411 research outputs found
An Analysis of requirements for undergraduate international students to attend U.S. universities
This paper is concerned with the requirements of international students who want to attend higher education here in the United States. The areas of concern are the requirements from the U.S. Universities that recruit international students as well as the requirements from the U.S. Government for “F” and “M” Visa students. This paper will also concern itself with the financial aid available to the international students, if needed
Stellar Iron Abundances: non-LTE Effects
We report new statistical equilibrium calculations for Fe I and Fe II in the
atmosphere of Late-Type stars. We used atomic models for Fe I and Fe II having
respectively 256 and 190 levels, as well as 2117 and 3443 radiative
transitions. Photoionization cross-sections are from the Iron Project. These
atomic models were used to investigate non-LTE effects in iron abundances of
Late-Type stars with different atmospheric parameters.
We found that most Fe I lines in metal-poor stars are formed in conditions
far from LTE. We derived metallicity corrections of about 0.3 dex with respect
to LTE values, for the case of stars with [Fe/H] ~ -3.0. Fe II is found not to
be affected by significant non-LTE effects. The main non-LTE effect invoked in
the case of Fe I is overionization by ultraviolet radiation, thus classical
ionization equilibrium is far to be satisfied. An important consequence is that
surface gravities derived by LTE analysis are in error and should be corrected
before final abundances corrections.
This apparently solves the observed discrepancy between spectroscopic surface
gravities derived by LTE analyses and those derived from Hipparcos parallaxes.
A table of non-LTE [Fe/H] and log g values for a sample of metal-poor late-type
stars is given.Comment: 22 pages, 9 figures, 1 table, ApJ style, accepte
OPTIMIZATION OF GROWTH OF TERNARY CuInS2 BY SPRAY PYROLYSIS FOR PHOTOVOLTAIC APPLICATION
CuInS2 ternary films is a promising absorber material for thin film solar cells. It has recently attracted considerable attention due to its high photovoltaic conversion efficiency, and the opportunity to be synthesized by low-cost techniques. In this work CuInS2 thin films have been deposited by chemical Spray pyrolysis onto glass substrate at ambient atmosphere without sulfurization. The effect of the [Cu]/[In] ration, substrate temperature and the time of spray, on the structural, chemical stoichiometry, topographical, and optical properties of CIS thin films were investigated. EDS result demonstrated that stoichiometric CuInS2 film can be adjusted [Cu]/[In] ration. Chalcopyrite structure of this film was confirmed by XRD analysis. The near stoichiometric CuInS2 film has the optical band gap Eg of 1.45eV
PREPARATION AND PROPERTIES OF CZTS THIN FILM PREPARED BY SPRAY PYROLYSIS
In this work we have developed thin-film CZTS (Cu2ZnSnS4) by "spray pyrolysis" technique on preheated glass substrates from an aqueous solution containing ions of copper, zinc, tin and sulfur at different temperatures. Then we performed structural analysis of samples prepared by different characterization methods such as X-ray diffraction (RXD), Raman microscopy, and transmission electron microscopy (SEM). Experimental results have verified that the thin films deposited CZTS are relatively uniform on the substrates. Structural analysis by X-ray diffraction showed that the deposited films are Kestrite structure with a bias in the direction  with the appearance of a second phase the binary Cu2-xS, which is confirmed by the analysis Raman spectroscopy
A toolkit of mechanism and context independent widgets
Most human-computer interfaces are designed to run on a static platform (e.g. a workstation with a monitor) in a static environment (e.g. an office). However, with mobile devices becoming ubiquitous and capable of running applications similar to those found on static devices, it is no longer valid to design static interfaces. This paper describes a user-interface architecture which allows interactors to be flexible about the way they are presented. This flexibility is defined by the different input and output mechanisms used. An interactor may use different mechanisms depending upon their suitability in the current context, user preference and the resources available for presentation using that mechanism
Abundances in Stars from the Red Giant Branch Tip to the Near Main Sequence in M71: II. Iron Abundance
We present [Ffe/H] abundance results that involve a sample of stars with a
wide range in luminosity from luminous giants to stars near the turnoff in a
globular cluster. Our sample of 25 stars in M71 includes 10 giant stars more
luminous than the RHB, 3 horizontal branch stars, 9 giant stars less luminous
than the RHB, and 3 stars near the turnoff. We analyzed both Fe I and Fe II
lines in high dispersion spectra observed with HIRES at the W. M. Keck
Observatory. We find that the [Fe/H] abundances from both Fe I and Fe II lines
agree with each other and with earlier determinations. Also the [Fe/H] obtained
