48 research outputs found
Conformally symmetric vacuum solutions of the gravitational field equations in the brane-world models
A class of exact solutions of the gravitational field equations in the vacuum
on the brane are obtained by assuming the existence of a conformal Killing
vector field, with non-static and non-central symmetry. In this case the
general solution of the field equations can be obtained in a parametric form in
terms of the Bessel functions. The behavior of the basic physical parameters
describing the non-local effects generated by the gravitational field of the
bulk (dark radiation and dark pressure) is also considered in detail, and the
equation of state satisfied at infinity by these quantities is derived. As a
physical application of the obtained solutions we consider the behavior of the
angular velocity of a test particle moving in a stable circular orbit. The
tangential velocity of the particle is a monotonically increasing function of
the radial distance and, in the limit of large values of the radial coordinate,
tends to a constant value, which is independent on the parameters describing
the model. Therefore a brane geometry admitting a one-parameter group of
conformal motions may provide an explanation for the dynamics of the neutral
hydrogen clouds at large distances from the galactic center, which is usually
explained by postulating the existence of the dark matter.Comment: 15 pages, 5 figures, to appear in Annals of Physic
Could pressureless dark matter have pressure?
A two-fluid dark matter model, in which dark matter is represented as a
two-component fluid thermodynamic system, without interaction between the
constituent particles of different species, and with each distinct component
having a different four-velocity, was recently proposed in Harko & Lobo, [Phys.
Rev. D83, 124051 (2011)]. In the present paper we further investigate the
two-fluid dark matter model, by assuming that the two dark matter components
are pressureless, non-comoving fluids. For this particular choice of the
equations of state the dark matter distribution can be described as a single
anisotropic fluid, with vanishing tangential pressure, and non-zero radial
pressure. We investigate the properties of this model in the region of constant
velocity galactic rotation curves, where the dynamics of the test particles is
essentially determined by the dark matter only. By solving the general
relativistic equations of mass continuity and hydrostatic equilibrium we obtain
the geometric and physical parameters of the dark matter halos in the constant
velocity region in an exact analytical form. The general, radial coordinate
dependent, functional relationship between the energy density and the radial
pressure is also determined, and it differs from a simple barotropic equation
of state.Comment: 14 pages, no figures, major revision, accepted for publication in
Astroparticle Physics. arXiv admin note: substantial text overlap with
arXiv:1106.264
Galactic rotation curves and brane world models
In the present investigation flat rotational curves of the galaxies are
considered under the framework of brane-world models where the 4d effective
Einstein equation has extra terms which arise from the embedding of the 3-brane
in the bulk. It has been shown here that these long range bulk
gravitational degrees of freedom can act as a mechanism to yield the observed
galactic rotation curves without the need for dark matter. The present model
has the advantage that the observed rotation curves result solely from
well-established non-local effects of gravitation, such as dark radiation and
dark pressure under a direct use of the condition of flat rotation curves and
does not invoke any exotic matter field.Comment: 7 Latex pages, 7 figures; Edited substantially with new figures and
references; Accepted for Mon. Not. R. Astron. So
Galactic rotation curves in brane world models
In the braneworld scenario the four dimensional effective Einstein equation
has extra source terms, which arise from the embedding of the 3-brane in the
bulk. These non-local effects, generated by the free gravitational field of the
bulk, may provide an explanation for the dynamics of the neutral hydrogen
clouds at large distances from the galactic center, which is usually explained
by postulating the existence of the dark matter. In the present paper we
consider the asymptotic behavior of the galactic rotation curves in the brane
world models, and we compare the theoretical results with observations of both
High Surface Brightness and Low Surface Brightness galaxies. For the chosen
sample of galaxies we determine first the baryonic parameters by fitting the
photometric data to the adopted galaxy model; then we test the hypothesis of
the Weyl fluid acting as dark matter on the chosen sample of spiral galaxies by
fitting the tangential velocity equation of the combined baryonic-Weyl model to
the rotation curves. We give an analytical expression for the rotational
velocity of a test particle on a stable circular orbit in the exterior region
to a galaxy, with Weyl fluid contributions included. The model parameter ranges
for which the test provides agreement (within 1 confidence
level) with observations on the velocity fields of the chosen galaxy sample are
then determined. There is a good agreement between the theoretical predictions
and observations, showing that extra-dimensional models can be effectively used
as a viable alternative to the standard dark matter paradigm.Comment: to be published in MNRAS, 17 pages, 31 figures, version including
corrections on the proo
Features of galactic halo in a brane world model and observational constraints
Several aspects of the 4d imprint of the 5d bulk Weyl radiation are
investigated within a recently proposed model solution. It is shown that the
solution has a number of physically interesting properties. The constraints on
the model imposed by combined measurements of rotation curve and lensing are
discussed. A brief comparison with a well known scalar field model is also
given.Comment: 17 pages. Accepted for publication in Monthly Notices of the Royal
Society; Figures available at
http://www.sfu.ca/~adebened/research/brane_gal_rot_curves
Line-of-sight velocity dispersions and a mass distribution model of the Sa galaxy NGC 4594
In the present paper we develop an algorithm allowing to calculate
line-of-sight velocity dispersions in an axisymmetric galaxy outside of the
galactic plane. When constructing a self-consistent model, we take into account
the galactic surface brightness distribution, stellar rotation curve and
velocity dispersions. This algorithm is applied to a Sa galaxy NGC 4594 = M
104, for which there exist velocity dispersion measurements outside of the
galactic major axis. The mass distribution model is constructed in two stages.
