. The inferred masses of the component stars, combined with the usual gravitational quadrupole formula, probably do not suffice to explain the inferred rate of mass transfer, even in quiescence. The secondary star does not lie on the computed evolutionary tracks in the period-mass diagram of Paczynski and Sienkiewicz.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.  相似文献   

A simple model of a supernova     

Richard C. Adams  Boyd B. Cary  Jeffrey M. Cohen 《Astrophysics and Space Science》1989,155(2):271-293

In this paper we formulate the problem of the collapse of a spherically-symmetric, radiating body in general relativity. The requirement that the metric and its normal derivative be continuous across the boundary imposes conditions upon the evolution of the star and allows identification of physical phenomena measured by a distant observer. A solution to Einstein's field equations for the exterior of a spherically-symmetric radiating body is that derived originally by Vaidya in 1951. By requiring the continuity described above we identify the mass, luminosity, velocity, and time increment measured by a distant observer in terms of the metric parameters evaluated in a frame comoving with the outer boundary. We also assume that the interior metric is a sum of products of functions of the radius and time. The continuity requirements allow the evolution of two of the three functions of time to be determined. The evolution of the third function, describing the motion of the core, is determined by the imposition of an equation of state at the center. The adiabatic index derived from the Baym-Bethe-Pethick equation of state was used to provide this last equation. A major result is obtaining an analytic solution to Einstein's field equations describing the core of a collapsing star. As a consequence of this solution we found that for the relatively small values of the adiabatic index (_max1.6), the star smoothly made the transition to a final collapsed state. Neither bounce nor shock wave was obtained. Also, there is a readily understood connection between the adiabatic index, and such parameters of the edge of the core as the velocity and acceleration. Finally, the analytic solutions provide the time-scales for the collapse which are significantly different from that of free-fall. The retarding effects of pressure upon the collapse are apparent. It is hoped that such analytic solutions will provide insight into more complicated dynamic systems in general relativity.  相似文献   

Comparison of Measured and Numerically Simulated Turbulence Statistics in a Convective Boundary Layer Over Complex Terrain     

Raj K. Rai  Larry K. Berg  Branko Kosović  Jeffrey D. Mirocha  Mikhail S. Pekour  William J. Shaw 《Boundary-Layer Meteorology》2017,163(1):69-89

The Weather Research and Forecasting (WRF) model can be used to simulate atmospheric processes ranging from quasi-global to tens of m in scale. Here we employ large-eddy simulation (LES) using the WRF model, with the LES-domain nested within a mesoscale WRF model domain with grid spacing decreasing from 12.15 km (mesoscale) to 0.03 km (LES). We simulate real-world conditions in the convective planetary boundary layer over an area of complex terrain. The WRF-LES model results are evaluated against observations collected during the US Department of Energy-supported Columbia Basin Wind Energy Study. Comparison of the first- and second-order moments, turbulence spectrum, and probability density function of wind speed shows good agreement between the simulations and observations. One key result is to demonstrate that a systematic methodology needs to be applied to select the grid spacing and refinement ratio used between domains, to avoid having a grid resolution that falls in the grey zone and to minimize artefacts in the WRF-LES model solutions. Furthermore, the WRF-LES model variables show large variability in space and time caused by the complex topography in the LES domain. Analyses of WRF-LES model results show that the flow structures, such as roll vortices and convective cells, vary depending on both the location and time of day as well as the distance from the inflow boundaries.  相似文献   

Mechanisms Responsible for the Observed Thermodynamic Structure in a Convective Boundary Layer Over the Hudson Valley of New York State     

Jeffrey M. Freedman  David R. Fitzjarrald 《Boundary-Layer Meteorology》2017,164(1):89-106

We examine cases of a regional elevated mixed layer (EML) observed during the Hudson Valley Ambient Meteorology Study (HVAMS) conducted in New York State, USA in 2003. Previously observed EMLs referred to topographic domains on scales of 10\(^{5}\)–10\(^{6}\) km\(^{2}\). Here, we present observational evidence of the mechanisms responsible for the development and maintenance of regional EMLs overlying a valley-based convective boundary layer (CBL) on much smaller spatial scales (<5000 km\(^{2})\). Using observations from aircraft-based, balloon-based, and surface-based platforms deployed during the HVAMS, we show that cross-valley horizontal advection, along-valley channelling, and fog-induced cold-air pooling are responsible for the formation and maintenance of the EML and valley-CBL coupling over New York State’s Hudson Valley. The upper layer stability of the overlying EML constrains growth of the valley CBL, and this has important implications for air dispersion, aviation interests, and fog forecasting.  相似文献   

Resource taxation and sustainability: A CGE model of the Czech Republic     

Wade E. Martin  R. Kenneth Skinner 《Natural Resources Research》1998,7(4):287-300

Resource use by a country is considered in the context of a production relationship. Resources include natural, produced, and human capital. The taxation of each of these resource groups has an impact on the efficient use of these inputs through changes in the relative prices. A computable general equilibrium model (CGE) of the Czech Republic is used to evaluate the impact that various revenue neutral tax structures have on the allocation of economic activity throughout the economy. A Hicksian welfare measure is used to determine the impact on society’s welfare of revenue neutral shifts in taxes. The results demonstrate that the change in the tax structure will result in a welfare improvement by as much as 5% for the Czech Republic. The results provide insight into the role that “getting the prices right” has on sustainability. Although the term “sustainable development” was first introduced in a report by the International Union for the Conservation of Nature and Natural Resources in 1980 it did not gain significant notoriety until the Brundtland Commission report in 1987. For a more detailed discussion see Jamieson, 1998.  相似文献   

Impacts of Land Degradation on Historical Temperature Records from the Sonoran Desert   总被引：1，自引：0，他引：1  

