Accretion disks around supermassive black holes are widely believed to be the
dominant source of the optical-ultraviolet continuum in many classes of active
galactic nuclei (AGN). We study here the relationship between the continuum
colors of AGN and the characteristic accretion disk temperature (T_max). Based
on NLTE models of accrection disks in AGN computed as described by Hubeny et
al. (2000), we find that continuum intensity ratios for several pairs of
wavelengths between 1350 and 5100 A should show a trend of bluer colors for
higher T_max, notwithstanding random disk inclinations. We compare this
theoretical expectation with observed colors of QSOs in the Sloan Digital Sky
Survey,deriving black hole mass and thence T_max from the width of the Mg II
broad emission line. The observed colors generally do not show the expected
trend and in some cases show a reverse trend of redder colors with increasing
T_max. The cause of this discrepancy does not appear to be dust reddening or
galaxy contamination but may relate to the accretion rate, as the offset
objects are accreting above ~30 % of the Eddington limit. The derived disk
temperature depends primarily on line width, with little or no dependence on
luminosity.Comment: 7 pages, 7 figures, accepted for publication in ApJ, uses
emulateapj.cl
A new test structure has been developed to identify unambiguously the main mechanism which determines the profiles of thin films deposited by low-pressure chemical vapor deposition (LPCVD) in structures such as steps, trenches, and via-holes. The two mechanisms considered are reemission due to a low surface reaction probability and surface diffusion. Experimental results using silane, diethylsilane (DES), tetraethoxysilane (TEOS), and tetramethylcyclotetrasiloxane (TMCTS) as the silicon sources for oxide deposition by LPCVD show that indirect deposition from reemission is the major contributing factor in determining the step coverage.
The models and algorithms used for simulation of chemical vapor deposition (CVD) profiles in Stanford profile emulator for etching and deposition in integrated circuit engineering are described. In the CVD simulation direct deposition, re-emission, and surface diffusion are considered as the mechanisms for near surface mass transport. The re-emission process is characterized by a single surface reaction coefficient (or sticking coefficient) which condenses the complex physico-chemical mechanisms (physisorption, chemisorption, desorption) in a single probability that defines the final attachment of a reactive to a surface. For desorption process, different desorption models can be selected (cosine re-emission, specular reflection). Surface diffusion is modeled with a gaussian distribution function characterized by the diffusion length. A Monte Carlo approach is used to determine the deposition rates in arbitrary topologies and a string algorithm is used for the evolution of the surface. Iteration between them allows taking into account self-shadowing effects. Comparisons of experimental versus simulation results for SiO2 trench filling, that can be characterized by direct deposition and re-emission with a pure cosine desorption law and no surface diffusion are included.
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