6,474 research outputs found
Numerical Simulations of Radiatively-Driven Dusty Winds
[abridged] Radiation pressure on dust grains may be an important mechanism in
driving winds in a wide variety of astrophysical systems. However, the
efficiency of the coupling between the radiation field and the dusty gas is
poorly understood in environments characterized by high optical depths. We
present a series of idealized numerical experiments, performed with the
radiation-hydrodynamic code ORION, in which we study the dynamics of such winds
and quantify their properties. We find that, after wind acceleration begins,
radiation Rayleigh-Taylor instability forces the gas into a configuration that
reduces the rate of momentum transfer from the radiation field to the gas by a
factor ~ 10 - 100 compared to an estimate based on the optical depth at the
base of the atmosphere; instead, the rate of momentum transfer from a driving
radiation field of luminosity L to the gas is roughly L/c multiplied by one
plus half the optical depth evaluated using the photospheric temperature, which
is far smaller than the optical depth one would obtain using the interior
temperature. When we apply our results to conditions appropriate to ULIRGs and
star clusters, we find that the asymptotic wind momentum flux from such objects
should not significantly exceed that carried by the direct radiation field,
L/c. This result constrains the expected mass loss rates from systems that
exceed the Eddington limit to be of order the so-called "single-scattering"
limit, and not significantly higher. We present an approximate fitting formula
for the rate of momentum transfer from radiation to dusty gas through which it
passes, which is suitable for implementation in sub-grid models of galaxy
formation. Finally, we provide a first map of the column density distribution
of gas in a radiatively-driven wind as a function of velocity, and velocity
dispersion.Comment: 19 pages, 17 figures, MNRAS in press; some additional discussion
compared to previous version, no changes in conclusion
The new paradigm in Europe: is Goldilocks going global?
European Union ; Labor market ; Technology ; Productivity
Investigations of stress appraisals and emotions, and their impact upon coping resources, performance, and neuroendocrine response among athletes
Both the Cognitive-Motivational-Relational (CMR) Theory of Emotions and the Broaden-and-Build (BaB) Theory of Emotions are seminal in the domain of psychology. Despite the widespread relevance of their combined core ingredients (that is, stress appraisals, emotions, and coping) to the field of sport psychology, there remain comprehensive gaps in their application to sporting populations, particularly in regards to psychological, neuroendocrine, and athlete performance response. Therefore, the contribution of this thesis was to investigate stress appraisals and emotions in relation to their impact upon psychophysiology and athletic performance through use of five interrelated studies. The CMR theory was examined through cross-sectional and laboratory research, whilst the BaB theory was tested via cross-sectional, longitudinal, and laboratory explorations. Regarding CMR theory, stress appraisals were found to be aligned with both subjective and objective measures of performance, as well as possessing a causal psychophysiological impact within athletes. The temporal orientation of stress appraisals may elicit distinct psychological, neuroendocrine, and performance profiles. Further, in relation to BaB theory, pleasant emotions may have short- and long-term performance and psychological benefits, including âbroadeningâ oneâs attention to facilitative coping strategies, âbuildingâ enduring coping resources, and âundoingâ psychological costs incurred from previous unpleasant emotional experiences. Cross-study evidence for the existence of physiological âsuccess stressâ was also discovered. Loss stress appraisals and unpleasant emotions may elicit a performance benefit in some athletes but are also linked with psychophysiological stress. Implications for athletes and their stakeholders, as well as future research recommendations are offered. This thesis represents the first causal examinations of both past-oriented stress appraisals and BaB theory within athletes and can be viewed as a novel contribution to the extant sport psychological literature
Market Forces and Price Ceilings: A Classroom Experiment
The effect of price controls on competitive equilibrium is a standard topic in many undergraduate economics courses. This classroom experiment demonstrates the effect of rent control (price ceilings) on the market for apartments. As participants in the experiment, students experience the effect of a price ceiling as buyers (renters) and sellers (landlords). The classroom-posted offer market exhibits a shortage under a binding price ceiling. Further, we explore a secondary response to rent control. When given the opportunity, landlords lower the quality of the apartments by reducing maintenance expenditures under the price ceiling, thus moving the market back to equilibrium. Since many students are themselves renters, they should relate to changes in quality due to lower maintenance by landlords. This experiment will stimulate discussion on market forces and on public policy aimed at restricting prices.
"Precious Metals-Exchange Rate Volatility Transmissions and Hedging Strategies"
This study examines the conditional volatility and correlation dependency and interdependency for the four major precious metals (that is, gold, silver, platinum and palladium), while accounting for geopolitics within a multivariate system. The implications of the estimated results for portfolio designs and hedging strategies are also analyzed. The results for the four metals system show significant short-run and long-run dependencies and interdependencies to news and past volatility. These results have become more pervasive when the exchange rate and FFR are included. Monetary policy also has a differential impact on the precious metals and the exchange rate volatilities. Finally, the applications of the results show the optimal weights in a two-asset portfolio and the hedging ratios for long positions.
The Maximum Flux of Star-Forming Galaxies
The importance of radiation pressure feedback in galaxy formation has been
extensively debated over the last decade. The regime of greatest uncertainty is
in the most actively star-forming galaxies, where large dust columns can
potentially produce a dust-reprocessed infrared radiation field with enough
pressure to drive turbulence or eject material. Here we derive the conditions
under which a self-gravitating, mixed gas-star disc can remain hydrostatic
despite trapped radiation pressure. Consistently taking into account the
self-gravity of the medium, the star- and dust-to-gas ratios, and the effects
of turbulent motions not driven by radiation, we show that galaxies can achieve
a maximum Eddington-limited star formation rate per unit area
pc Myr,
corresponding to a critical flux of
kpc similar to previous estimates; higher fluxes eject mass in bulk,
halting further star formation. Conversely, we show that in galaxies below this
limit, our one-dimensional models imply simple vertical hydrostatic equilibrium
and that radiation pressure is ineffective at driving turbulence or ejecting
matter. Because the vast majority of star-forming galaxies lie below the
maximum limit for typical dust-to-gas ratios, we conclude that infrared
radiation pressure is likely unimportant for all but the most extreme systems
on galaxy-wide scales. Thus, while radiation pressure does not explain the
Kennicutt-Schmidt relation, it does impose an upper truncation on it. Our
predicted truncation is in good agreement with the highest observed gas and
star formation rate surface densities found both locally and at high redshift.Comment: Version accepted for publication in MNRAS. 12 pages, 8 figures. New
appendix on photon tirin
Measuring the Regional Economic Response to Hurricane Katrina
Naturkatastrophe; Sturm; Makroökonomischer Einfluss; USA
Precious Metals-Exchange Rate Volatility Transmissions and Hedging Strategies
This study examines the conditional volatility and correlation dependency and interdependency for the four major precious metals (that is, gold, silver, platinum and palladium), while accounting for geopolitics within a multivariate system. The implications of the estimated results for portfolio designs and hedging strategies are also analyzed. The results for the four metals system show significant short-run and long-run dependencies and interdependencies to news and past volatility. These results have become more pervasive when the exchange rate and FFR are included. Monetary policy also has a differential impact on the precious metals and the exchange rate volatilities. Finally, the applications of the results show the optimal weights in a two-asset portfolio and the hedging ratios for long positions.
- âŠ