863 research outputs found
Classical Nucleation Theory of the One-Component Plasma
We investigate the crystallization rate of a one-component plasma (OCP) in
the context of classical nucleation theory. From our derivation of the free
energy of an arbitrary distribution of solid clusters embedded in a liquid
phase, we derive the steady-state nucleation rate of an OCP as a function of
the Coulomb coupling parameter. Our result for the rate is in accord with
recent molecular dynamics simulations, but it is greater than that of previous
analytical estimates by many orders of magnitude. Further molecular dynamics
simulations of the nucleation rate of a supercooled liquid OCP for several
values of the coupling parameter would clarify the physics of this process.Comment: 6 pages, 1 figure, accepted by PR
A Novel Massage Therapy Technique for Management of Chronic Cervical Pain: A Case Series
Background: Neck pain is a generalized condition resulting from a complex etiology with presentation of a wide variety of symptoms. Neck pain is most often accompanied by decreased range of motion (ROM), muscle and joint stiffness, and limitations in functional capabilities. This condition may result in significant personal and societal burden.
Purpose: We evaluated the effectiveness of a novel massage therapy intervention by following the treatment regimen and outcomes of two patients experiencing chronic neck pain.
Participants: Two patients (46 and 53 years old) experienced chronic (>5 years) neck pain. Both patients reported pain, limited ROM, and muscle and joint stiffness. Additionally, the first patient reported a lack of sleep, and both patients stated their pain interfered with their quality of life and activities of daily living.
Intervention: Patients received the Integrative Muscular Movement Technique (IMMT) intervention approximately twice a week for a total of eight treatments, each approximately 20 minutes in duration.
Results: Both patients experienced a reduction in pain and an increase in cervical ROM in flexion, extension, rotation, and sidebending. The first patient also reported an increased ability to sleep. Both patients reported an increased ability to perform activities of daily living, including work-related responsibilities.
Conclusions: For the two patients included in this report, therapist observations and patient reports indicate that inclusion of the IMMT treatment in a treatment regimen for chronic neck pain may lead to decreased pain and increased cervical ROM. These positive effects of the IMMT intervention may have a role in enhancing functional outcomes of these patients
Theoretical Models of Superbursts on Accreting Neutron Stars
We carry out a general-relativistic global linear stability analysis of the
amassed carbon fuel on the surface of an accreting neutron star to determine
the conditions under which superbursts occur. We reproduce the general
observational characteristics of superbursts, including burst fluences,
recurrence times, and the absence of superbursts on stars with accretion rates
below 10% of the Eddington limit. By comparing our results with observations,
we are able to set constraints on neutron star parameters such as the stellar
radius and neutrino cooling mechanism in the core. Specifically, we find that
accreting neutron stars with ordered crusts and highly efficient neutrino
emission in their cores (due to direct URCA or pionic reactions, for example)
produce extremely energetic (> 10^44 ergs) superbursts which are inconsistent
with observations, in agreement with previous investigations. Also, because of
pycnonuclear burning of carbon, they do not have superbursts in the range of
accretion rates at which superbursts are actually observed unless the crust is
very impure. Stars with less efficient neutrino emission (due to modified URCA
reactions, for example) produce bursts that agree better with observations.
Stars with highly inefficient neutrino emission in their cores produce bursts
that agree best with observations. All systems that accrete primarily hydrogen
and in which superbursts are observed show evidence of H- and He-burning
delayed mixed bursts. We speculate that delayed mixed bursts provide sufficient
amounts of carbon fuel for superbursts and are thus a prerequisite for having
superbursts. We compare our global stability analysis to approximate one-zone
criteria used by other authors and identify a particular set of approximations
that give accurate results for most choices of parameters. (abstract truncated)Comment: 43 pages, 18 figures, accepted by Ap
Ignition column depths of helium-rich thermonuclear bursts from 4U 1728-34
We analysed thermonuclear (type-I) X-ray bursts observed from the low-mass
X-ray binary 4U1728-34 by RXTE, Chandra and INTEGRAL. We compared the variation
in burst energy and recurrence times as a function of accretion rate with the
predictions of a numerical ignition model including a treatment of the heating
and cooling in the crust. We found that the measured burst ignition column
depths are significantly below the theoretically predicted values, regardless
of the assumed thermal structure of the neutron star interior. While it is
possible that the accretion rate measured by Chandra is underestimated, due to
additional persistent spectral components outside the sensitivity band, the
required correction factor is typically 3.6 and as high as 6, which is
implausible. Furthermore, such underestimation is even more unlikely for RXTE
and INTEGRAL, which have much broader bandpasses. Possible explanations for the
observed discrepancy include shear-triggered mixing of the accreted helium to
larger column depths, resulting in earlier ignition, or the fractional covering
of the accreted fuel on the neutron star surface.Comment: 12 pages, 5 figures, accepted for publication in Ap
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Molecular Determinants and Dynamics of Hepatitis C Virus Secretion
