20,939 research outputs found
Magnetic Doppler imaging of the roAp star HD 24712
We present the first magnetic Doppler images of a rapidly oscillating Ap
(roAp) star.
We deduce information about magnetic field geometry and abundance
distributions of a number of chemical elements on the surface of the hitherto
best studied roAp star, HD 24712, using the magnetic Doppler imaging (MDI)
code, INVERS10, which allows us to reconstruct simultaneously and consistently
the magnetic field geometry and elemental abundance distributions on a stellar
surface. For this purpose we analyse time series spectra obtained in Stokes I
and V parameters with the SOFIN polarimeter at the Nordic Optical Telescope and
recover surface abundance structures of sixteen different chemical elements,
respectively ions, including Mg, Ca, Sc, Ti, Cr, Fe, Co, Ni, Y, La, Ce, Pr, Nd,
Gd, Tb, and Dy. For the rare earth elements (REE) Pr and Nd separate maps were
obtained using lines of the first and the second ionization stage.
We find and confirm a clear dipolar structure of the surface magnetic field
and an unexpected correlation of elemental abundances with respect to this
field: one group of elements accumulates solely where the positive magnetic
pole is visible, whereas the other group avoids this region and is enhanced
where the magnetic equatorial region dominates the visible stellar surface. We
also observe relative shifts of abundance enhancement- or depletion regions
between the various elements exhibiting otherwise similar behaviour.Comment: 13 pages, 9 figures, to be published in Astronomy and Astrophysic
Ground-state energy and depletions for a dilute binary Bose gas
When calculating the ground-state energy of a weakly interacting Bose gas
with the help of the customary contact pseudopotential, one meets an artifical
ultraviolet divergence which is caused by the incorrect treatment of the true
interparticle interactions at small distances. We argue that this problem can
be avoided by retaining the actual, momentum-dependent interaction matrix
elements, and use this insight for computing both the ground-state energy and
the depletions of a binary Bose gas mixture. Even when considering the
experimentally relevant case of equal masses of both species, the resulting
expressions are quite involved, and no straightforward generalizations of the
known single-species formulas. On the other hand, we demonstrate in detail how
these latter formulas are recovered from our two-species results in the limit
of vanishing interspecies interaction.Comment: 11 pages, Phys. Rev. A in pres
Lattice Black Holes
We study the Hawking process on lattices falling into static black holes. The
motivation is to understand how the outgoing modes and Hawking radiation can
arise in a setting with a strict short distance cutoff in the free-fall frame.
We employ two-dimensional free scalar field theory. For a falling lattice with
a discrete time-translation symmetry we use analytical methods to establish
that, for Killing frequency and surface gravity satisfying
in lattice units, the continuum Hawking spectrum
is recovered. The low frequency outgoing modes arise from exotic ingoing modes
with large proper wavevectors that "refract" off the horizon. In this model
with time translation symmetry the proper lattice spacing goes to zero at
spatial infinity. We also consider instead falling lattices whose proper
lattice spacing is constant at infinity and therefore grows with time at any
finite radius. This violation of time translation symmetry is visible only at
wavelengths comparable to the lattice spacing, and it is responsible for
transmuting ingoing high Killing frequency modes into low frequency outgoing
modes.Comment: 26 pages, LaTeX, 2 figures included with psfig. Several improvements
in the presentation. One figure added. Final version to appear in Phys.Rev.
p21 is decreased in polycystic kidney disease and leads to increased epithelial cell cycle progression: roscovitine augments p21 levels.
BackgroundAutosomal dominant polycystic kidney disease (ADPKD) is a common genetic disease with few treatment options other than renal replacement therapy. p21, a cyclin kinase inhibitor which has pleiotropic effects on the cell cycle, in many cases acts to suppress cell cycle progression and to prevent apoptosis. Because defects in cell cycle arrest and apoptosis of renal tubular epithelial cells occur in PKD, and in light of earlier reports that polycystin-1 upregulates p21 and that the cyclin-dependent kinase inhibitor roscovitine arrests progression in a mouse model, we asked whether (1) p21 deficiency might underlie ADPKD and (2) the mechanism of the salutary roscovitine effect on PKD involves p21.Methodsp21 levels in human and animal tissue samples as well as cell lines were examined by immunoblotting and/or immunohistochemisty. Apoptosis was assessed by PARP cleavage. p21 expression was attenuated in a renal tubular epithelial cell line by antisense methods, and proliferation in response to p21 attenuation and to roscovitine was assessed by the MTT assay.ResultsWe show that p21 is decreased in human as well as a non-transgenic rat model of ADPKD. In addition, hepatocyte growth factor, which induces transition from a cystic to a tubular phenotype, increases p21 levels. Furthermore, attenuation of p21 results in augmentation of cell cycle transit in vitro. Thus, levels of p21 are inversely correlated with renal tubular epithelial cell proliferation. Roscovitine, which has been shown to arrest progression in a murine model of PKD, increases p21 levels and decreases renal tubular epithelial cell proliferation, with no affect on apoptosis.ConclusionThe novelty of our study is the demonstration in vivo in humans and rat models of a decrement of p21 in cystic kidneys as compared to non-cystic kidneys. Validation of a potential pathogenetic model of increased cyst formation due to enhanced epithelial proliferation and apoptosis mediated by p21 suggests a mechanism for the salutary effect of roscovitine in ADPKD and supports further investigation of p21 as a target for future therapy
Dynamics of Interacting Scalar Fields in Expanding Space-Time
The effective equation of motion is derived for a scalar field interacting
with other fields in a Friedman-Robertson-Walker background space-time. The
dissipative behavior reflected in this effective evolution equation is studied
both in simplified approximations as well as numerically. The relevance of our
results to inflation are considered both in terms of the evolution of the
inflaton field as well as its fluctuation spectrum. A brief examination also is
made of supersymmetric models that yield dissipative effects during inflation.Comment: 36 pages, 12 figures. Version published in the Physical Review
A nanomechanical resonator shuttling single electrons at radio frequencies
We observe transport of electrons through a metallic island on the tip of a
nanomechanical pendulum. The resulting tunneling current shows distinct
features corresponding to the discrete mechanical eigenfrequencies of the
pendulum. We report on measurements covering the temperature range from 300 K
down to 4.2 K. We explain the I-V curve, which differs from previous
theoretical predictions, with model calculations based on a Master equation
approach.Comment: 5 pages, 4 jpeg-figure
Reconstitution of T cell receptor signaling in ZAP-70-deficient cells by retroviral transduction of the ZAP-70 gene.
