6,906 research outputs found
The Influence of Dichromate Ions on Aluminum Dissolution Kinetics in Artificial Crevice Electrode Cells
Dissolution kinetics for pits and crevices in aluminum and the effect of dichromate ions on the dissolution kinetics were investigated by using artificial crevice electrodes. The aluminum artificial crevice electrodes were potentiostatically polarized over a range of potential in 0.1 M NaCl solution with and without dichromate ions. The anodic dissolution charge, and cathodic charges for the hydrogen and dichromate reduction reactions, were measured. The addition of dichromate ions did not suppress the active dissolution. This indicates that the mechanism of localized corrosion inhibition by dichromates is something other than anodic inhibition of Al dissolution in the pit or crevice environment. The relative amount of local cathodic reactions on Al was increased by the addition of dichromate because of the dichromate reduction. The initial dissolution of aluminum in a crevice was ohmic controlled. From the change in the dissolution current with time, the conductivity of the crevice and potential at the bottom of crevice were estimated. The conductivity and the bottom potential decreased with the ratio of cathodic charge of hydrogen evolution to anodic dissolution charge. The conductivity in the crevice and thus the dissolution current seem to be controlled by hydrogen evolution and only indirectly by dichromate concentration.This work was supported by the Air Force Office of Scientific Research under contract no. F49620-96-1-0479
Inapproximability of maximal strip recovery
In comparative genomic, the first step of sequence analysis is usually to
decompose two or more genomes into syntenic blocks that are segments of
homologous chromosomes. For the reliable recovery of syntenic blocks, noise and
ambiguities in the genomic maps need to be removed first. Maximal Strip
Recovery (MSR) is an optimization problem proposed by Zheng, Zhu, and Sankoff
for reliably recovering syntenic blocks from genomic maps in the midst of noise
and ambiguities. Given genomic maps as sequences of gene markers, the
objective of \msr{d} is to find subsequences, one subsequence of each
genomic map, such that the total length of syntenic blocks in these
subsequences is maximized. For any constant , a polynomial-time
2d-approximation for \msr{d} was previously known. In this paper, we show that
for any , \msr{d} is APX-hard, even for the most basic version of the
problem in which all gene markers are distinct and appear in positive
orientation in each genomic map. Moreover, we provide the first explicit lower
bounds on approximating \msr{d} for all . In particular, we show that
\msr{d} is NP-hard to approximate within . From the other
direction, we show that the previous 2d-approximation for \msr{d} can be
optimized into a polynomial-time algorithm even if is not a constant but is
part of the input. We then extend our inapproximability results to several
related problems including \cmsr{d}, \gapmsr{\delta}{d}, and
\gapcmsr{\delta}{d}.Comment: A preliminary version of this paper appeared in two parts in the
Proceedings of the 20th International Symposium on Algorithms and Computation
(ISAAC 2009) and the Proceedings of the 4th International Frontiers of
Algorithmics Workshop (FAW 2010
Nonrigid chiral soliton for the octet and decuplet baryons
Systematic treatment of the collective rotation of the nonrigid chiral
soliton is developed in the SU(3) chiral quark soliton model and applied to the
octet and decuplet baryons. The strangeness degrees of freedom are treated by a
simplified bound-state approach which omits the locality of the kaon wave
function. Then, the flavor rotation is divided into the isospin rotation and
the emission and absorption of the kaon. The kaon Hamiltonian is diagonalized
by the Hartree approximation. The soliton changes the shape according to the
strangeness. The baryons appear as the rotational bands of the combined system
of the soliton and the kaon.Comment: 11 pages(LaTex), 1 figures(eps
X-Ray bright optically faint active galactic nuclei in the Subaru Hyper Suprime-Cam wide survey
We construct a sample of X-ray bright optically faint active galactic nuclei
by combining Subaru Hyper Suprime-Cam, XMM-Newton, and infrared source
catalogs. 53 X-ray sources satisfying i band magnitude fainter than 23.5 mag
and X-ray counts with EPIC-PN detector larger than 70 are selected from 9.1
deg^2, and their spectral energy distributions (SEDs) and X-ray spectra are
analyzed. 44 objects with an X-ray to i-band flux ratio F_X/F_i>10 are
classified as extreme X-ray-to-optical flux sources. SEDs of 48 among 53 are
represented by templates of type 2 AGNs or starforming galaxies and show
signature of stellar emission from host galaxies in the optical in the source
rest frame. Infrared/optical SEDs indicate significant contribution of emission
from dust to infrared fluxes and that the central AGN is dust obscured.
Photometric redshifts determined from the SEDs are in the range of 0.6-2.5.
