1,236 research outputs found
Non-LTE Spectra of Accretion Disks Around Intermediate-Mass Black Holes
We have calculated the structures and the emergent spectra of stationary,
geometrically thin accretion disks around 100 and 1000 M_sun black holes in
both the Schwarzschild and extreme Kerr metrics. Equations of radiative
transfer, hydrostatic equilibrium, energy balance, ionization equilibrium, and
statistical equilibrium are solved simultaneously and consistently. The six
most astrophysically abundant elements (H, He, C, N, O, and Fe) are included,
as well as energy transfer by Comptonization. The observed spectrum as a
function of viewing angle is computed incorporating all general relativistic
effects. We find that, in contrast with the predictions of the commonly-used
multi-color disk (MCD) model, opacity associated with photoionization of heavy
elements can significantly alter the spectrum near its peak. These ionization
edges can create spectral breaks visible in the spectra of slowly-spinning
black holes viewed from almost all angles and in the spectra of
rapidly-spinning black holes seen approximately pole-on. For fixed mass and
accretion rate relative to Eddington, both the black hole spin and the viewing
angle can significantly shift the observed peak energy of the spectrum,
particularly for rapid spin viewed obliquely or edge-on. We present a detailed
test of the approximations made in various forms of the MCD model. Linear
limb-darkening is confirmed to be a reasonable approximation for the integrated
flux, but not for many specific frequencies of interest.Comment: 30 pages, 11 eps figures, accepted for publication in Ap
Understanding the disorder of the DNA base cytosine on the Au(111) surface
Using ultrahigh vacuum scanning tunneling microscopy (STM) and ab initio density functional theory, we have investigated in detail structures formed by cytosine on the Au(111) surface in clean ultrahigh vacuum conditions. In spite of the fact that the ground state of this DNA base on the surface is shown to be an ordered arrangement of cytosine one-dimensional branches (filaments), this structure has never been observed in our STM experiments. Instead, disordered structures are observed, which can be explained by only a few elementary structural motifs: filaments, five- and sixfold rings, which randomly interconnect with each other forming bent chains, T junctions, and nanocages. The latter may have trapped smaller structures inside. The formation of such an unusual assembly is explained by simple kinetic arguments as a liquid-glass transition. © 2008 American Institute of Physics
Non-LTE Models and Theoretical Spectra of Accretion Disks in Active Galactic Nuclei
We present self-consistent models of the vertical structure and emergent
spectrum of AGN accretion disks. The central object is assumed to be a
supermassive Kerr black hole. We demonstrate that NLTE effects and the effects
of a self-consistent vertical structure of a disk play a very important role in
determining the emergent radiation, and therefore should be taken into account.
In particular, NLTE models exhibit a largely diminished H I Lyman discontinuity
when compared to LTE models, and the He II discontinuity appears strongly in
emission for NLTE models. Consequently, the number of ionizing photons in the
He II Lyman continuum predicted by NLTE disk models is by 1 - 2 orders of
magnitude higher than that following from the black-body approximation. This
prediction has important implications for ionization models of AGN broad line
regions, and for models of the intergalactic radiation field and the ionization
of helium in the intergalactic medium.Comment: 11 pages; 2 postscript figures; LaTeX, AASPP4 macro; to appear in the
Astrophysical Journal (Letters
Non-LTE Models and Theoretical Spectra of Accretion Disks in Active Galactic Nuclei. III. Integrated Spectra for Hydrogen-Helium Disks
We have constructed a grid of non-LTE disk models for a wide range of black
hole mass and mass accretion rate, for several values of viscosity parameter
alpha, and for two extreme values of the black hole spin: the maximum-rotation
Kerr black hole, and the Schwarzschild (non-rotating) black hole. Our procedure
calculates self-consistently the vertical structure of all disk annuli together
with the radiation field, without any approximations imposed on the optical
thickness of the disk, and without any ad hoc approximations to the behavior of
the radiation intensity. The total spectrum of a disk is computed by summing
the spectra of the individual annuli, taking into account the general
relativistic transfer function. The grid covers nine values of the black hole
mass between M = 1/8 and 32 billion solar masses with a two-fold increase of
mass for each subsequent value; and eleven values of the mass accretion rate,
each a power of 2 times 1 solar mass/year. The highest value of the accretion
