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Ionization Fraction (ionization + fraction)

Distribution by Scientific Domains
Physics and Astronomy75%

Selected Abstracts

A MATLAB toolbox for solving acid-base chemistry problems in environmental engineering applications

COMPUTER APPLICATIONS IN ENGINEERING EDUCATION, Issue 4 2005
Chetan T. Goudar
Abstract A MATLAB toolbox incorporating several computer programs has been developed in an attempt to automate laborious calculations in acid-base chemistry. Such calculations are routinely used in several environmental engineering applications including the design of wastewater treatment systems and for predicting contaminant fate and transport in the subsurface. The computer programs presented in this study do not replace student thinking involved in formulating the problem solving strategy but are merely tools that simplify the actual problem solving process. They encompass a wide variety of acid-base chemistry topics including equilibrium constant calculations, construction of distribution diagrams for mono and multiprotic systems, ionic strength and activity coefficient calculations, and buffer index calculations. All programs are characterized by an intuitive graphical user interface where the user supplies input information. Program outputs are either numerical or graphical depending upon the nature of the problem. The application of this approach to solving actual acid-base chemistry problems is illustrated by computing the pH and equilibrium composition of a 0.1 M Na2CO3 system at 30°C using several programs in the toolbox. As these programs simplify lengthy computations such as ionization fraction and activity coefficient calculations, it is hoped they will help bring more complicated problems to the environmental engineering classroom and enhance student understanding of important concepts that are applicable to real-world systems. The programs are available free of charge for academic use from the authors. © 2005 Wiley Periodicals, Inc. Comput Appl Eng Educ 13: 257,265, 2005; Published online in Wiley InterScience (www.interscience.wiley.com); DOI 10.1002/cae.20051 [source]

Cosmological simulations of intergalactic medium enrichment from galactic outflows

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
Benjamin D. Oppenheimer
ABSTRACT We investigate models of self-consistent chemical enrichment of the intergalactic medium (IGM) from z= 6.0 , 1.5, based on hydrodynamic simulations of structure formation that explicitly incorporate outflows from star-forming galaxies. Our main result is that outflow parametrizations derived from observations of local starburst galaxies, in particular momentum-driven wind scenarios, provide the best agreement with observations of C iv absorption at z, 2,5. Such models sufficiently enrich the high- z IGM to produce a global mass density of C iv absorbers that is relatively invariant from z= 5.5 , 1.5, in agreement with observations. This occurs despite continual IGM enrichment causing an increase in volume-averaged metallicity by ,× 5,10 over this redshift range, because energy input accompanying the enriching outflows causes a drop in the global ionization fraction of C iv. Comparisons to observed C iv column density and linewidth distributions and C iv -based pixel optical depth ratios provide significant constraints on wind models. Our best-fitting outflow models show mean IGM temperatures only slightly above our no-outflow case, metal filling factors of just a few per cent with volume-weighted metallicities around 10,3 at z, 3, significant amounts of collisionally ionized C iv absorption and a metallicity,density relationship that rises rapidly at low overdensities and flattens at higher ones. In general, we find that outflow speeds must be high enough to enrich the low-density IGM at early times but low enough not to overheat it, and concurrently must significantly suppress early star formation while still producing enough early metals. It is therefore non-trivial that locally calibrated momentum-driven wind scenarios naturally yield the desired strength and evolution of outflows, and suggest that such models represent a significant step towards understanding the impact of galactic outflows on galaxies and the IGM across cosmic time. [source]

A high-resolution radio survey of Class I protostars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2000
P. W. Lucas
We report the results of a survey of low-mass Class I protostars in the cm continuum. In the initial survey, seven sources in the Taurus star formation region were observed with the VLA at 0.25-arcsec resolution. All seven sources drive CO outflows and display Herbig,Haro flows in the optical or near-infrared (NIR) wavebands. Four out of seven sources were detected, two of which are new discoveries in systems of very low luminosity, one being the lowest luminosity system detected to date in the cm continuum. Notably, three sources were not detected to a 3, limit of 0.10 mJy/beam, which indicates that significant cm continuum emission is not a universal feature of Class I systems with outflow activity. Subsequent observations of HH30, a more evolved Class II system, found no emission to a 3, limit of 0.03 mJy/beam. After comparison with near infrared data, we suggest that the discriminating feature of the detected systems is a relatively high ionization fraction in the stellar wind. Temporal variability of the outflow may also play a role: only recently ejected knots may have sufficiently dense plasma to be optically thick to free,free emission, and hence produce detectable flux. The one relatively bright source, IRAS 04016+2610 (L1489 IRS), is clearly resolved on a 0.4-arcsec scale at 2 and 3.5 cm. Additional imaging with MERLIN did not detect this source with a 0.04-arcsec beam, indicating that the radio emission is generated in a region with a radius of ,25 au, which is broadly similar to the radius of the bipolar cavities inferred from models of NIR data. Interpretation of this system is complicated by the existence of a quadrupolar outflow, i.e. two bipolar outflows along roughly perpendicular axes, which we originally detected through polarimetric imaging. We present an NIR H2 image in which a bow shock in the secondary outflow is clearly seen. This complicated structure may have been caused by a gravitational interaction between two protostars. [source]

The energy cascade from warm dark matter decays

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008
M. Valdés
ABSTRACT We use a set of Monte Carlo simulations to follow the cascade produced by a primary electron of energy Ein in the intergalactic medium. We choose Ein= 3,10 keV as expected from the decay of one of the most popular warm dark matter (WDM) candidates, sterile neutrinos. Our simulation takes into account processes previously neglected such as free,free interactions with ions and recombinations, and uses the best available cross-sections for collisional ionizations and excitations with H and He and for electron,electron collisions. We precisely derive the fraction of the primary electron energy that heats the gas, ionizes atoms and produces line and continuum photons as a function of the ionization fraction. Handy fitting formulae for all the above energy depositions are provided. By keeping track of the individual photons, we can distinguish between photons in the Ly, resonance and those with energy E < 10.2 eV that do not interact further with gas. This separation is important because a Ly, background can heat or cool the gas depending on the nature of the photons, and can have effects on the 21-cm radiation emitted by neutral H, which will probably become detectable at z > 6 in the near future by the next generation radio interferometers. [source]