684 research outputs found
Mechanisms for Decreased Function of B Cells in Aged Mice and Humans
Abstract
The immune system has been known for some time to be compromised in aged individuals, e.g., both mice and humans, and in both humoral and cellular responses. Our studies have begun to elucidate intrinsic B lymphocyte defects in Ig class switch recombination, activation-induced cytidine deaminase, and E47 transcription factor expression. These defects occur in both mice and humans. Our studies have also shown that tristetraprolin is one of the key players in regulating the decreased E47 mRNA stability in aged B lymphocytes. These and current studies should lead to improvements in B lymphocyte function in aged populations
Theoretical values of convective turnover times and Rossby numbers for solar-like, pre-main sequence stars
Magnetic fields are at the heart of the observed stellar activity in
late-type stars, and they are presumably generated by a dynamo mechanism at the
interface layer between the radiative and the convective stellar regions. Since
dynamo models are based on the interaction between differential rotation and
convective motions, the introduction of rotation in the ATON 2.3 stellar code
allows for explorations regarding a physically consistent treatment of magnetic
effects in stellar structure and evolution, even though there are formidable
mathematical and numerical challenges involved. As examples, we present
theoretical estimates for both the local (tau_c) and global (tau_g) convective
turnover times for rotating pre-main sequence solar-type stars, based on
up-to-date input physics for stellar models. Our theoretical predictions are
compared with the previous ones available in the literature. In addition, we
investigate the dependence of the convective turnover time on convection
regimes, the presence of rotation and atmospheric treatment. Those estimates,
this quantities can be used to calculate the Rossby number, Ro, which is
related to the magnetic activity strength in dynamo theories and, at least for
main-sequence stars, shows an observational correlation with stellar activity.
More important, they can also contribute for testing stellar models against
observations. Our theoretical values of tau_c, tau_g and Ro qualitatively agree
with those published by Kim & Demarque (1996). By increasing the convection
efficiency, tau_g decreases for a given mass. FST models show still lower
values. The presence of rotation shifts tau_g towards slightly higher values
when compared with non-rotating models. The use of non-gray boundary conditions
in the models yields values of tau_g smaller than in the gray approximation.Comment: 10 pages, 14 figures, accepted for publication in A&
Exciton and negative trion dissociation by an external electric field in vertically coupled quantum dots
We study the Stark effect for an exciton confined in a pair of vertically
coupled quantum dots. A single-band approximation for the hole and a parabolic
lateral confinement potential are adopted which allows for the separation of
the lateral center-of-mass motion and consequently for an exact numerical
solution of the Schr\"odinger equation. We show that for intermediate tunnel
coupling the external electric field leads to the dissociation of the exciton
via an avoided crossing of bright and dark exciton energy levels which results
in an atypical form of the Stark shift. The electric-field-induced dissociation
of the negative trion is studied using the approximation of frozen lateral
degrees of freedom. It is shown that in a symmetric system of coupled dots the
trion is more stable against dissociation than the exciton. For an asymmetric
system of coupled dots the trion dissociation is accompanied by a positive
curvature of the recombination energy line as a function of the electric field.Comment: PRB - in prin
Exploiting exciton-exciton interactions in semiconductor quantum dots for quantum-information processing
We propose an all-optical implementation of quantum-information processing in
semiconductor quantum dots, where electron-hole excitations (excitons) serve as
the computational degrees of freedom (qubits). We show that the strong dot
confinement leads to an overall enhancement of Coulomb correlations and to a
strong renormalization of the excitonic states, which can be exploited for
performing conditional and unconditional qubit operations.Comment: 5 pages revtex, 2 encapsulated postscript figures. Accepted for
publication in Phys. Rev. B (Rapid Communication
Absence of correlation between built-in electric dipole moment and quantum Stark effect in InAs/GaAs self-assembled quantum dots
We report significant deviations from the usual quadratic dependence of the
ground state interband transition energy on applied electric fields in
InAs/GaAs self-assembled quantum dots. In particular, we show that conventional
second-order perturbation theory fails to correctly describe the Stark shift
for electric field below kV/cm in high dots. Eight-band calculations demonstrate this effect is predominantly due to
the three-dimensional strain field distribution which for various dot shapes
and stoichiometric compositions drastically affects the hole ground state. Our
conclusions are supported by two independent experiments.Comment: 4 pages, 4 figure
Time-resolved spectroscopy of multi-excitonic decay in an InAs quantum dot
The multi-excitonic decay process in a single InAs quantum dot is studied
through high-resolution time-resolved spectroscopy. A cascaded emission
sequence involving three spectral lines is seen that is described well over a
wide range of pump powers by a simple model. The measured biexcitonic decay
rate is about 1.5 times the single-exciton decay rate. This ratio suggests the
presence of selection rules, as well as a significant effect of the Coulomb
interaction on the biexcitonic wavefunction.Comment: one typo fixe
Combined effects of tidal and rotational distortions on the equilibrium configuration of low-mass, pre-main sequence stars
In close binary systems, rotation and tidal forces of the component stars
deform each other and destroy their spherical symmetry. We present new models
for low-mass, pre-main sequence stars that include the combined distortion
effects of tidal and rotational forces on the equilibrium configuration of
stars. We investigate the effects of interaction between tides and rotation on
the stellar structure and evolution. The Kippenhahn & Thomas (1970)
approximation, along with the Clairaut-Legendre expansion for the gravitational
potential of a self-gravitating body, is used to take the distortion effects
into account. We obtained values of internal structure constants for low-mass,
pre-main sequence stars from stellar evolutionary models that consider the
combined effects of rotation and tidal forces due to a companion star. We also
derived a new expression for the rotational inertia of a tidally and
rotationally distorted star. Our distorted models were successfully used to
analyze the eclipsing binary system EK Cep, reproducing the stellar radii,
effective temperature ratio, lithium depletion, rotational velocities, and the
apsidal motion rate in the age interval of 15.5-16.7 Myr. In the low-mass
range, the assumption that harmonics greater than j=2 can be neglected seems
not to be fully justified, although it is widely used when analyzing the
apsidal motion of binary systems. The non-standard evolutionary tracks are
cooler than the standard ones, mainly for low-mass stars. Distorted models
predict more mass-concentrated stars at the zero-age main-sequence than
standard models
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