12,580 research outputs found
Chaotic string-capture by black hole
We consider a macroscopic charge-current carrying (cosmic) string in the
background of a Schwarzschild black hole. The string is taken to be circular
and is allowed to oscillate and to propagate in the direction perpendicular to
its plane (that is parallel to the equatorial plane of the black hole).
Nurmerical investigations indicate that the system is non-integrable, but the
interaction with the gravitational field of the black hole anyway gives rise to
various qualitatively simple processes like "adiabatic capture" and "string
transmutation".Comment: 13 pages Latex + 3 figures (not included), Nordita 93/55
Stable and Unstable Circular Strings in Inflationary Universes
It was shown by Garriga and Vilenkin that the circular shape of nucleated
cosmic strings, of zero loop-energy in de Sitter space, is stable in the sense
that the ratio of the mean fluctuation amplitude to the loop radius is
constant. This result can be generalized to all expanding strings (of non-zero
loop-energy) in de Sitter space. In other curved spacetimes the situation,
however, may be different.
In this paper we develop a general formalism treating fluctuations around
circular strings embedded in arbitrary spatially flat FRW spacetimes. As
examples we consider Minkowski space, de Sitter space and power law expanding
universes. In the special case of power law inflation we find that in certain
cases the fluctuations grow much slower that the radius of the underlying
unperturbed circular string. The inflation of the universe thus tends to wash
out the fluctuations and to stabilize these strings.Comment: 15 pages Latex, NORDITA 94/14-
Continuous star cluster formation in the spiral NGC 45
We determined ages for 52 star clusters with masses < 10^6 solar masses in
the low surface brightness spiral galaxy NGC 45. Four of these candidates are
old globular clusters located in the bulge. The remaining ones span a large age
range. The cluster ages suggest a continuous star/cluster formation history
without evidence for bursts, consistent with the galaxy being located in a
relatively unperturbed environment in the outskirts of the Sculptor group.Comment: 4 pages, 3 figures. To appear in "Island Universes - Structure and
Evolution of Disk Galaxies", Terschelling (Netherlands), July 200
Seasonal Patterns of Flight and Attack of Maple Saplings by the Ambrosia Beetle \u3ci\u3eCorthylus Punctatissimus\u3c/i\u3e (Coleoptera: Scolytidae) in Central Michigan
Window traps with ethanol were used to observe seasonal flight patterns of Corthylus punctatissimus in central Michigan. Flights peaked in early July with a second peak seven weeks later in late August. Similarly, wilting of attacked maple (Acer) saplings began to appear a week after initial Corthylus flights, and showed twopeaks, one in mid-July and again with another peak, seven weeks later, in early September. The second peak of activity is presumably from reemerged adults, and not a second generation
Dynamics of cosmic strings and springs; a covariant formulation
A general family of charge-current carrying cosmic string models is
investigated. In the special case of circular configurations in arbitrary
axially symmetric gravitational and electromagnetic backgrounds the dynamics is
determined by simple point particle Hamiltonians. A certain "duality"
transformation relates our results to previous ones, obtained by Carter et.
al., for an infinitely long open stationary string in an arbitrary stationary
background.Comment: 11 pages, Latex, Nordita preprint 93/28
Valuation of aircraft noise by time of day: a comparison of two approaches
This paper reports an innovative application of stated preference techniques to derive values of aircraft noise by time of day and day of week. Revealed preference techniques cannot provide such segmentations which would clearly be of use in policy development especially relating to airport operations. Given the lack of research on this issue the work reported here is highly experimental. Two stated preference experiments were designed. The first focussed on a single time period whilst the second asked respondents to trade between time periods. Both approaches yielded results that are plausible and mutually consistent in terms of relative values by time period. We conclude that stated preference techniques are particularly useful in this context where the use of aggregated values may lead to non-optimal policy decisions
Null Strings in Schwarzschild Spacetime
The null string equations of motion and constraints in the Schwarzschild
spacetime are given. The solutions are those of the null geodesics of General
Relativity appended by a null string constraint in which the "constants of
motion" depend on the world-sheet spatial coordinate. Because of the extended
nature of a string, the physical interpretation of the solutions is completely
different from the point particle case. In particular, a null string is
generally not propagating in a plane through the origin, although each of its
individual points is. Some special solutions are obtained and their physical
interpretation is given. Especially, the solution for a null string with a
constant radial coordinate moving vertically from the south pole to the
north pole around the photon sphere, is presented. A general discussion of
classical null/tensile strings as compared to massless/massive particles is
given. For instance, tensile circular solutions with a constant radial
coordinate do not exist at all. The results are discussed in relation to
the previous literature on the subject.Comment: 16 pages, REVTEX, no figure
Photoemission electron microscopy of localized surface plasmons in silver nanostructures at telecommunication wavelengths
We image the field enhancement at Ag nanostructures using femtosecond laser
pulses with a center wavelength of 1.55 micrometer. Imaging is based on
non-linear photoemission observed in a photoemission electron microscope
(PEEM). The images are directly compared to ultra violet PEEM and scanning
electron microscopy (SEM) imaging of the same structures. Further, we have
carried out atomic scale scanning tunneling microscopy (STM) on the same type
of Ag nanostructures and on the Au substrate. Measuring the photoelectron
spectrum from individual Ag particles shows a larger contribution from higher
order photoemission process above the work function threshold than would be
predicted by a fully perturbative model, consistent with recent results using
shorter wavelengths. Investigating a wide selection of both Ag nanoparticles
and nanowires, field enhancement is observed from 30% of the Ag nanoparticles
and from none of the nanowires. No laser-induced damage is observed of the
nanostructures neither during the PEEM experiments nor in subsequent SEM
analysis. By direct comparison of SEM and PEEM images of the same
nanostructures, we can conclude that the field enhancement is independent of
the average nanostructure size and shape. Instead, we propose that the
variations in observed field enhancement could originate from the wedge
interface between the substrate and particles electrically connected to the
substrate
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