5,930 research outputs found
X-ray emission from O-type stars : DH Cep and HD 97434
We present X-ray emission characteristics of the massive O-type stars DH Cep
and HD 97434 using archival XMM-Newton observations. There is no convincing
evidence for short term variability in the X-ray intensity during the
observations. However, the analysis of their spectra reveals X-ray structure
being consistent with two-temperature plasma model. The hydrogen column
densities derived from X-ray spectra of DH Cep and HD 97434 are in agreement
with the reddening measurements for their corresponding host clusters NGC 7380
and Trumpler 18, indicating that the absorption by stellar wind is negligible.
The X-ray emission from these hot stars is interpreted in terms of the standard
instability-driven wind shock model.Comment: 13 pages ; 2 figures; 2 tables (Accepted for publication in New
Astronomy
Answer Set Programming Modulo `Space-Time'
We present ASP Modulo `Space-Time', a declarative representational and
computational framework to perform commonsense reasoning about regions with
both spatial and temporal components. Supported are capabilities for mixed
qualitative-quantitative reasoning, consistency checking, and inferring
compositions of space-time relations; these capabilities combine and synergise
for applications in a range of AI application areas where the processing and
interpretation of spatio-temporal data is crucial. The framework and resulting
system is the only general KR-based method for declaratively reasoning about
the dynamics of `space-time' regions as first-class objects. We present an
empirical evaluation (with scalability and robustness results), and include
diverse application examples involving interpretation and control tasks
Bound Magnetic Polaron Interactions in Insulating Doped Diluted Magnetic Semiconductors
The magnetic behavior of insulating doped diluted magnetic semiconductors
(DMS) is characterized by the interaction of large collective spins known as
bound magnetic polarons. Experimental measurements of the susceptibility of
these materials have suggested that the polaron-polaron interaction is
ferromagnetic, in contrast to the antiferromagnetic carrier-carrier
interactions that are characteristic of nonmagnetic semiconductors. To explain
this behavior, a model has been developed in which polarons interact via both
the standard direct carrier-carrier exchange interaction (due to virtual
carrier hopping) and an indirect carrier-ion-carrier exchange interaction (due
to the interactions of polarons with magnetic ions in an interstitial region).
Using a variational procedure, the optimal values of the model parameters were
determined as a function of temperature. At temperatures of interest, the
parameters describing polaron-polaron interactions were found to be nearly
temperature-independent. For reasonable values of these constant parameters, we
find that indirect ferromagnetic interactions can dominate the direct
antiferromagnetic interactions and cause the polarons to align. This result
supports the experimental evidence for ferromagnetism in insulating doped DMS.Comment: 11 pages, 7 figure
Conductivity of Metallic Si:B near the Metal-Insulator Transition: Comparison between Unstressed and Uniaxially Stressed Samples
The low-temperature dc conductivities of barely metallic samples of p-type
Si:B are compared for a series of samples with different dopant concentrations,
n, in the absence of stress (cubic symmetry), and for a single sample driven
from the metallic into the insulating phase by uniaxial compression, S. For all
values of temperature and stress, the conductivity of the stressed sample
collapses onto a single universal scaling curve. The scaling fit indicates that
the conductivity of si:B is proportional to the square-root of T in the
critical range. Our data yield a critical conductivity exponent of 1.6,
considerably larger than the value reported in earlier experiments where the
transition was crossed by varying the dopant concentration. The larger exponent
is based on data in a narrow range of stress near the critical value within
which scaling holds. We show explicitly that the temperature dependences of the
conductivity of stressed and unstressed Si:B are different, suggesting that a
direct comparison of the critical behavior and critical exponents for stress-
tuned and concentration-tuned transitions may not be warranted
Two-component approach for thermodynamic properties in diluted magnetic semiconductors
We examine the feasibility of a simple description of Mn ions in III-V
diluted magnetic semiconductors (DMSs) in terms of two species (components),
motivated by the expectation that the Mn-hole exchange couplings are widely
distributed, expecially for low Mn concentrations. We find, using distributions
indicated by recent numerical mean field studies, that the thermodynamic
properties (magnetization, susceptibility, and specific heat) cannot be fit by
a single coupling as in a homogeneous model, but can be fit well by a
two-component model with a temperature dependent number of ``strongly'' and
``weakly'' coupled spins. This suggests that a two-component description may be
a minimal model for the interpretation of experimental measurements of
thermodynamic quantities in III-V DMS systems.Comment: 10 pages, 9 figures, 1 new figure, substantial revision
Tripartite phase separation of two signal effectors with vesicles priming B cell responsiveness.
Antibody-mediated immune responses rely on antigen recognition by the B cell antigen receptor (BCR) and the proper engagement of its intracellular signal effector proteins. Src homology (SH) 2 domain-containing leukocyte protein of 65 kDa (SLP65) is the key scaffold protein mediating BCR signaling. In resting B cells, SLP65 colocalizes with Cbl-interacting protein of 85 kDa (CIN85) in cytoplasmic granules whose formation is not fully understood. Here we show that effective B cell activation requires tripartite phase separation of SLP65, CIN85, and lipid vesicles into droplets via vesicle binding of SLP65 and promiscuous interactions between nine SH3 domains of the trimeric CIN85 and the proline-rich motifs (PRMs) of SLP65. Vesicles are clustered and the dynamical structure of SLP65 persists in the droplet phase in vitro. Our results demonstrate that phase separation driven by concerted transient interactions between scaffold proteins and vesicles is a cellular mechanism to concentrate and organize signal transducers
Glassy Roughness of a Crystalline Surface Upon a Disordered Substrate
The discrete Gaussian model for the surface of a crystal deposited on a
disordered substrate is studied by Monte Carlo simulations. A continuous
transition is found from a phase with a thermally-induced roughness to a glassy
one in which the roughness is driven by the disorder. The behavior of the
height-height correlations is consistent with the one-step replica symmetry
broken solution of the variational approximation. The results differ from the
renormalization group predictions and from recent simulations of a 2D
vortex-glass model which belongs to the same universality class.Comment: 12 pages (RevTeX) & 3 figures (PS) uuencode
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