from Fe I and Fe II lines is constant within the rather small uncertainties for
this group of stars over the full range in Teff and luminosity, suggesting that
NLTE effects are negligible in our iron abundance determination. In this
globular cluster, there is no difference among the mean [Fe/H] of giant stars
located at or above the RHB, RHB stars, giant stars located below the RHB and
stars near the turnoff.Comment: Minor changes to conform to version accepted for publication, with
several new figures (Paper 2 of a pair
Effect of annealing on structural and optical properties of ZnO thin films prepared by Sol-Gel technique
Zinc oxide thin films were prepared by sol gel method and spin coating technique, using zinc acetate as precursor solution on glass substrate. The prepared films were annealed at three different temperatures to study the effect of annealing on the structural and optical properties of ZnO thin films. The deposited and annealed films were characterized by X-ray diffraction (XRD), UV-Vis-NIR spectroscopy and scanning electron microscopy (SEM) coupled with microanalysis (EDX). The XRD pattern shows that ZnO films are polycrystalline in nature and crystallite size increases with the increase in annealing temperature. Optical transmittance measurements were taken using UV-Vis-NIR spectrophotometer and the calculated values of the direct band gap energy, Eg was between 3, 28 and 3, 42 eV
Th Ages for Metal-Poor Stars
With a sample of 22 metal-poor stars, we demonstrate that the heavy element
abundance pattern (Z > 55) is the same as the r-process contributions to the
solar nebula. This bolsters the results of previous studies that there is a
universal r-process production pattern. We use the abundance of thorium in five
metal-poor stars, along with an estimate of the initial Th abundance based on
the abundances of stable r-process elements, to measure their ages. We have
four field red giants with errors of 4.2 Gyr in their ages and one M92 giant
with an error of 5.6 Gyr, based on considering the sources of observational
error only. We obtain an average age of 11.4 Gyr, which depends critically on
the assumption of an initial production ratio of Th/Eu of 0.496. If the
Universe is 15 Gyr old, then the initial Th/Eu value should be 0.590, in
agreement with some theoretical models of the r-process.Comment: 26 pages, to be published in Ap
On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk
We present homogeneous and accurate iron abundances for 42 Galactic Cepheids
based on high-spectral resolution (R~38,000) high signal-to-noise ratio
(SNR>100) optical spectra collected with UVES at VLT (128 spectra). The above
abundances were complemented with high-quality iron abundances provided either
by our group (86) or available in the literature. We paid attention in deriving
a common metallicity scale and ended up with a sample of 450 Cepheids. We also
estimated for the entire sample accurate individual distances by using
homogeneous near-infrared photometry and the reddening free Period-Wesenheit
relations. The new metallicity gradient is linear over a broad range of
Galactocentric distances (Rg~5-19 kpc) and agrees quite well with similar
estimates available in the literature (-0.060+/-0.002 dex/kpc). We also uncover
evidence which suggests that the residuals of the metallicity gradient are
tightly correlated with candidate Cepheid Groups (CGs). The candidate CGs have
been identified as spatial overdensities of Cepheids located across the thin
disk. They account for a significant fraction of the residual fluctuations, and
in turn for the large intrinsic dispersion of the metallicity gradient. We
performed a detailed comparison with metallicity gradients based on different
tracers: OB stars and open clusters. We found very similar metallicity
gradients for ages younger than 3 Gyrs, while for older ages we found a
shallower slope and an increase in the intrinsic spread. The above findings
rely on homogeneous age, metallicity and distance scales. Finally we found, by
using a large sample of Galactic and Magellanic Cepheids for which are
available accurate iron abundances, that the dependence of the luminosity
amplitude on metallicity is vanishing.Comment: Accepted in A&A, 11 figures, 7 table
The Carina dSph galaxy: where is the edge?
Recent cosmological N-body simulations suggest that current empirical
estimates of tidal radii in dSphs might be underestimated by at least one order
of magnitude. To constrain the plausibility of this theoretical framework, we
undertook a multiband (U,B,V,I) survey of the Carina dSph. Deep B,V data of
several fields located at radial distances from the Carina center ranging from
0.5 to 4.5 degrees show a sizable sample of faint blue objects with the same
magnitudes and colors of old, Turn-Off stars detected across the center. We
found that the (U-V,B-I) color-color plane is a robust diagnostic to split
stars from background galaxies. Unfortunately, current U,I-band data are too
shallow to firmly constrain the real extent of Carina.Comment: To be published on the proceedings of the XLIX meeting of the Italian
Astronomical Society. Requires mem.cl
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