3,793 research outputs found
An optimality criterion for sizing members of heated structures with temperature constraints
A thermal optimality criterion is presented for sizing members of heated structures with multiple temperature constraints. The optimality criterion is similar to an existing optimality criterion for design of mechanically loaded structures with displacement constraints. Effectiveness of the thermal optimality criterion is assessed by applying it to one- and two-dimensional thermal problems where temperatures can be controlled by varying the material distribution in the structure. Results obtained from the optimality criterion agree within 2 percent with results from a closed-form solution and with results from a mathematical programming technique. The thermal optimality criterion augments existing optimality criteria for strength and stiffness related constraints and offers the possibility of extension of optimality techniques to sizing structures with combined thermal and mechanical loading
The effects of an extra U(1) axial condensate on the radiative decay eta' --> gamma gamma at finite temperature
Supported by recent lattice results, we consider a scenario in which a
U(1)-breaking condensate survives across the chiral transition in QCD. This
scenario has important consequences on the pseudoscalar-meson sector, which can
be studied using an effective Lagrangian model. In particular, generalizing the
results obtained in a previous paper (where the zero-temperature case was
considered), we study the effects of this U(1) chiral condensate on the
radiative decay eta' --> gamma gamma at finite temperature.Comment: 15 pages, LaTeX fil
Periodicity makes galactic shocks unstable - I. Linear analysis
We study the dynamical stability of stationary galactic spiral shocks. The
steady-state equilibrium flow contains a shock of the type derived by Roberts
in the tightly wound approximation. We find that boundary conditions are
critical in determining whether the solutions are stable or not. Shocks are
unstable if periodic boundary conditions are imposed. For intermediate
strengths of the spiral potential, the instability disappears if boundary
conditions are imposed such that the upstream flow is left unperturbed as in
the classic analysis of D'yakov and Kontorovich. This reconciles apparently
contradictory findings of previous authors regarding the stability of spiral
shocks. This also shows that the instability is distinct from the
Kelvin-Helmholtz instability, confirming the findings of Kim et al. We suggest
that instability is a general characteristics of periodic shocks, regardless of
the presence of shear, and provide a physical picture as to why this is the
case. For strong spiral potentials, high post-shock shear makes the system
unstable also to parasitic Kelvin-Helmholtz instability regardless of the
boundary conditions. Our analysis is performed in the context of a simplified
problem that, while preserving all the important characteristics of the
original problem, strips it from unnecessary complications, and assumes that
the gas is isothermal, non self-gravitating, non-magnetised.Comment: Accepted for publication in MNRA
Dark-State Polaritons for multi-component and stationary light fields
We present a general scheme to determine the loss-free adiabatic
eigensolutions (dark-state polaritons) of the interaction of multiple probe
laser beams with a coherently driven atomic ensemble under conditions of
electromagnetically induced transparency. To this end we generalize the
Morris-Shore transformation to linearized Heisenberg-Langevin equations
describing the coupled light-matter system in the weak excitation limit. For
the simple lambda-type coupling scheme the generalized Morris-Shore
transformation reproduces the dark-state polariton solutions of slow light.
Here we treat a closed-loop dual-V scheme wherein two counter-propagating
control fields generate a quasi stationary pattern of two counter-propagating
probe fields -- so-called stationary light. We show that contrary to previous
predictions,there exists a single unique dark-state polariton; it obeys a
simple propagation equation.Comment: 6 pages, 2 figure
The spectroscopic evolution of the -ray emitting classical nova Nova Mon 2012. I. Implications for the ONe subclass of classical novae
Nova Mon 2012 was the first classical nova to be detected as a high energy
-ray transient, by Fermi-LAT, before its optical discovery. We study a
time sequence of high resolution optical echelle spectra (Nordic Optical
Telescope) and contemporaneous NOT, STIS UV, and CHIRON echelle spectra (Nov
20/21/22). We use [O III] and H line fluxs to constrain the properties
of the ejecta. We derive the structure from the optical and UV line profiles
and compare our measured line fluxes for with predictions using Cloudy with
abundances from other ONe novae. Mon 2012 is confirmed as an ONe nova. We find
E(B-V)=0.850.05 and hydrogen column density
cm. The corrected continuum luminosity is nearly the same in the entire
observed energy range as V1974 Cyg, V382 Mon, and Nova LMC 2000 at the same
epoch after outburst. The distance, about 3.6 kpc, is quite similar to V1974
Cyg. The line profiles can be modeled using an axisymmetric bipolar geometry
for the ejecta with various inclinations of the axis to the line of sight, 60
\le i \le 80 degrees, an opening angle of \approx\Delta
R/R(t)\approx 0.4f\approx 0.1-0.3\leq 6\times
10^{-5}_\odot\gamma$-ray emission may be a generic phenomenon, common to all ONe novae,
possibly to all classical novae, and connected with acceleration and emission
processes within the ejecta (abstract severely truncated).Comment: Submitted to A&A 9/1/2013; Accepted 27/2/2013 (in press
Pseudoscalar Glueball, eta'-meson and its Excitation in the Chiral Effective Lagrangian
A generalization of the chiral effective lagrangian of order is
proposed which involves the -meson, its excitation, and the pseudoscalar
(PS) glueball. Model-independent constraints are found for the contributions to
the lagrangian of the above singlet states. Those allow one to independently
identify the nature of these singlet states in the framework of the approach.
The mixing among the iso-singlet states (including -state) is analysed,
and the hierarchy of the mixing angles is described which is defined by the
chiral and large- expansions. The recent PCAC results are reproduced,
which are related to the problem of the renormalization-group invariant
description of the and the PS glueball, and a further analysis of this
problem is performed.Comment: 19 pages LaTeX, no figures. Revised version accepted in Phis.Rev.
