1,849 research outputs found
Metastable states, the adiabatic theorem and parity violating geometric phases II
We discuss and calculate parity conserving (PC) and parity violating (PV)
geometric phases for the metastable 2S states of hydrogen and deuterium. The
atoms are supposed to be subjected to slowly varying electric and magnetic
fields which act as external parameters for the atoms. Geometric flux density
fields are introduced which allow for an easy overview how to choose the paths
in parameter space in order to obtain only PC or only PV geometric phases. The
PV phases are calculated in the Standard Model of particle physics. Even if
numerically they come out small they have interest of principle as a new
manifestation of parity violation in atomic physics.Comment: 63 pages, 8 figures, 10 table
A search for variable white dwarfs in large area time domain surveys: a pilot study in SDSS Stripe 82
We present a method to reliably select variable white dwarfs from large area
time domain surveys and apply this method in a pilot study to search for
pulsating white dwarfs in the Sloan Digital Sky Survey Stripe 82. From a sample
400 high-confidence white dwarf candidates, we identify 24 which show
significant variability in their multi-epoch Stripe 82 data. Using colours, we
further selected a sample of pulsating white dwarf (ZZ Ceti) candidates and
obtained high cadence follow up for six targets. We confirm five of our
candidates as cool ZZ Cetis, three of which are new discoveries. Among our 24
candidates we also identify: one eclipsing binary, two magnetic white dwarfs
and one pulsating PG1159 star. Finally we discuss the possible causes for the
variability detected in the remaining targets. Even with sparse multi-epoch
data over the limited area of Stripe 82, we demonstrate that our selection
method can successfully identify various types of variable white dwarfs and
efficiently select high-confidence ZZ Ceti candidates.Comment: Accepted for publication in MNRAS, 14 pages, 11 figure
Efficient Implementation of Elastohydrodynamics via Integral Operators
The dynamics of geometrically non-linear flexible filaments play an important
role in a host of biological processes, from flagella-driven cell transport to
the polymeric structure of complex fluids. Such problems have historically been
computationally expensive due to numerical stiffness associated with the
inextensibility constraint, as well as the often non-trivial boundary
conditions on the governing high-order PDEs. Formulating the problem for the
evolving shape of a filament via an integral equation in the tangent angle has
recently been found to greatly alleviate this numerical stiffness. The
contribution of the present manuscript is to enable the simulation of non-local
interactions of multiple filaments in a computationally efficient manner using
the method of regularized stokeslets within this framework. The proposed method
is benchmarked against a non-local bead and link model, and recent code
utilizing a local drag velocity law. Systems of multiple filaments (1) in a
background fluid flow, (2) under a constant body force, and (3) undergoing
active self-motility are modeled efficiently. Buckling instabilities are
analyzed by examining the evolving filament curvature, as well as by
coarse-graining the body frame tangent angles using a Chebyshev approximation
for various choices of the relevant non-dimensional parameters. From these
experiments, insight is gained into how filament-filament interactions can
promote buckling, and further reveal the complex fluid dynamics resulting from
arrays of these interacting fibers. By examining active moment-driven
filaments, we investigate the speed of worm- and sperm-like swimmers for
different governing parameters. The MATLAB(R) implementation is made available
as an open-source library, enabling flexible extension for alternate
discretizations and different surrounding flows.Comment: 37 pages, 17 figure
Does the group leader matter? The impact of monitoring activities and social ties of group leaders on the repayment performance of groupbased lending Eritrea
When flux standards go wild: white dwarfs in the age of Kepler
White dwarf stars have been used as flux standards for decades, thanks to
their staid simplicity. We have empirically tested their photometric stability
by analyzing the light curves of 398 high-probability candidates and
spectroscopically confirmed white dwarfs observed during the original Kepler
mission and later with K2 Campaigns 0-8. We find that the vast majority (>97
per cent) of non-pulsating and apparently isolated white dwarfs are stable to
better than 1 per cent in the Kepler bandpass on 1-hr to 10-d timescales,
confirming that these stellar remnants are useful flux standards. From the
cases that do exhibit significant variability, we caution that binarity,
magnetism, and pulsations are three important attributes to rule out when
establishing white dwarfs as flux standards, especially those hotter than
30,000 K.Comment: Accepted for publication in MNRAS; 7 pages, 4 figures, 2 table
Does the group leader matter? The impact of monitoring activities and social ties of group leaders on the repayment performance of groupbased lending Eritrea
- …