Robert C. Balling Jr.  Jeffrey M. Klopatek  Mark L. Hildebrandt  Cherie K. Moritz  Christopher J. Watts 《Climatic change》1998,40(3-4):669-681

Previous research revealed that severe overgrazing and resultant land degradation in the semiarid areas of northern Mexico created significantly higher temperatures in the border area. In this investigation, the temperature and precipitation records from ten homogeneous stations are identified in the arid and hyperarid areas of northwest Sonora and are compared with the records from ten stations in southwestern Arizona. Our data show that the Mexican stations are again consistently warmer than the Arizona stations when statistical controls are applied to correct for the linear or non-linear effects of latitude and/or elevation. The stations in Sonora warm at a statistically significantly faster pace than the stations in Arizona during the study period. Furthermore, and consistent with other dryland areas undergoing land degradation, the stations in Sonora reveal a significant increase in the diurnal temperature range during the summer season. Local precipitation reduces the temperature differential between nations on the time scale of days, but enhances the differential on the time scale of months and seasons. Among other findings, the results show how land degradation in dryland areas appears to influence local historical temperature records.  相似文献   

Heat Flux in the Coastal Zone   总被引：5，自引：4，他引：1  

L. Mahrt  Dean Vickers  Jim Edson  Jielun Sun  Jørgen Højstrup  Jeffrey Hare  James M. Wilczak 《Boundary-Layer Meteorology》1998,86(3):421-446

Various difficulties with application of Monin–Obukhov similarity theory are surveyed including the influence of growing waves, advection and internal boundary-layer development. These complications are normally important with offshore flow. The transfer coefficient for heat is computed from eddy correlation data taken at a mast two kilometres off the Danish coast in RASEX. For these coastal zone data, the thermal roughness length shows no well-defined relation to the momentum roughness length or roughness Reynolds number, in contrast to previous theories. The variation of the momentum roughness length is dominated by wave state. In contrast, the thermal roughness length shows significant dependence on wave state only for small values of wave age where the mixing is apparently enhanced by wave breaking. The development of thin internal boundary layers with offshore flow substantially reduces the heat transfer and thermal roughness length but has no obvious influence on momentum roughness length. A new formulation of the thermal roughness length based on the internal boundary-layer depth is calibrated to the RASEX data. For the very stable case, the turbulence is mainly detached from the surface and existing formulations do not apply.As an alternative to adjusting the thermal roughness length, the transfer coefficient is related directly to the stability and the internal boundary-layer depth. This avoids specification of roughness lengths resulting from the usual integration of the non-dimensional temperature function. The resulting stability function is simpler than previous ones and satisfies free convection similarity theory without introduction of the gustiness factor. The internal boundary layer also influences the moisture transfer coefficient.  相似文献   

Nitrogen Stable Isotopes of Macrophytes Assess Stormwater Nitrogen Inputs to an Urbanized Estuary   总被引：1，自引：0，他引：1  

Kevin S. Dillon  Jeffrey P. Chanton 《Estuaries and Coasts》2008,31(2):360-370

The ¹⁵N composition of seagrass and benthic macroalgae from shallow waters of Sarasota Bay was measured to determine if stable N isotopes can be used to trace stormwater N into macrophyte production within an urbanized estuary. Results show isotopically enriched macroalgae at the landward stations near creeks and bayous in the central Bay and in the southern portion of the Bay. A known sewage outfall at Whitaker Bayou resulted in δ ¹⁵NO₃ values from 0 to +9‰. Isotopically enriched NH₄ values in Phillippi Creek (+10 to +17‰) were similar to the stormwater ¹⁵NH₄ values from the watershed (+7 to +18‰). Enriched N sources supported a significant portion of macroalgae N demands in the southern reaches of the Bay while isotopically depleted N sources (i.e., atmospheric deposition and/or fertilizers) appear to be more important for macroalgae in the northern portion of the Bay. Macroalgae were typically more enriched than seagrass and appear to be better indicators of anthropogenic loadings near creeks and bayous that receive large volumes of stormwater and other anthropogenic N sources. Historically, studies have used enriched ¹⁵N in macrophytes to infer wastewater influences. This study shows that stormwater N inputs need to be considered in nitrogen budgets for aquatic systems that show anthropogenic ¹⁵N enrichment.  相似文献   

Lake basin response to tectonic drainage diversion: Eocene Green River Formation, Wyoming   总被引：1，自引：0，他引：1  

Jeffrey T. Pietras  Alan R. Carroll  Meredith K. Rhodes 《Journal of Paleolimnology》2003,30(2):115-125

A previously unidentified major sequence boundary within the Eocene Green River Formation separates fluctuating profundal facies of the Tipton Shale Member from evaporative facies of the Wilkins Peak Member. During deposition of the Tipton Shale Member, rivers entered the basin from the north, across the subdued Wind River Mountains, and deposited the southward prograding deltaic complex of the Farson Sandstone Member. Boulder-rich alluvial fan deposits overlie the Farson Sandstone adjacent to the Continental Fault, and correlate basinward to hypersaline lacustrine deposits of the Wilkins Peak Member. The alluvial fan deposits record a period of reverse motion on the Continental Fault and uplift of the southeastern Wind River Range, which diverted drainage away from the greater Green River Basin. This decreased inflow caused Lake Gosiute to shrink, exposing its bed to desiccation and erosion, and contributed to hydrologically-closed conditions and periodic evaporite deposition thereafter. This study is one of the first to demonstrate a direct relationship between movement along a specific basin-bounding structure, and a change in the overall style of lacustrine sedimentation. The identification of similar relationships elsewhere may challenge conventional interpretations of climate as the dominant factor influencing the character of lake deposits, and provide an important, but previously unexploited, approach to interpreting continental deformation and regional drainage organization.  相似文献