14,673 research outputs found
The chemical evolution of gas-rich dwarf galaxies
A numerical double burst model of the chemical evolution of gas-rich dwarf
galaxies has been developed. The model is fitted to a sample of N/O, O/H, Y and
gas fraction observations, where N/O and O/H are the relative abundances by
number of nitrogen to oxygen and oxygen to hydrogen, respectively. Y is the
abundance by mass of helium. Closed models as well as models including enriched
outflow, ordinary outflow and ordinary outflow combined with inflow are
considered.The bursts are assumed to be instantaneous but ordered in pairs to
explain the scatter in N/O-O/H. The method of gas fraction fitting is revised,
and it is found that it is very important to specify whether dwarf irregulars
(dIrrs) or blue compact galaxies (BCGs) are considered. Effective enriched
winds fail when fitting N/O, whereas closed models, models with ordinary winds
or a combination of ordinary winds and inflow are all viable.Comment: 22 pages, 25 figures, MNRAS LaTeX forma
An LC inventory based on representative and coherent farm types
There is a need for valid and representative data regarding the production, resource use and emissions from typical farming systems in Denmark for analysis of the environmental impact of different systems and as input to product oriented analyses such as Life Cycle Assess-ments of basic food items. An inventory of 31 farm types was constructed on the basis of 2138 farm accounts from 1999 selected and weighted to be representative for the Danish farming sector. The farm accounts were grouped according to the major soil types, the num-ber of standard working hours, the most important enterprise (dairy, pig, different cash crops) and the stocking rate (livestock units per hectare). For each group the account data on the average inputs and outputs, land use and herd structure was used to establish a farm type model with coherency between livestock production, total feed use, land use, yields, im-ported feed, home-grown feed, manure production, fertiliser use and crop production. The set of farm types were scaled up to national level thus representing the whole Danish agricul-tural sector for the included products. The sum of area and yield by crop, number and pro-duction by livestock type and the use of fertiliser, energy and concentrated feed was checked against national level statistics and corrected accordingly across all farm types. Resource use and emissions in each farm type was established using standard nutrient concentrations and models for nutrient cycling, energy use and emissions of e.g. ammonia, nitrous oxides and methane. For LCA the product oriented inventory was established using system expan-sion rather than allocations to account for the secondary enterprises in the livestock farm types. Data are made available on a web-based database and may be used for analyses of the primary production systems or as input for LCA across the whole production chain
Space processing of chalcogenide glass
The manner in which the weightless, containerless nature of in-space processing can be successfully utilized to improve the quality of infrared transmitting chalcogenide glasses is determined. The technique of space processing chalcogenide glass was developed, and the process and equipment necessary to do so was defined. Earthbound processing experiments with As2S3 and G28Sb12Se60 glasses were experimented with. Incorporated into these experiments is the use of an acoustic levitation device
Geostrophic adjustment in a shallow-water numerical model as it relates to thermospheric dynamics
The theory of geostrophic adjustment and its application to the dynamics of the high latitude thermosphere have been discussed in previous papers based on a linearized treatment of the fluid dynamical equations. However, a linearized treatment is only valid for small Rossby numbers given by Ro = V/fL, where V is the wind speed, f is the local value of the Coriolis parameter, and L is a characteristic horizontal scale for the flow. For typical values in the auroral zone, the approximation is not reasonable for wind speeds greater than 25 m/s or so. A shallow-water (one layer) model was developed that includes the spherical geometry and full nonlinear dynamics in the momentum equations in order to isolate the effects of the nonlinearities on the adjustment process. A belt of accelerated winds between 60 deg and 70 deg latitude was used as the initial condition. The adjustment process was found to proceed as expected from the linear formulation, but that an asymmetry between the response for an eastward and westward flow results from the nonlineawr curvature (centrifugal) terms. In general, the amplitude of an eastward flowing wind will be less after adjustment than a westward wind. For instance, if the initial wind velocity is 300 m/s, the linearized theory predicts a final wind speed of 240 m/s, regardless of the flow direction. However, the nonlinear curvature terms modify the response and produce a final wind speed of only 200 m/s for an initial eastward wind and a final wind speed of almost 300 m/s for an initial westward flow direction. Also, less gravity wave energy is produced by the adjustment of the westward flow than by the adjustment of the eastward flow. The implications are that the response of the thermosphere should be significantly different on the dawn and dusk sides of the auroral oval. Larger flow velocities would be expected on the dusk side since the plasma will accelerate the flow in a westward direction in that sector