X-ray spectra are fitted by an absorbed power law model, and the intrinsic
absorption column densities are modest (best-fit log N_H = 20.5-23.5 cm^-2 in
most cases). The absorption corrected X-ray luminosities are in the range of
6x10^42 - 2x10^45 erg s^-1. 20 objects are classified as type 2 quasars based
on X-ray luminsosity and N_H. The optical faintness is explained by a
combination of redshifts (mostly z>1.0), strong dust extinction, and in part a
large ratio of dust/gas.Comment: 25 pages, 14 figures, 5 tables, accepted for publication in PAS
Modeling Seven Years of Event Horizon Telescope Observations with Radiatively Inefficient Accretion Flow Models
An initial three-station version of the Event Horizon Telescope, a
millimeter-wavelength very-long baseline interferometer, has observed
Sagittarius A* (Sgr A*) repeatedly from 2007 to 2013, resulting in the
measurement of a variety of interferometric quantities. Of particular
importance, there is now a large set of closure phases, measured over a number
of independent observing epochs. We analyze these observations within the
context of a realization of semi-analytic radiatively inefficient disk models,
implicated by the low luminosity of Sgr A*. We find a broad consistency among
the various observing epochs and between different interferometric data types,
with the latter providing significant support for this class of models of Sgr
A*. The new data significantly tighten existing constraints on the spin
magnitude and its orientation within this model context, finding a spin
magnitude of , an inclination with respect to
the line of sight of
, and a position
angle of east of
north. These are in good agreement with previous analyses. Notably, the
previous degeneracy in the position angle has now been conclusively
broken by the inclusion of the closure phase measurements. A reflection
degeneracy in the inclination remains, permitting two localizations of the spin
vector orientation, one of which is in agreement with the orbital angular
momentum of the infrared gas cloud G2 and the clockwise disk of young stars.
This possibly supports a relationship between Sgr A*'s accretion flow and these
larger-scale features.Comment: 16 pages, 11 figures, accepted to Ap
Spatial Relationship between Solar Flares and Coronal Mass Ejections
We report on the spatial relationship between solar flares and coronal mass
ejections (CMEs) observed during 1996-2005 inclusive. We identified 496
flare-CME pairs considering limb flares (distance from central meridian > 45
deg) with soft X-ray flare size > C3 level. The CMEs were detected by the Large
Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric
Observatory (SOHO). We investigated the flare positions with respect to the CME
span for the events with X-class, M-class, and C-class flares separately. It is
found that the most frequent flare site is at the center of the CME span for
all the three classes, but that frequency is different for the different
classes. Many X-class flares often lie at the center of the associated CME,
while C-class flares widely spread to the outside of the CME span. The former
is different from previous studies, which concluded that no preferred flare
site exists. We compared our result with the previous studies and conclude that
the long-term LASCO observation enabled us to obtain the detailed spatial
relation between flares and CMEs. Our finding calls for a closer flare-CME
relationship and supports eruption models typified by the CSHKP magnetic
reconnection model.Comment: 7 pages; 4 figures; Accepted by the Astrophysical Journa
Gain in a quantum wire laser of high uniformity
A multi-quantum wire laser operating in the 1-D ground state has been
achieved in a very high uniformity structure that shows free exciton emission
with unprecedented narrow width and low lasing threshold. Under optical pumping
the spontaneous emission evolves from a sharp free exciton peak to a
red-shifted broad band. The lasing photon energy occurs about 5 meV below the
free exciton. The observed shift excludes free excitons in lasing and our
results show that Coulomb interactions in the 1-D electron-hole system shift
the spontaneous emission and play significant roles in laser gain.Comment: 4 pages, 4 figures, prepared by RevTe
Yeast autonomously replicating sequence binding factor is involved in nucleotide excision repair
Nucleotide excision repair (NER) in yeast is effected by the concerted action of a large complex of proteins. Recently, we identified a stable subcomplex containing the yeast Rad7 and Rad16 proteins. Here, we report the identification of autonomously replicating sequence binding factor 1 (ABF1) as a component of the Rad7/Rad16 NER subcomplex. Yeast ABF1 protein is encoded by an essential gene required for DNA replication, transcriptional regulation, and gene silencing. We show that ABF1 plays a direct role in NER in vitro. Additionally, consistent with a role of ABF1 protein in NER in vivo, we show that certain temperature-sensitive abf1 mutant strains that are defective in DNA replication are specifically defective in the removal of photoproducts by NER and are sensitive to killing by ultraviolet (UV) radiation. These studies define a novel and unexpected role for ABF1 protein during NER in yeast
Radiative Lifetimes of Single Excitons in Semiconductor Quantum Dots- Manifestation of the Spatial Coherence Effect
Using time correlated single photon counting combined with temperature
dependent diffraction limited confocal photoluminescence spectroscopy we
accurately determine, for the first time, the intrinsic radiative lifetime of
single excitons confined within semiconductor quantum dots. Their lifetime is
one (two) orders of magnitude longer than the intrinsic radiative lifetime of
single excitons confined in semiconductor quantum wires (wells) of comparable
confining dimensions. We quantitatively explain this long radiative time in
terms of the reduced spatial coherence between the confined exciton dipole
moment and the radiation electromagnetic field.Comment: 4 pages, 3 figure
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