763 research outputs found
Long-Baseline Interferometric Multiplicity Survey of the Sco-Cen OB Association
We present the first multiplicity-dedicated long baseline optical
interferometric survey of the Scorpius-Centaurus-Lupus-Crux association. We
used the Sydney University Stellar Interferometer to undertake a survey for new
companions to 58 Sco-Cen B- type stars and have detected 24 companions at
separations ranging from 7-130mas, 14 of which are new detections. Furthermore,
we use a Bayesian analysis and all available information in the literature to
determine the multiplicity distribution of the 58 stars in our sample, showing
that the companion frequency is F = 1.35 and the mass ratio distribution is
best described as a power law with exponent equal to -0.46, agreeing with
previous Sco-Cen high mass work and differing significantly from lower-mass
stars in Tau-Aur. Based on our analysis, we estimate that among young B-type
stars in moving groups, up to 23% are apparently single stars. This has strong
implications for the understanding of high-mass star formation, which requires
angular momentum dispersal through some mechanism such as formation of multiple
systems.Comment: 7 figures, 5 tables, accepted for publication in MNRA
Remanence effects in the electrical resistivity of spin glasses
We have measured the low temperature electrical resistivity of Ag : Mn
mesoscopic spin glasses prepared by ion implantation with a concentration of
700 ppm. As expected, we observe a clear maximum in the resistivity (T ) at a
temperature in good agreement with theoretical predictions. Moreover, we
observe remanence effects at very weak magnetic fields for the resistivity
below the freezing temperature Tsg: upon Field Cooling (fc), we observe clear
deviations of (T ) as compared with the Zero Field Cooling (zfc); such
deviations appear even for very small magnetic fields, typically in the Gauss
range. This onset of remanence for very weak magnetic fields is reminiscent of
the typical signature on magnetic susceptibility measurements of the spin glass
transition for this generic glassy system
Dynamical Casimir-Polder interaction between an atom and surface plasmons
We investigate the time-dependent Casimir-Polder potential of a polarizable
two-level atom placed near a surface of arbitrary material, after a sudden
change in the parameters of the system. Different initial conditions are taken
into account. For an initially bare ground-state atom, the time-dependent
Casimir-Polder energy reveals how the atom is "being dressed" by virtual,
matter-assisted photons. We also study the transient behavior of the
Casimir-Polder interaction between the atom and the surface starting from a
partially dressed state, after an externally induced change in the atomic level
structure or transition dipoles. The Heisenberg equations are solved through an
iterative technique for both atomic and field operators in the medium-assisted
electromagnetic field quantization scheme. We analyze in particular how the
time evolution of the interaction energy depends on the optical properties of
the surface, in particular on the dispersion relationof surface plasmon
polaritons. The physical significance and the limits of validity of the
obtained results are discussed in detail.Comment: 12 pages, 8 figure
Nonlocal field correlations and dynamical Casimir-Polder forces between one excited- and two ground-state atoms
The problem of nonlocality in the dynamical three-body Casimir-Polder
interaction between an initially excited and two ground-state atoms is
considered. It is shown that the nonlocal spatial correlations of the field
emitted by the excited atom during the initial part of its spontaneous decay
may become manifest in the three-body interaction. The observability of this
new phenomenon is discussed.Comment: 17 pages, 1 figure, sub. to Phys. Rev.
Eclipsing binaries in the open cluster Ruprecht 147. II: EPIC 219568666
We report our spectroscopic monitoring of the detached, grazing, and slightly eccentric 12 day double-lined eclipsing binary EPIC 219568666 in the old nearby open cluster Ruprecht 147. This is the second eclipsing system to be analyzed in this cluster, following our earlier study of EPIC 219394517. Our analysis of the radial velocities combined with the light curve from the K2 mission yields absolute masses and radii for EPIC 219568666 of M₁ = 1.121 ± 0.013 M☉ and R₁ = 1.1779 ± 0.0070 R☉ for the F8 primary and M₂ = 0.7334 ± 0.0050 M☉ and R₂ = 0.640 ± 0.017 R☉ for the faint secondary. Comparison with current stellar evolution models calculated for the known metallicity of the cluster points to a primary star that is oversized, as is often seen in active M dwarfs, but this seems rather unlikely for a star of its mass and with a low level of activity. Instead, we suspect a subtle bias in the radius ratio inferred from the photometry, despite our best efforts to avoid it, which may be related to the presence of spots on one or both stars. The radius sum for the binary, which bypasses this possible problem, indicates an age of 2.76 ± 0.61 Gyr, which is in good agreement with a similar estimate from the binary in our earlier study
Can spacetime curvature induced corrections to Lamb shift be observable?
The Lamb shift results from the coupling of an atom to vacuum fluctuations of
quantum fields, so corrections are expected to arise when the spacetime is
curved since the vacuum fluctuations are modified by the presence of spacetime
curvature. Here, we calculate the curvature-induced correction to the Lamb
shift outside a spherically symmetric object and demonstrate that this
correction can be remarkably significant outside a compact massive
astrophysical body. For instance, for a neutron star or a stellar mass black
hole, the correction is 25% at a radial distance of ,
16% at and as large as 1.6% even at , where is
the mass of the object, the Newtonian constant, and the speed of light.
In principle, we can look at the spectra from a distant compact super-massive
body to find such corrections. Therefore, our results suggest a possible way of
detecting fundamental quantum effects in astronomical observations.Comment: 13 pages, 3 figures, slight title change, clarifications and more
discussions added, version to be published in JHE
The Mitochondrial Ca(2+) Uniporter: Structure, Function, and Pharmacology.
Mitochondrial Ca(2+) uptake is crucial for an array of cellular functions while an imbalance can elicit cell death. In this chapter, we briefly reviewed the various modes of mitochondrial Ca(2+) uptake and our current understanding of mitochondrial Ca(2+) homeostasis in regards to cell physiology and pathophysiology. Further, this chapter focuses on the molecular identities, intracellular regulators as well as the pharmacology of mitochondrial Ca(2+) uniporter complex
A multiscale hybrid model for pro-angiogenic calcium signals in a vascular endothelial cell
Cytosolic calcium machinery is one of the principal signaling mechanisms by which endothelial cells (ECs) respond to external stimuli during several biological processes, including vascular progression in both physiological and pathological conditions. Low concentrations of angiogenic factors (such as VEGF) activate in fact complex pathways involving, among others, second messengers arachidonic acid (AA) and nitric oxide (NO), which in turn control the activity of plasma membrane calcium channels. The subsequent increase in the intracellular level of the ion regulates fundamental biophysical properties of ECs (such as elasticity, intrinsic motility, and chemical strength), enhancing their migratory capacity. Previously, a number of continuous models have represented cytosolic calcium dynamics, while EC migration in angiogenesis has been separately approached with discrete, lattice-based techniques. These two components are here integrated and interfaced to provide a multiscale and hybrid Cellular Potts Model (CPM), where the phenomenology of a motile EC is realistically mediated by its calcium-dependent subcellular events. The model, based on a realistic 3-D cell morphology with a nuclear and a cytosolic region, is set with known biochemical and electrophysiological data. In particular, the resulting simulations are able to reproduce and describe the polarization process, typical of stimulated vascular cells, in various experimental conditions.Moreover, by analyzing the mutual interactions between multilevel biochemical and biomechanical aspects, our study investigates ways to inhibit cell migration: such strategies have in fact the potential to result in pharmacological interventions useful to disrupt malignant vascular progressio
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