The current model of hepatitis C virus (HCV) production involves the assembly of virions on or near the surface of lipid droplets, envelopment at the ER in association with components of VLDL synthesis, and egress via the secretory pathway. However, the cellular requirements for and a mechanistic understanding of HCV secretion are incomplete at best. We combined an RNA interference (RNAi) analysis of host factors for infectious HCV secretion with the development of live cell imaging of HCV core trafficking to gain a detailed understanding of HCV egress. RNAi studies identified multiple components of the secretory pathway, including ER to Golgi trafficking, lipid and protein kinases that regulate budding from the trans-Golgi network (TGN), VAMP1 vesicles and adaptor proteins, and the recycling endosome. Our results support a model wherein HCV is infectious upon envelopment at the ER and exits the cell via the secretory pathway. We next constructed infectious HCV with a tetracysteine (TC) tag insertion in core (TC-core) to monitor the dynamics of HCV core trafficking in association with its cellular cofactors. In order to isolate core protein movements associated with infectious HCV secretion, only trafficking events that required the essential HCV assembly factor NS2 were quantified. TC-core traffics to the cell periphery along microtubules and this movement can be inhibited by nocodazole. Sub-populations of TC-core localize to the Golgi and co-traffic with components of the recycling endosome. Silencing of the recycling endosome component Rab11a results in the accumulation of HCV core at the Golgi. The majority of dynamic core traffics in association with apolipoprotein E (ApoE) and VAMP1 vesicles. This study identifies many new host cofactors of HCV egress, while presenting dynamic studies of HCV core trafficking in infected cells.</p
The Latitude of Type I X-Ray Burst Ignition on Rapidly Rotating Neutron Stars
We investigate the latitude at which type I X-ray bursts are ignited on
rapidly rotating accreting neutron stars. We find that, for a wide range of
accretion rates, ignition occurs preferentially at the equator, in accord with
the work of Spitkovsky et al. However, for a range of accretion rates below the
critical rate above which bursts cease, ignition occurs preferentially at
higher latitudes. The range of accretion rates over which nonequatorial
ignition occurs is an increasing function of the neutron star spin frequency.
These findings have significant implications for thermonuclear flame
propagation, and they may explain why oscillations during the burst rise are
detected predominantly when the accretion rate is high. They also support the
suggestion of Bhattacharyya & Strohmayer that non-photospheric radius expansion
double-peaked bursts and the unusual harmonic content of oscillations during
the rise of some bursts result from ignition at or near a rotational pole.Comment: 4 pages, 2 figures, accepted by ApJ
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RNA Interference and Single Particle Tracking Analysis of Hepatitis C Virus Endocytosis
Hepatitis C virus (HCV) enters hepatocytes following a complex set of receptor interactions, culminating in internalization via clathrin-mediated endocytosis. However, aside from receptors, little is known about the cellular molecular requirements for infectious HCV entry. Therefore, we analyzed a siRNA library that targets 140 cellular membrane trafficking genes to identify host genes required for infectious HCV production and HCV pseudoparticle entry. This approach identified 16 host cofactors of HCV entry that function primarily in clathrin-mediated endocytosis, including components of the clathrin endocytosis machinery, actin polymerization, receptor internalization and sorting, and endosomal acidification. We next developed single particle tracking analysis of highly infectious fluorescent HCV particles to examine the co-trafficking of HCV virions with cellular cofactors of endocytosis. We observe multiple, sequential interactions of HCV virions with the actin cytoskeleton, including retraction along filopodia, actin nucleation during internalization, and migration of internalized particles along actin stress fibers. HCV co-localizes with clathrin and the ubiquitin ligase c-Cbl prior to internalization. Entering HCV particles are associated with the receptor molecules CD81 and the tight junction protein, claudin-1; however, HCV-claudin-1 interactions were not restricted to Huh-7.5 cell-cell junctions. Surprisingly, HCV internalization generally occurred outside of Huh-7.5 cell-cell junctions, which may reflect the poorly polarized nature of current HCV cell culture models. Following internalization, HCV particles transport with GFP-Rab5a positive endosomes, which is consistent with trafficking to the early endosome. This study presents technical advances for imaging HCV entry, in addition to identifying new host cofactors of HCV infection, some of which may be antiviral targets.</p
Gauge-invariant fluctuations of scalar branes
A generalization of the Bardeen formalism to the case of warped geometries is
presented. The system determining the gauge-invariant fluctuations of the
metric induced by the scalar fluctuations of the brane is reduced to a set of
Schr\"odinger-like equations for the Bardeen potentials and for the canonical
normal modes of the scalar-tensor action. Scalar, vector and tensor modes of
the geometry are classified according to four-dimensional Lorentz
transformations. While the tensor modes of the geometry live on the brane
determining the corrections to Newton law, the scalar and and vector
fluctuations exhibit non normalizable zero modes and are, consequently, not
localized on the brane. The spectrum of the massive modes of the fluctuations
is analyzed using supersymmetric quantum mechanics.Comment: 29 pages in Latex styl
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