Perturbative Evaluation of Interacting Scalar Fields on a Curved Manifold with Boundary
The effects of quantum corrections to a conformally invariant scalar field
theory on a curved manifold of positive constant curvature with boundary are
considered in the context of a renormalisation procedure. The renormalisation
of the theory to second order in the scalar self-coupling pursued herein
involves explicit calculations of up to third loop-order and reveals that, in
addition to the renormalisation of the scalar self-coupling and scalar field,
the removal of all divergences necessitates the introduction of conformally
non-invariant counterterms proportional to and in the
bare scalar action as well as counterterms proportional to , and
in the gravitational action. The substantial backreaction effects and
their relevance to the renormalisation procedure are analysed.Comment: 25 pages, 1 figure. Minor elucidations in the Appendix regarding the
cut-off and in p.4 regarding the gravitational action. Certain
reference-related ommission corrected. To appear in Classical and Quantum
Gravit
Rigid invariance as derived from BRS invariance: The abelian Higgs model
Consequences of a symmetry, e.g.\ relations amongst Green functions, are
renormalization scheme independently expressed in terms of a rigid Ward
identity. The corresponding local version yields information on the respective
current. In the case of spontaneous breakdown one has to define the theory via
the BRS invariance and thus to construct rigid and current Ward identity
non-trivially in accordance with it. We performed this construction to all
orders of perturbation theory in the abelian Higgs model as a prelude to the
standard model. A technical tool of interest in itself is the use of a doublet
of external scalar ``background'' fields. The Callan-Symanzik equation has an
interesting form and follows easily once the rigid invariance is established.Comment: 33 pages, Plain Te
Caspase cleavage product of BAP31 induces mitochondrial fission through endoplasmic reticulum calcium signals, enhancing cytochrome c release to the cytosol
Stimulation of cell surface death receptors activates caspase-8, which targets a limited number of substrates including BAP31, an integral membrane protein of the endoplasmic reticulum (ER). Recently, we reported that a caspase-resistant BAP31 mutant inhibited several features of Fas-induced apoptosis, including the release of cytochrome c (cyt.c) from mitochondria (Nguyen, M., D.G. Breckenridge, A. Ducret, and G.C. Shore. 2000. Mol. Cell. Biol. 20:6731–6740), implicating ER-mitochondria crosstalk in this pathway. Here, we report that the p20 caspase cleavage fragment of BAP31 can direct pro-apoptotic signals between the ER and mitochondria. Adenoviral expression of p20 caused an early release of Ca2+ from the ER, concomitant uptake of Ca2+ into mitochondria, and mitochondrial recruitment of Drp1, a dynamin-related protein that mediates scission of the outer mitochondrial membrane, resulting in dramatic fragmentation and fission of the mitochondrial network. Inhibition of Drp1 or ER-mitochondrial Ca2+ signaling prevented p20-induced fission of mitochondria. p20 strongly sensitized mitochondria to caspase-8–induced cyt.c release, whereas prolonged expression of p20 on its own ultimately induced caspase activation and apoptosis through the mitochondrial apoptosome stress pathway. Therefore, caspase-8 cleavage of BAP31 at the ER stimulates Ca2+-dependent mitochondrial fission, enhancing the release of cyt.c in response to this initiator caspase
Skin microvascular vasodilatory capacity in offspring of two parents with Type 2 diabetes
Aims<br/> Microvascular dysfunction occurs in Type 2 diabetes and in subjects with fasting hyperglycaemia. It is unclear whether this dysfunction relates to dysglycaemia. This study investigated in normogylcaemic individuals whether a genetic predisposition to diabetes, or indices of insulin resistance including endothelial markers, were associated with impaired microvascular function.<br/> Methods<br/> Maximum microvascular hyperaemia to local heating of the skin was measured using laser Doppler flowmetry in 21 normoglycaemic subjects with no family history of diabetes (Group 1) and 21 normoglycaemic age, sex and body mass index-matched offspring of two parents with Type 2 diabetes (Group 2). <br/>Results<br/> Although Group 2 had normal fasting plasma glucose and glucose tolerance tests, the 120-min glucose values were significantly higher at 6.4 (5.3-6.6) mmol/l (median (25th-75th centile)) than the control group at 4.9 (4.6-5.9) mmol/l (P=0.005) and the insulinogenic index was lower at 97.1 (60.9-130.8) vs. 124.0 (97.2-177.7) (P=0.027). Skin maximum microvascular hyperaemia (Group 1: 1.56 (1.39- 1.80) vs. Group 2: 1.53 (1.30-1.98) V, P=0.99) and minimum microvascular resistance which normalizes the hyperaemia data for blood pressure (Group 1: 52.0 (43.2-67.4) vs. Group 2: 56.0 (43.7-69.6) mmHgN, P=0.70) did not differ in the two groups. Significant positive associations occurred between minimum microvascular resistance and indices of the insulin resistance syndrome; plasminogen activator inhibitor type 1 (R-s=0.46, P=0.003), t-PA (R-s=0.36, P=0.03), total cholesterol (R-s=0.35, P=0.02), and triglyceride concentration (R-s=0.35, P=0.02), and an inverse association with insulin sensitivity (R-s=-0.33, P=0.03).<br/> Conclusions<br/> In normoglycaemic adults cutaneous microvascular vasodilatory capacity is associated with features of insulin resistance syndrome, particularly with plasminogen activator inhibitor type 1. A strong family history of Type 2 diabetes alone does not result in impairment in the maximum hyperaemic response
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