In the first stage we construct a luminosity distribution model, where only
galactic surface brightness distribution is taken into account. Thereafter, in
the second stage we develop on the basis of the Jeans equations a detailed mass
distribution model and calculate line-of-sight velocity dispersions and the
stellar rotation curve. Here a dark matter halo is added to visible components.
Calculated dispersions are compared with observations along different slit
positions perpendicular and parallel to the galactic major axis. In the
best-fitting model velocity dispersion ellipsoids are radially elongated.
Outside the galactic plane velocity dispersion behaviour is more sensitive to
the dark matter density distribution and allows to estimate dark halo
parameters.Comment: 12 pages, 13 figures, accepted for publication in MNRA
Clostridial Glucosylating Toxins Enter Cells via Clathrin-Mediated Endocytosis
Clostridium difficile toxin A (TcdA) and toxin B (TcdB), C. sordellii lethal toxin (TcsL) and C. novyi α-toxin (TcnA) are important pathogenicity factors, which represent the family of the clostridial glucosylating toxins (CGTs). Toxin A and B are associated with antibiotic-associated diarrhea and pseudomembraneous colitis. Lethal toxin is involved in toxic shock syndrome after abortion and α-toxin in gas gangrene development. CGTs enter cells via receptor-mediated endocytosis and require an acidified endosome for translocation of the catalytic domain into the cytosol. Here we studied the endocytic processes that mediate cell internalization of the CGTs. Intoxication of cells was monitored by analyzing cell morphology, status of Rac glucosylation in cell lysates and transepithelial resistance of cell monolayers. We found that the intoxication of cultured cells by CGTs was strongly delayed when cells were preincubated with dynasore, a cell-permeable inhibitor of dynamin, or chlorpromazine, an inhibitor of the clathrin-dependent endocytic pathway. Additional evidence about the role of clathrin in the uptake of the prototypical CGT family member toxin B was achieved by expression of a dominant-negative inhibitor of the clathrin-mediated endocytosis (Eps15 DN) or by siRNA against the clathrin heavy chain. Accordingly, cells that expressed dominant-negative caveolin-1 were not protected from toxin B-induced cell rounding. In addition, lipid rafts impairment by exogenous depletion of sphingomyelin did not decelerate intoxication of HeLa cells by CGTs. Taken together, our data indicate that the endocytic uptake of the CGTs involves a dynamin-dependent process that is mainly governed by clathrin
A randomised pragmatic trial of corticosteroid optimization in severe asthma using a composite biomarker algorithm to adjust corticosteroid dose versus standard care: study protocol for a randomised trial
Background: Patients with difficult-to-control asthma consume 50–60% of healthcare costs attributed to asthma and cost approximately five-times more than patients with mild stable disease. Recent evidence demonstrates that not all patients with asthma have a typical type 2 (T2)-driven eosinophilic inflammation. These asthmatics have been called ‘T2-low asthma’ and have a minimal response to corticosteroid therapy. Adjustment of corticosteroid treatment using sputum eosinophil counts from induced sputum has demonstrated reduced severe exacerbation rates and optimized corticosteroid dose. However, it has been challenging to move induced sputum into the clinical setting. There is therefore a need to examine novel algorithms to target appropriate levels of corticosteroid treatment in difficult asthma, particularly in T2-low asthmatics. This study examines whether a composite non-invasive biomarker algorithm predicts exacerbation risk in patients with asthma on high-dose inhaled corticosteroids (ICS) (± long-acting beta agonist) treatment, and evaluates the utility of this composite score to facilitate personalized biomarker-specific titration of corticosteroid therapy.Methods/design: Patients recruited to this pragmatic, multi-centre, single-blinded randomised controlled trial are randomly allocated into either a biomarker controlled treatment advisory algorithm or usual care group in a ratio of 4:1. The primary outcome measure is the proportion of patients with any reduction in ICS or oral corticosteroid dose from baseline to week 48. Secondary outcomes include the rate of protocol-defined severe exacerbations per patient per year, time to first severe exacerbation from randomisation, dose of inhaled steroid at the end of the study, cumulative dose of inhaled corticosteroid during the study, proportion of patients on oral corticosteroids at the end of the study, proportion of patients who decline to progress to oral corticosteroids despite composite biomarker score of 2, frequency of hospital admission for asthma, change in the 7-item Asthma Control Questionnaire (ACQ-7), Asthma Quality of Life Questionnaire (AQLQ), forced expiratory volume in 1 s (FEV1), exhaled nitric oxide, blood eosinophil count, and periostin levels from baseline to week 48. Blood will also be taken for whole blood gene expression; serum, plasma, and urine will be stored for validation of additional biomarkers.Discussion: Multi-centre trials present numerous logistical issues that have been addressed to ensure minimal bias and robustness of study conduct.Trial registration: ClinicalTrials.gov, NCT02717689. Registered on 16 March 2016