Entropy of Thermally Excited Black Rings
A string theory description of near extremal black rings is proposed. The
entropy is computed and the thermodynamic properties are derived for a large
family of black rings that have not yet been constructed in supergravity. It is
also argued that the most general black ring in N=8 supergravity has 21
parameters up to duality.Comment: 17 pages; v2: minor edits and refs adde
Ventilatory function in young adults and dietary antioxidant intake
ArtĂculo de publicaciĂłn ISIDietary antioxidants may protect against poor ventilatory function. We assessed the relation between ventilatory function and antioxidant components of diet in young Chileans. Forced expiratory volume in 1 s (FEV1), forced vital capacity (FVC), and the ratio FEV1/FVC were measured in 1232 adults aged 22-28 years, using a Vitalograph device. Dietary intake was ascertained with a food frequency questionnaire (FFQ) designed for this study, from which nutrient and flavonoid intakes were estimated. Dietary patterns were derived with Principal Component Analysis (PCA). After controlling for potential confounders, dietary intake of total catechins was positively associated with FVC (Regression coefficient (RC) of highest vs. lowest quintile of intake 0.07; 95% CI 0.01 to 0.15; p per trend 0.006). Total fruit intake was related to FVC (RC of highest vs. lowest quintile 0.08; 95% CI 0.003 to 0.15; p per trend 0.02). Intake of omega 3 fatty acids was associated with a higher FEV1 (RC for highest vs. lowest quintile 0.08; 95% CI 0.01 to 0.15 L; p per trend 0.02) and with FVC 0.08 (RC in highest vs. lowest quintile of intake 0.08, 95% CI 0.001 to 0.16; p per trend 0.04). Our results show that fresh fruits, flavonoids, and omega 3 fatty acids may contribute to maintain ventilatory function.Wellcome Trust 059448Z
Regulated Monopolies in Urban Public Transport Can we Design Proper Regulations and Incentives?
A model of the public transport company in Oslo is used to the design a system of price regulations and subsidies. The objective is to provide incentives for optimum provision of public transport services both for peak and off-peak demand. Optimum is defined in terms of fares, level of service and average capacity per revenue kilometre. The cost of public funds and the fact that car traffic is priced below marginal cost in peak periods are taken care of in the model. The regulator determines the fares, the transit operator receives a subsidy per revenue kilometre, differentiated between basic services and additional services operated only in peak periods. There is also a subsidy per passenger in peak periods due to underpriced car traffic. The results indicate that it should be possible to have regulated monopolies in local public transport that gives a proper supply of services based only on commercial incentives.Institute of Transport and Logistics Studies. Faculty of Economics and Business. The University of Sydne
Area products for stationary black hole horizons
Area products for multi-horizon stationary black holes often have intriguing
properties, and are often (though not always) independent of the mass of the
black hole itself (depending only on various charges, angular momenta, and
moduli). Such products are often formulated in terms of the areas of inner
(Cauchy) horizons and outer (event) horizons, and sometimes include the effects
of unphysical "virtual" horizons. But the conjectured mass-independence
sometimes fails. Specifically, for the Schwarzschild-de Sitter [Kottler] black
hole in (3+1) dimensions it is shown by explicit exact calculation that the
product of event horizon area and cosmological horizon area is not mass
independent. (Including the effect of the third "virtual" horizon does not
improve the situation.) Similarly, in the Reissner-Nordstrom-anti-de Sitter
black hole in (3+1) dimensions the product of inner (Cauchy) horizon area and
event horizon area is calculated (perturbatively), and is shown to be not mass
independent. That is, the mass-independence of the product of physical horizon
areas is not generic. In spherical symmetry, whenever the quasi-local mass m(r)
is a Laurent polynomial in aerial radius, r=sqrt{A/4\pi}, there are
significantly more complicated mass-independent quantities, the elementary
symmetric polynomials built up from the complete set of horizon radii (physical
and virtual). Sometimes it is possible to eliminate the unphysical virtual
horizons, constructing combinations of physical horizon areas that are mass
independent, but they tend to be considerably more complicated than the simple
products and related constructions currently being mooted in the literature.Comment: V1: 16 pages; V2: 9 pages (now formatted in PRD style). Minor change
in title. Extra introduction, background, discussion. Several additional
references; other references updated. Minor typos fixed. This version
accepted for publication in PRD; V3: Minor typos fixed. Published versio
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