429 research outputs found
Radiative transfer with scattering for domain-decomposed 3D MHD simulations of cool stellar atmospheres
We present the implementation of a radiative transfer solver with coherent
scattering in the new BIFROST code for radiative magneto-hydrodynamical (MHD)
simulations of stellar surface convection. The code is fully parallelized using
MPI domain decomposition, which allows for large grid sizes and improved
resolution of hydrodynamical structures. We apply the code to simulate the
surface granulation in a solar-type star, ignoring magnetic fields, and
investigate the importance of coherent scattering for the atmospheric
structure. A scattering term is added to the radiative transfer equation,
requiring an iterative computation of the radiation field. We use a
short-characteristics-based Gauss-Seidel acceleration scheme to compute
radiative flux divergences for the energy equation. The effects of coherent
scattering are tested by comparing the temperature stratification of three 3D
time-dependent hydrodynamical atmosphere models of a solar-type star: without
scattering, with continuum scattering only, and with both continuum and line
scattering. We show that continuum scattering does not have a significant
impact on the photospheric temperature structure for a star like the Sun.
Including scattering in line-blanketing, however, leads to a decrease of
temperatures by about 350\,K below log tau < -4. The effect is opposite to that
of 1D hydrostatic models in radiative equilibrium, where scattering reduces the
cooling effect of strong LTE lines in the higher layers of the photosphere.
Coherent line scattering also changes the temperature distribution in the high
atmosphere, where we observe stronger fluctuations compared to a treatment of
lines as true absorbers.Comment: A&A, in pres
Role of interface charges on high-k based poly-Si and aetal gate nano-scale MOSFETs
The characteristics of typical sub-100 nm high K gate dielectrics MOSFET with different gate materials are simulated by two dimensional device simulators (ATLAS and ATHENA). The impact of interface charges on the characteristics of Poly-Si and TiN metal gate MOSFETs are investigated. The simulation results shows that, at high interface charge densities, the devices with Poly-Si gate degrade much compared to metal gate MOSFET structures. Emphasis is given to study the mobility degradation which stands as a major hurdle with the implementation of high-k dielectrics in nano-scale devices. The advantages of using Watt model over other models for the extraction of channel mobility is also clearly explained. The performance of the high-k MOSFET with metal electrode and poly-silicon electrode is also compared for various interface state charges.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/2788
An Empirical Ultraviolet Template for Iron Emission in Quasars as Derived from I Zw 1
We present an empirical template spectrum suitable for fitting/subtracting
and studying the FeII and FeIII line emission in the restframe UV spectra of
active galatic nuclei (AGNs), the first empirical UV iron template to cover the
full 1250 - 3090 A range. Iron emission is often a severe contaminant in
optical--UV spectra of AGNs. Its presence complicates and limits the accuracy
of measurements of both strong and weak emission lines and the continuum
emission, affecting studies of line and continuum interrelations, the
ionization structure, and elemental abundances in AGNs. Despite the wealth of
work on modeling the AGN FeII emission and the need to account for it in
observed AGN spectra, there is no UV template electronically available to aid
this process. The iron template we present is based on HST spectra of the
Narrow Line Seyfert 1, IZw1. Its intrinsic narrow lines (~900 km/s) and rich
iron spectrum make the template particularly suitable for use with most AGN
spectra. The iron emission spectrum and the line identifications and
measurements are presented and compared with the work of Laor et al. We
illustrate the application of the derived FeII and FeIII templates by fitting
and subtracting the iron emission from UV spectra of four high-z quasars and of
the nearby quasar, 3C273. We briefly discuss the small discrepancies between
this observed iron emission and the UV template, and compare the template with
previously published ones. We discuss the advantages and limitations of the
templates and of the template fitting method. We conclude that the templates
work sufficiently well to be a valuable and important tool for eliminating and
studying the iron emission in AGNs, at least until accurate theoretical iron
emission models are developed. (Abridged)Comment: 73 pages including 7 figures, 6 tables. To appear in ApJS. Preprint
is also available at
http://www.astronomy.ohio-state.edu/~vester/IronEmission
Hydrogen production from reforming of biogas: Review of technological advances and an Indian perspective
This paper examines the benefits and potential of biogas generation and uses in India, with focus on advances made in hydrogen production by catalytic reforming technologies with steam, O2, and CO2 as the oxidants. Utilization of biomass by means of generating biogas is one of the easiest and cost effective methods of harnessing renewable energy. As India is an agricultural country, a tremendous amount of biomass and livestock waste is generated every year. Likewise the large population of India generates much food and municipal solid waste that is not being utilized for biogas generation. Fuel cells are one of the effective means of utilizing biogas, although the synthesis gas or hydrogen generated from biogas can also be used in internal combustion engines. Higher efficiency of fuel cells (45%) compared to internal combustion engines (30%) and even higher with co-generation systems (70%) make them highly desirable for biogas utilization for power generation. Investigations of hydrogen productionfor fuel cell use are in their infancy stage in India. Apart from a few scattered investigations not much work has been undertaken in the area of research. Large scale hydrogen production is achieved by steam reforming of hydrocarbons, in particular natural gas. Although the process is a proven technology it has come under considerable scrutiny due to its environmental impact and the energy intensive nature of the process. Several technical and scientific challenges have to be overcome for assimilation of the technology for hydrogen production from biogas and wide scale application in India. These challenges include development of less energy intensive features, highly active and poisons-resistant catalyst, and fuel cell development for utilization of biogas generated hydrogen
