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Critical Density (critical + density)
Selected AbstractsMapping low- and high-density clouds in astrophysical nebulae by imaging forbidden line emission,MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009J. E. Steiner ABSTRACT Emission line ratios have been essential for determining physical parameters such as gas temperature and density in astrophysical gaseous nebulae. With the advent of panoramic spectroscopic devices, images of regions with emission lines related to these physical parameters can, in principle, also be produced. We show that, with observations from modern instruments, it is possible to transform images taken from density-sensitive forbidden lines into images of emission from high- and low-density clouds by applying a transformation matrix. In order to achieve this, images of the pairs of density-sensitive lines as well as the adjacent continuum have to be observed and combined. We have computed the critical densities for a series of pairs of lines in the infrared, optical, ultraviolet and X-rays bands, and calculated the pair line intensity ratios in the high- and low-density limit using a four- and five-level atom approximation. In order to illustrate the method, we applied it to Gemini Multi-Object Spectrograph (GMOS) Integral Field Unit (GMOS-IFU) data of two galactic nuclei. We conclude that this method provides new information of astrophysical interest, especially for mapping low- and high-density clouds; for this reason, we call it ,the ld/hd imaging method'. [source] Using the Steepened Plasma Profile and Wave Breaking Threshold in Laser-Plasma InteractionCONTRIBUTIONS TO PLASMA PHYSICS, Issue 8 2008P. Zobdeh Abstract In this work we evaluate the interaction of high intense laser beam with a steepened density profile. During laser interaction with underdense plasma by freely expanding plasma regime, modification of density profile is possible. In this paper we have investigated the ultra short laser pulse interaction with nonisothermal and collisionless plasma. We consider self,focusing as an effective nonlinear phenomenon that tends to increase when the laser power is more than critical rate. By leading the expanded plasma to a preferred location near to critical density, laser reflection is obtained, so the density profile will be locally steepened. The electromagnetic fields are evaluated in this new profile. We show the amplitude and period of electrical field oscillation are increased by reducing the steepened scale length. Also our numerical results identify that by reducing the steepened scale length, the electrical field is increased to wave breaking threshold limit. This high gradient electrical field causes the effective beam loading during the wave breaking phenomenon. The wave breaking can be the initial point for other acceleration regime as cavity or channel guiding regime. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] A new approach to prioritizing marine fish and shellfish populations for conservationFISH AND FISHERIES, Issue 4 2001Einar Eg Nielsen Abstract There has been increasing awareness of the vulnerability of marine organisms to population extirpation and species extinction. While very few documented cases of species extinction exist in the marine environment, it is anticipated that managers will face the dilemma of prioritizing populations of marine fish and shellfish for protection in the near future. Current prioritization procedures have been developed from salmonid models with the intent of applying them to all marine organisms, and in some cases to freshwater and terrestrial taxa. In this review we provide evidence for the relevance of such a process for marine species and further suggest five broad categories of marine organisms that have distinctive traits influencing their genetic structure. The current prioritization models have been adapted to account for each of these species groups. Emphasis is placed on ,Classical Marine Species' which represent the opposite end of the continuum from the salmon model, displaying high within-population genetic variance. From this category, three cod (Gadus morhua) stocks were selected to evaluate a revised scheme developed specifically for ,Classical Marine Species' that includes performance measures such as (i) reduction in number of spawning populations; (ii) reduction of Ne : Nc (ratio of effective to census population size); (iii) changes in life-history traits; (iv) critical density for spawning success; and (v) patchy vs. continuous distribution pattern. When the salmonid scheme was applied, the cod examples were allocated low values, indicating that they were not under threat. However, when the revised scheme was applied, all three cod stocks were allocated high values, indicating that the revised scheme was more reflective of the particular life-history traits of this category of organisms. [source] A macroscopic traffic model for highway work zones: Formulations and numerical resultsJOURNAL OF ADVANCED TRANSPORTATION, Issue 3 2006Gang-len Chang This study presents a multilane model for analyzing the dynamic traffic properties of a highway segment under a lane-closure operation that often incurs complex interactions