528 research outputs found
An L^2-Index Theorem for Dirac Operators on S^1 * R^3
An expression is found for the -index of a Dirac operator coupled to a
connection on a vector bundle over . Boundary
conditions for the connection are given which ensure the coupled Dirac operator
is Fredholm. Callias' index theorem is used to calculate the index when the
connection is independent of the coordinate on . An excision theorem due
to Gromov, Lawson, and Anghel reduces the index theorem to this special case.
The index formula can be expressed using the adiabatic limit of the
-invariant of a Dirac operator canonically associated to the boundary. An
application of the theorem is to count the zero modes of the Dirac operator in
the background of a caloron (periodic instanton).Comment: 14 pages, Latex, to appear in the Journal of Functional Analysi
Enhancement of charged macromolecule capture by nanopores in a salt gradient
Nanopores spanning synthetic membranes have been used as key components in
proof-of-principle nanofluidic applications, particularly those involving
manipulation of biomolecules or sequencing of DNA. The only practical way of
manipulating charged macromolecules near nanopores is through a voltage
difference applied across the nanopore-spanning membrane. However, recent
experiments have shown that salt concentration gradients applied across
nanopores can also dramatically enhance charged particle capture from a low
concentration reservoir of charged molecules at one end of the nanopore. This
puzzling effect has hitherto eluded a physically consistent theoretical
explanation. Here, we propose an electrokinetic mechanism of this enhanced
capture that relies on the electrostatic potential near the pore mouth. For
long pores with diameter much greater than the local screening length, we
obtain accurate analytic expressions showing how salt gradients control the
local conductivity which can lead to increased local electrostatic potentials
and charged analyte capture rates. We also find that the attractive
electrostatic potential may be balanced by an outward, repulsive electroosmotic
flow (EOF) that can in certain cases conspire with the salt gradient to further
enhance the analyte capture rate.Comment: 10 pages, 6 Figure
Cover Crop Selection and Management for Agronomic Farming Systems
Cover crops can extend the season of active nutrient uptake and living soil cover and thereby reduce nutrient losses in water and sediment. The conversion of the prairies or other native vegetation ecosystems to summer annual grain crops resulted in a shortening of the season of living plant cover and nutrient uptake. Summer annual grain crops, like corn and soybean, accumulate water and nutrients and provide living cover for only about four months (mid-May to mid-September), whereas in natural systems, some living plants are actively accumulating nutrients and water whenever the ground is not frozen (at least 7 months; April-October). As a result, soil nutrients in summer annual cropping systems are susceptible to losses in part because there are periods during each year when active plant uptake and soil cover are absent
Effects of Streambed Morphology and Biofilm Growth on the Transient Storage of Solutes
Microbial biofilms are the prime site of nutrient and contaminant
removal in streams. It is therefore essential to understand
how biofilms affect hydrodynamic exchange, solute transport,
and retention in systems where geomorphology and induced
hydrodynamics shape their growth and structure. We
experimented with large-scale streamside flumes with
streambed landscapes constructed from graded bedforms of
constant height and wavelength. Each flume had a different
bedform height and was covered with a layer of gravel
as substratum for benthic microbial biofilms. Biofilms developed
different biomass and physical structures in response to the
hydrodynamic conditions induced by the streambed morphology.
Step injections of conservative tracers were performed at
different biofilm growth stages. The experimental breakthrough
curves were analyzed with the STIR model, using a residence
time approach to characterize the retention effects associated
with biofilms. The retained mass of the solute increased with
biofilm biomass and the biofilm-associated retention was
furthermore related to bedform height. We tentatively relate
this behavior to biofilm structural differentiation induced by bed
morphology, which highlights the strong linkage between
geomorphology, hydrodynamics, and biofilms in natural streams
and provide important clues for stream restoration
The First Two Years of Electromagnetic Follow-Up with Advanced LIGO and Virgo
We anticipate the first direct detections of gravitational waves (GWs) with
Advanced LIGO and Virgo later this decade. Though this groundbreaking technical
achievement will be its own reward, a still greater prize could be observations
of compact binary mergers in both gravitational and electromagnetic channels
simultaneously. During Advanced LIGO and Virgo's first two years of operation,
2015 through 2016, we expect the global GW detector array to improve in
sensitivity and livetime and expand from two to three detectors. We model the
detection rate and the sky localization accuracy for binary neutron star (BNS)
mergers across this transition. We have analyzed a large, astrophysically
motivated source population using real-time detection and sky localization
codes and higher-latency parameter estimation codes that have been expressly
built for operation in the Advanced LIGO/Virgo era. We show that for most BNS
events the rapid sky localization, available about a minute after a detection,
is as accurate as the full parameter estimation. We demonstrate that Advanced
Virgo will play an important role in sky localization, even though it is
anticipated to come online with only one-third as much sensitivity as the
Advanced LIGO detectors. We find that the median 90% confidence region shrinks
from ~500 square degrees in 2015 to ~200 square degrees in 2016. A few distinct
scenarios for the first LIGO/Virgo detections emerge from our simulations.Comment: 17 pages, 11 figures, 5 tables. For accompanying data, see
http://www.ligo.org/scientists/first2year
Dirichlet process Gaussian-mixture model:an application to localizing coalescing binary neutron stars with gravitational-wave observations
We reconstruct posterior distributions for the position (sky area and distance) of a simulated set of binary neutron star gravitational-waves signals observed with Advanced LIGO and Advanced Virgo. We use a Dirichlet process Gaussian-mixture model, a fully Bayesian non-parametric method that can be used to estimate probability density functions with a flexible set of assumptions. The ability to reliably reconstruct the source position is important for multimessenger astronomy, as recently demonstrated with GW170817. We show that for detector networks comparable to the early operation of Advanced LIGO and Advanced Virgo, typical localization volumes are similar to 10(4)-10(5 similar to)Mpc(3) corresponding to similar to 10(2)-10(3) potential host galaxies. The localization volume is a strong function of the network signal-to-noise ratio, scaling roughly proportional to e(net)(-6). Fractional localizations improve with the addition of further detectors to the network. Our Dirichlet process Gaussian-mixture model can be adopted for localizing events detected during future gravitational-wave observing runs and used to facilitate prompt multimessenger follow-up
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