A variant of severe combined immunodeficiency syndrome (SCID) with a selective inability to produce CD8 single positive T cells and a signal transduction defect in peripheral CD4+ cells has recently been shown to be the result of mutations in the ZAP-70 gene. T cell receptor (TCR) signaling requires the association of the ZAP-70 protein tyrosine kinase with the TCR complex. Human T cell leukemia virus type I-transformed CD4+ T cell lines were established from ZAP-70-deficient patients and normal controls. ZAP-70 was expressed and appropriately phosphorylated in normal T cell lines after TCR engagement, but was not detected in T cell lines from ZAP-70-deficient patients. To determine whether signaling could be reconstituted, wild-type ZAP-70 was introduced into deficient cells with a ZAP-70 retroviral vector. High titer producer clones expressing ZAP-70 were generated in the Gibbon ape leukemia virus packaging line PG13. After transduction, ZAP-70 was detected at levels equivalent to those observed in normal cells, and was appropriately phosphorylated on tyrosine after receptor engagement. The kinase activity of ZAP-70 in the reconstituted cells was also appropriately upregulated by receptor aggregation. Moreover, normal and transduced cells, but not ZAP-70-deficient cells, were able to mobilize calcium after receptor ligation, indicating that proximal TCR signaling was reconstituted. These results indicate that this form of SCID may be corrected by gene therapy
Z_N Phases in Hot Gauge Theories
We argue that the \zn phases of hot gauge theories cannot be realized as a
real system with an Hermitean density matrix.Comment: 7 page
Damage-free single-mode transmission of deep-UV light in hollow-core PCF
Transmission of UV light with high beam quality and pointing stability is
desirable for many experiments in atomic, molecular and optical physics. In
particular, laser cooling and coherent manipulation of trapped ions with
transitions in the UV require stable, single-mode light delivery. Transmitting
even ~2 mW CW light at 280 nm through silica solid-core fibers has previously
been found to cause transmission degradation after just a few hours due to
optical damage. We show that photonic crystal fiber of the kagom\'e type can be
used for effectively single-mode transmission with acceptable loss and bending
sensitivity. No transmission degradation was observed even after >100 hours of
operation with 15 mW CW input power. In addition it is shown that
implementation of the fiber in a trapped ion experiment significantly increases
the coherence times of the internal state transfer due to an increase in beam
pointing stability
The MHD Kelvin-Helmholtz Instability II: The Roles of Weak and Oblique Fields in Planar Flows
We have carried out high resolution MHD simulations of the nonlinear
evolution of Kelvin-Helmholtz unstable flows in 2 1/2 dimensions. The modeled
flows and fields were initially uniform except for a thin shear layer with a
hyperbolic tangent velocity profile and a small, normal mode perturbation. The
calculations consider periodic sections of flows containing magnetic fields
parallel to the shear layer, but projecting over a full range of angles with
respect to the flow vectors. They are intended as preparation for fully 3D
calculations and to address two specific questions raised in earlier work: 1)
What role, if any, does the orientation of the field play in nonlinear
evolution of the MHD Kelvin-Helmholtz instability in 2 1/2 D. 2) Given that the
field is too weak to stabilize against a linear perturbation of the flow, how
does the nonlinear evolution of the instability depend on strength of the
field. The magnetic field component in the third direction contributes only
through minor pressure contributions, so the flows are essentially 2D. Even a
very weak field can significantly enhance the rate of energy dissipation. In
all of the cases we studied magnetic field amplification by stretching in the
vortex is limited by tearing mode, ``fast'' reconnection events that isolate
and then destroy magnetic flux islands within the vortex and relax the fields
outside the vortex. If the magnetic tension developed prior to reconnection is
comparable to Reynolds stresses in the flow, that flow is reorganized during
reconnection. Otherwise, the primary influence on the plasma is generation of
entropy. The effective expulsion of flux from the vortex is very similar to
that shown by Weiss for passive fields in idealized vortices with large
magnetic Reynolds numbers. We demonstrated that thisComment: 23 pages of ApJ Latex (aaspp4.sty) with 10 figures, high resolution
postscript images for figs 4-9 available through anonymous at
ftp://ftp.msi.umn.edu/pub/twj To appear in the June 10, 1997 Ap
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