rate corresponds to 0.3 Eddington. We show the vertical structure of individual
annuli within the set of accretion disk models, along with their local emergent
flux, and discuss the internal physical self-consistency of the models. We then
present the full disk-integrated spectra, and discuss a number of
observationally interesting properties of the models, such as
optical/ultraviolet colors, the behavior of the hydrogen Lyman limit region,
polarization, and number of ionizing photons. Our calculations are far from
definitive in terms of the input physics, but generally we find that our models
exhibit rather red optical/UV colors. Flux discontinuities in the region of the
hydrogen Lyman limit are only present in cool, low luminosity models, while
hotter models exhibit blueshifted changes in spectral slope.Comment: 20 pages, 31 figures, ApJ in press, spectral models are available for
downloading at http://www.physics.ucsb.edu/~blaes/habk
Gas-induced segregation in Pt-Rh alloy nanoparticles observed by in-situ Bragg coherent diffraction imaging
Bimetallic catalysts can undergo segregation or redistribution of the metals
driven by oxidizing and reducing environments. Bragg coherent diffraction
imaging (BCDI) was used to relate displacement fields to compositional
distributions in crystalline Pt-Rh alloy nanoparticles. 3D images of internal
composition showed that the radial distribution of compositions reverses
partially between the surface shell and the core when gas flow changes between
O2 and H2. Our observation suggests that the elemental segregation of
nanoparticle catalysts should be highly active during heterogeneous catalysis
and can be a controlling factor in synthesis of electrocatalysts. In addition,
our study exemplifies applications of BCDI for in situ 3D imaging of internal
equilibrium compositions in other bimetallic alloy nanoparticles
Hydrogen bonding and orientation effects on the accommodation of methylamine at the air-water interface
Methylamine is an abundant amine compound detected in the atmosphere which can affect the nature of atmospheric aerosol surfaces, changing their chemical and optical properties. Molecular dynamics simulation results show that methylamine accommodation on water is close to unity with the hydrophilic head group solvated in the interfacial environment and the methyl group pointing into the air phase. A detailed analysis of the hydrogen bond network indicates stronger hydrogen bonds between water and the primary amine group at the interface, suggesting that atmospheric trace gases will likely react with the methyl group instead of the solvated amine site. These findings suggest new chemical pathways for methylamine acting on atmospheric aerosols in which the methyl group is the site of orientation specific chemistry involving its conversion into a carbonyl site providing hydrophilic groups for uptake of additional water. This conversion may explain the tendency of aged organic aerosols to form cloud condensation nuclei. At the same time, formation of NH2 radical and formaldehyde is suggested to be a new source for NH2 radicals at aerosol surfaces, other than by reaction of absorbed NH3. The results have general implications for the chemistry of other amphiphilic organics, amines in particular, at the surface of atmospherically relevant aerosol
Hydrogen bonding and orientation effects on the accommodation of methylamine at the air-water interface
Methylamine is an abundant amine compound detected in the atmosphere which can affect the nature of atmospheric aerosol surfaces, changing their chemical and optical properties. Molecular dynamics simulation results show that methylamine accommodation on water is close to unity with the hydrophilic head group solvated in the interfacial environment and the methyl group pointing into the air phase. A detailed analysis of the hydrogen bond network indicates stronger hydrogen bonds between water and the primary amine group at the interface, suggesting that atmospheric trace gases will likely react with the methyl group instead of the solvated amine site. These findings suggest new chemical pathways for methylamine acting on atmospheric aerosols in which the methyl group is the site of orientation specific chemistry involving its conversion into a carbonyl site providing hydrophilic groups for uptake of additional water. This conversion may explain the tendency of aged organic aerosols to form cloud condensation nuclei. At the same time, formation of NH2 radical and formaldehyde is suggested to be a new source for NH2 radicals at aerosol surfaces, other than by reaction of absorbed NH3. The results have general implications for the chemistry of other amphiphilic organics, amines in particular, at the surface of atmospherically relevant aerosol
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