between mandatory lane-changing vehicles and traffic at unblocked lanes. The proposed traffic flow formulations employ the hyperbolic model used in the non-Newtonian fluid dynamics, and assume the lane-changing intensity between neighboring lanes as a function of their difference in density. The results of extensive simulation experiments indicate that the proposed model is capable of realistically replicating the impacts of lane-changing maneuvers from the blocked lanes on the overall traffic conditions, including the interrelations between the approaching flow density, the resulting congestion level, and the exiting flow rate from the lane-closure zone. Our extensive experimental analyses also confirm that traffic conditions will deteriorate dramatically and evolve to the state of traffic jam if the density has exceeded its critical level that varies with the type of lane-closure operations. This study also provides a convenient way for computing such a critical density under various lane-closure conditions, and offers a theoretical basis for understanding the formation as well as dissipation of traffic jam. [source] Experimental demonstration of population extinction due to a predator-driven Allee effectJOURNAL OF ANIMAL ECOLOGY, Issue 3 2010Andrew M. Kramer Summary 1. Allee effects may result in negative growth rates at low population density, with important implications for conservation and management of exploited populations. Theory predicts prey populations will exhibit Allee effects when their predator exhibits a Type II functional response, but empirical evidence linking this positively density-dependent variation in predator-induced individual mortality to population growth rate and probability of extinction is lacking. 2. Here, we report a demonstration of extinction due to predator-driven Allee effects in an experimental Daphnia-Chaoborus system. A component Allee effect caused by higher predation rates at low Daphnia density led to positive density dependence in per capita growth rate and accelerated extinction rate at low density. 3. A stochastic model of the process revealed how the critical density below which population growth is negative depends on the mechanistic details of the predator,prey interaction. 4. The ubiquity of predator,prey interactions and saturating functional responses suggests predator-driven Allee effects are potentially important in determining extinction risk of a large number of species. [source] Centrally condensed turbulent cores: massive stars or fragmentation?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2005Clare L. Dobbs ABSTRACT We present numerical investigations into the formation of massive stars from turbulent cores of density structure ,,r,1.5. The results of five hydrodynamical simulations are described, following the collapse of the core, fragmentation and the formation of small clusters of protostars. We generate two different initial turbulent velocity fields corresponding to power-law spectra P,k,4 and P,k,3.5, and we apply two different initial core radii. Calculations are included for both completely isothermal collapse, and a non-isothermal equation of state above a critical density (10,14 g cm,3). Our calculations reveal the preference of fragmentation over monolithic star formation in turbulent cores. Fragmentation was prevalent in all the isothermal cases. Although disc fragmentation was largely suppressed in the non-isothermal runs due to the small dynamic range between the initial density and the critical density, our results show that some fragmentation still persisted. This is inconsistent with previous suggestions that turbulent cores result in the formation of a single massive star. We conclude that turbulence cannot be measured as an isotropic pressure term. [source] The mass function of the stellar component of galaxies in the Sloan Digital Sky SurveyMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2004Benjamin Panter ABSTRACT Using the moped algorithm, we determine non-parametrically the stellar mass function of 96 545 galaxies from the Sloan Digital Sky Survey Data Release One. By using the reconstructed spectrum due to starlight, we can eliminate contamination from either emission lines or active galactic nuclei components. Our results give excellent agreement with previous works, but extend their range by more than two decades in mass to 107.5,Ms/h,2 M,, 1012. We present both a standard Schechter fit and a fit modified to include an extra, high-mass contribution, possibly from cluster central dominant galaxies. The Schechter fit parameters are ,,= (7.8 ± 0.1) × 10,3 h3 Mpc,3, M,= (7.64 ± 0.09) × 1010 h,2 M, and ,=,1.159 ± 0.008. Our sample also yields an estimate for the contribution from baryons in stars to the critical density of ,b*h= (2.39 ± 0.08) × 10,3, in good agreement with other indicators. Error bars are statistical and a Salpeter initial mass function is assumed throughout. We find no evolution of the mass function in the redshift range 0.05 < z < 0.34, indicating that almost all stars were already formed at z, 0.34 with little or no star formation activity since then and that the evolution seen in the luminosity function must be largely due to stellar fading. [source] | ||||||||||