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Sanitation coverage in Bangladesh since the millennium: consistency matters
Household surveys in Bangladesh between 1994 and 2009 assessed sanitation access using questions that differed significantly over time, resulting in apparently inconsistent findings. Applying the WHO and UNICEF Joint Monitoring Programme's 2008 definition for open defecation and improved sanitation facilities excluding shared facilities to the compiled data set, sensible sanitation coverage trends emerge. The percentage of households openly defecating declined at a rate of about 1.8% per year from 30% in 1994 to 6.8% in 2009, primarily due to changes in rural areas. Access to individual improved sanitation facilities nearly doubled from about 30% in 2006 to 57% in 2009, with both rural and urban areas showing impressive progress. Access to shared improved latrines also nearly doubled from about 13% in 2006 to 24% in 2009, with the urban slums recording the greatest gain from 17% in 2006 to 65% in 2009. Shared improved latrines are only slightly less clean than individual ones. Dependence on shared improved latrines increases with population density. In 2007, 20% of the poorest households still openly defecated, although more of them (38%) shared a latrine of any type. A poverty reduction program is recommended to address this equity issue, although applying consistent definitions is crucial to documenting progress
Infrared FeII Emission in Narrow-Line Seyfert 1 Galaxies
We obtained 0.8-2.4 micron spectra at a resolution of 320 km/s of four
narrow-line Seyfert 1 galaxies in order to study the near-infrared properties
of these objects. We focus on the analysis of the FeII emission in that region
and the kinematics of the low-ionization broad lines. We found that the 1
micron FeII lines (9997 A, 10501 A, 10863 A and 11126 A) are the strongest FeII
lines in the observed interval. For the first time, primary cascade lines of
FeII arising from the decay of upper levels pumped by Ly-alpha fluorescence are
resolved and identified in active galactic nuclei. Excitation mechanisms
leading to the emission of the 1 micron FeII features are discussed. A
combination of Ly-alpha fluorescence and collisional excitation are found to be
the main contributors. The flux ratio between near-IR FeII lines varies from
object to object, in contrast to what is observed in the optical region. A good
correlation between the 1 micron and optical FeII emission is found. This
suggests that the upper z4Fo and z4Do levels from which the bulk of the optical
lines descend are mainly populated by the transitions leading to the 1 micron
lines. The width and profile shape of FeII 11127, CaII 8642 and OI 8446 are
very similar but significantly narrower than Pa-beta, giving strong
observational support to the hypothesis that the region where FeII, CaII and OI
are produced are co-spatial, interrelated kinematically and most probably
located in the outermost portion of the BLR.Comment: Accepted for publication in ApJ - 35 page
Quantifying uncertainty in the measurement of arsenic in suspended particulate matter by Atomic Absorption Spectrometry with hydride generator
Arsenic is the toxic element, which creates several problems in human being specially when inhaled through air. So the accurate and precise measurement of arsenic in suspended particulate matter (SPM) is of prime importance as it gives information about the level of toxicity in the environment, and preventive measures could be taken in the effective areas. Quality assurance is equally important in the measurement of arsenic in SPM samples before making any decision. The quality and reliability of the data of such volatile elements depends upon the measurement of uncertainty of each step involved from sampling to analysis. The analytical results quantifying uncertainty gives a measure of the confidence level of the concerned laboratory. So the main objective of this study was to determine arsenic content in SPM samples with uncertainty budget and to find out various potential sources of uncertainty, which affects the results. Keeping these facts, we have selected seven diverse sites of Delhi (National Capital of India) for quantification of arsenic content in SPM samples with uncertainty budget following sampling by HVS to analysis by Atomic Absorption Spectrometer-Hydride Generator (AAS-HG). In the measurement of arsenic in SPM samples so many steps are involved from sampling to final result and we have considered various potential sources of uncertainties. The calculation of uncertainty is based on ISO/IEC17025: 2005 document and EURACHEM guideline. It has been found that the final results mostly depend on the uncertainty in measurement mainly due to repeatability, final volume prepared for analysis, weighing balance and sampling by HVS. After the analysis of data of seven diverse sites of Delhi, it has been concluded that during the period from 31st Jan. 2008 to 7th Feb. 2008 the arsenic concentration varies from 1.44 ± 0.25 to 5.58 ± 0.55 ng/m3 with 95% confidence level (k = 2)
Excitation of emission lines by fluorescence and recombination in IC 418
We predict intensities of lines of CII, NI, NII, OI and OII and compare them
with a deep spectroscopic survey of IC 418 to test the effect of excitation of
nebular emission lines by continuum fluorescence of starlight. Our calculations
use a nebular model and a synthetic spectrum of its central star to take into
account excitation of the lines by continuum fluorescence and recombination.
The NII spectrum is mostly produced by fluorescence due to the low excitation
conditions of the nebula, but many CII and OII lines have more excitation by
fluorescence than recombination. In the neutral envelope, the NI permitted
lines are excited by fluorescence, and almost all the OI lines are excited by
recombination. Electron excitation produces the forbidden optical lines of OI,
but continuum fluorescence excites most of the NI forbidden line intensities.
Lines excited by fluorescence of light below the Lyman limit thus suggest a new
diagnostic to explore the photodissociation region of a nebula.Comment: 2 pages, 4 figures, to appear in proceedings of the IAU Symposium
283: "Planetary Nebulae: An Eye to the Future", Eds.: A. Manchado, L.
Stanghellini and D. Schoenberne
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