1,397 research outputs found
Stability and Evolution of Supernova Fallback Disks
We show that thin accretion disks made of Carbon or Oxygen are subject to the
same thermal ionization instability as Hydrogen and Helium disks. We argue that
the instability applies to disks of any metal content. The relevance of the
instability to supernova fallback disks probably means that their power-law
evolution breaks down when they first become neutral. We construct simple
analytical models for the viscous evolution of fallback disks to show that it
is possible for these disks to become neutral when they are still young (ages
of a few 10^3 to 10^4 years), compact in size (a few 10^9 cm to 10^11 cm) and
generally accreting at sub-Eddington rates (Mdot ~ a few 10^14 - 10^18 g/s).
Based on recent results on the nature of viscosity in the disks of close
binaries, we argue that this time may also correspond to the end of the disk
activity period. Indeed, in the absence of a significant source of viscosity in
the neutral phase, the entire disk will likely turn to dust and become passive.
We discuss various applications of the evolutionary model, including anomalous
X-ray pulsars and young radio pulsars. Our analysis indicates that metal-rich
fallback disks around newly-born neutron stars and black holes become neutral
generally inside the tidal truncation radius (Roche limit) for planets, at
\~10^11 cm. Consequently, the efficiency of the planetary formation process in
this context will mostly depend on the ability of the resulting disk of rocks
to spread via collisions beyond the Roche limit. It appears easier for the
merger product of a doubly degenerate binary, whether it is a massive white
dwarf or a neutron star, to harbor planets because it can spread beyond the
Roche limit before becoming neutral.[Abridged]Comment: 34 pages, 2 figures, accepted for publication in Ap
Structure of the semiquinone form of flavodoxin from Clostridium MP : Extension of 1.8 A resolution and some comparisons with the oxidized state
As part of a series of comparisons of the structures of the three oxidation states of flavodoxin from Clostridium MP, phases for the semiquinono form were determined to 2.0 A resolution by isomorphous replacement (m > = 0.725). Subsequently, the structure was refined at 1.8 A resolution by a combination of difference Fourier, real space and reciprocal space methods. After refining to an R of 0.194, we explored the conformation of the FMN binding site by real space refinement versus maps with Fourier coefficients of the form (2|Fo|- |Fc|) exp (i[alpha]c). To minimize bias in the fitting, groups of atoms were systematically omitted from the structure factors used in computation of the (2|Fo - |Fc|) maps.One-electron reduction of oxidized flavodoxin is accompanied by several changes at the FMN binding site: the conformation of residues in the reverse bend formed by Met56-Gly57-Asp58-Glu59 differs in the crystal structures of the oxidized and semiquinone species; further, backbone atoms in residues 55 and 89 shift by more than 0.5 A and the indole ring of Trp90 undergoes a significant displacement. The orientation of the peptide unit connecting Gly57 and Asp58 is consistent with the presence of a hydrogen bond between the carbonyl oxygen of Gly57 and the flavin N(5) in flavodoxin semiquinone. No equivalent bond is found in oxidized flavodoxin. In both the oxidized and semiquinone species of clostridial flavodoxin, the isoalloxazine ring is essentially planar : the bending angles about N(5)---N(10) are ~2.5 [deg] for the semiquinone structure and ~0 [deg] in oxidized flavodoxin.The intensity changes resulting from the oxidized agsemiquinone conversion (RI = 0.33) arise in part from changes in molecular packing. Intermolecular contacts, including neighbors of the prosthetic group, are altered in the repacking. Maps or models of the two oxidation states can be brought into approximate coincidence by a rigid body motion. The required transformation, determined for the isomorphous replacement maps by the method of Cox (1967), is equivalent to a screw motion with a rotation of 1.18 [deg] and a translation of -0.34 A. The molecular structures of oxidized and semiquinone flavodoxins have been compared after superposition of models with idealized co-ordinates and discrepancy indices Rox = 0.213 and Rsq = 0.200. The root-mean-square distance between 523 backbone atoms (excluding sequences 56 to 59 and 89 to 91) is 0.308 A.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22809/1/0000366.pd
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Untangling a Web of Lies: Exploring Automated Detection of Deception in Computer-Mediated Communication
Safeguarding organizations against opportunism and severe deception in computer-mediated communication (CMC) presents a major challenge to CIOs and IT managers. New insights into linguistic cues of deception derive from the speech acts innate to CMC. Applying automated text analysis to archival email exchanges in a CMC system as part of a reward program, we assess the ability of word use (micro-level), message development (macro-level), and intertextual exchange cues (meta-level) to detect severe deception by business partners. We empirically assess the predictive ability of our framework using an ordinal multilevel regression model. Results indicate that deceivers minimize the use of referencing and self-deprecation but include more superfluous descriptions and flattery. Deceitful channel partners also over structure their arguments and rapidly mimic the linguistic style of the account manager across dyadic e-mail exchanges. Thanks to its diagnostic value, the proposed framework can support firmsâ decision-making and guide compliance monitoring system development
Electronic sculpting of ligand-GPCR subtype selectivity:the case of angiotensin II
GPCR subtypes possess distinct functional
and pharmacological profiles,
and thus development of subtype-selective ligands has immense therapeutic
potential. This is especially the case for the angiotensin receptor
subtypes AT1R and AT2R, where a functional negative control has been
described and AT2R activation highlighted as an important cancer drug
target. We describe a strategy to fine-tune ligand selectivity for
the AT2R/AT1R subtypes through electronic control of ligand aromatic-prolyl
interactions. Through this strategy an AT2R high affinity (<i>K</i><sub>i</sub> = 3 nM) agonist analogue that exerted 18,000-fold
higher selectivity for AT2R versus AT1R was obtained. We show that
this compound is a negative regulator of AT1R signaling since it is
able to inhibit MCF-7 breast carcinoma cellular proliferation in the
low nanomolar range
Single hadron response measurement and calorimeter jet energy scale uncertainty with the ATLAS detector at the LHC
The uncertainty on the calorimeter energy response to jets of particles is
derived for the ATLAS experiment at the Large Hadron Collider (LHC). First, the
calorimeter response to single isolated charged hadrons is measured and
compared to the Monte Carlo simulation using proton-proton collisions at
centre-of-mass energies of sqrt(s) = 900 GeV and 7 TeV collected during 2009
and 2010. Then, using the decay of K_s and Lambda particles, the calorimeter
response to specific types of particles (positively and negatively charged
pions, protons, and anti-protons) is measured and compared to the Monte Carlo
predictions. Finally, the jet energy scale uncertainty is determined by
propagating the response uncertainty for single charged and neutral particles
to jets. The response uncertainty is 2-5% for central isolated hadrons and 1-3%
for the final calorimeter jet energy scale.Comment: 24 pages plus author list (36 pages total), 23 figures, 1 table,
submitted to European Physical Journal
Standalone vertex ďŹnding in the ATLAS muon spectrometer
A dedicated reconstruction algorithm to find decay vertices in the ATLAS muon spectrometer is presented. The algorithm searches the region just upstream of or inside the muon spectrometer volume for multi-particle vertices that originate from the decay of particles with long decay paths. The performance of the algorithm is evaluated using both a sample of simulated Higgs boson events, in which the Higgs boson decays to long-lived neutral particles that in turn decay to bbar b final states, and pp collision data at âs = 7 TeV collected with the ATLAS detector at the LHC during 2011
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