254 research outputs found
NS Branes in Type I Theory
We consider novel nonperturbative effects of type I theories compactified on
singular ALE spaces obtained by adding NS branes. Such effects include a
description of small instantons at singularities.Comment: 6 pages, Harvmac.tex, updated referenc
Calculation of Nonperturbative Terms in Open String Models
Nonperturbative corrections in type II string theory corresponding to Riemann
surfaces with one boundary are calculated in several noncompact geometries of
desingularized orbifolds. One of these models has a complicated phase structure
which is explored. A general condition for integrality of the numerical
invariants is discussed
Spatial and Temporal Relationships Between Forest Bird Declines and Prevalence of the Hemlock Woolly Adelgid in the Northeastern United States
The eastern hemlock (Tsuga canadensis) is a vital foundation tree species throughout the eastern United States, providing essential structural diversity and habitat for more than 120 different animal species. Within the past few decades, T. canadensis has undergone significant declines that are largely associated with the hemlock woolly adelgid (HWA; Adelges tsugae), an exotic, aphid-like insect native to East Asia. From the 1970s to present day, the HWA has spread throughout southern New England, large portions of the Mid-Atlantic region, and parts of Tennessee and the Carolinas. Research has shown that loss of the eastern hemlock is drastically altering forest community structures, potentially impacting a wide variety of forest fauna, including avian populations strongly associated with hemlock forests. Here we present research investigating the correlation between HWA prevalence and recent declines of hemlock-associated forest birds in the Eastern US. We analyzed bird population trends data from the North American Breeding Bird Survey (BBS), US Forest Service HWA data, and land cover data to analyze the population trends of hemlock-associated and forest generalist species in association with the arrival of HWA, taking hemlock density into account. We found a significant correlation between the timing of HWA arrival and declines of conifer forest specialist birds. The Black-throated Green Warbler and the Blue-headed Vireo exhibited significant decline along survey routes after HWA arrival. Populations of some forest generalists (Tufted Titmouse, White-Breasted Nuthatch) were unaffected and continued to increase linearly, while others (Red-Eyed Vireo, Ovenbird) showed minor decrease in population
The Suborbital Particle Aggregation and Collision Experiment (SPACE): Studying the Collision Behavior of Submillimeter-Sized Dust Aggregates on the Suborbital Rocket Flight REXUS 12
The Suborbital Particle Aggregation and Collision Experiment (SPACE) is a
novel approach to study the collision properties of submillimeter-sized, highly
porous dust aggregates. The experiment was designed, built and carried out to
increase our knowledge about the processes dominating the first phase of planet
formation. During this phase, the growth of planetary precursors occurs by
agglomeration of micrometer-sized dust grains into aggregates of at least
millimeters to centimeters in size. However, the formation of larger bodies
from the so-formed building blocks is not yet fully understood. Recent
numerical models on dust growth lack a particular support by experimental
studies in the size range of submillimeters, because these particles are
predicted to collide at very gentle relative velocities of below 1 cm/s that
can only be achieved in a reduced-gravity environment.
The SPACE experiment investigates the collision behavior of an ensemble of
silicate-dust aggregates inside several evacuated glass containers which are
being agitated by a shaker to induce the desired collisions at chosen
velocities. The dust aggregates are being observed by a high-speed camera,
allowing for the determination of the collision properties of the
protoplanetary dust analog material. The data obtained from the suborbital
flight with the REXUS (Rocket Experiments for University Students) 12 rocket
will be directly implemented into a state-of-the-art dust growth and collision
model
Consistency Conditions for Branes at Orbifold Singularities
We discuss consistency conditions for branes at orbifold singularities. The
conditions have a world-sheet interpretation in terms of tadpole cancellation
and a space-time interpretation in terms of anomalies. As examples, we consider
type II and type I branes on orbifolds. We give orientifold
constructions of phases of type I or heterotic string theory, involving
branches with extra tensor multiplets, which arise when small SO(32) instantons
sit on orbifold singularities.Comment: harvmac, 14 pages, added reference
New Phases of String Theory and 6d RG Fixed Points via Branes at Orbifold Singularities
We discuss type II and type I branes at general ADE type orbifold
singularities. We show that there are new phases of type I or heterotic string
theory in six dimensions, involving extra tensor multiplets, which arise when
small instantons sit on orbifold singularities. The theories with extra tensor
multiplets are explicitly constructed via orientifolds. The world-volume
theories in type IIB or type I five-branes at orbifold singularities lead to
the existence of several infinite classes of six dimensional, interacting,
renormalization group fixed point theories.Comment: Harvmac, 28 pages, small typo correcte
H-Dyons and S-Duality
We present a relatively simple argument showing that the H-dyon states
required by S-duality of the heterotic string on are present provided
that the BPS dyons required by S-duality of N=4 supersymmetric Yang-Mills
theory are present. We also conjecture and provide evidence that H-dyons at
singularities where the nonperturbative gauge symmetry is completely broken are
actually BPS dyons.Comment: 16 pages, uses harvmac.tex, arguments are drastically changed but the
conclusions remain unchange
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