4,410 research outputs found
Electrodynamic Radiation Reaction and General Relativity
We argue that the well-known problem of the instabilities associated with the
self-forces (radiation reaction forces) in classical electrodynamics are
possibly stabilized by the introduction of gravitational forces via general
relativity
Stellar science from a blue wavelength range - A possible design for the blue arm of 4MOST
From stellar spectra, a variety of physical properties of stars can be
derived. In particular, the chemical composition of stellar atmospheres can be
inferred from absorption line analyses. These provide key information on large
scales, such as the formation of our Galaxy, down to the small-scale
nucleosynthesis processes that take place in stars and supernovae. By extending
the observed wavelength range toward bluer wavelengths, we optimize such
studies to also include critical absorption lines in metal-poor stars, and
allow for studies of heavy elements (Z>38) whose formation processes remain
poorly constrained. In this context, spectrographs optimized for observing blue
wavelength ranges are essential, since many absorption lines at redder
wavelengths are too weak to be detected in metal-poor stars. This means that
some elements cannot be studied in the visual-redder regions, and important
scientific tracers and science cases are lost. The present era of large public
surveys will target millions of stars. Here we describe the requirements
driving the design of the forthcoming survey instrument 4MOST, a multi-object
spectrograph commissioned for the ESO VISTA 4m-telescope. We focus here on
high-density, wide-area survey of stars and the science that can be achieved
with high-resolution stellar spectroscopy. Scientific and technical
requirements that governed the design are described along with a thorough line
blending analysis. For the high-resolution spectrograph, we find that a
sampling of >2.5 (pixels per resolving element), spectral resolution of 18000
or higher, and a wavelength range covering 393-436 nm, is the most
well-balanced solution for the instrument. A spectrograph with these
characteristics will enable accurate abundance analysis (+/-0.1 dex) in the
blue and allow us to confront the outlined scientific questions. (abridged)Comment: 14 pages, 8 figures, accepted for publication in A
Uniform generation in trace monoids
We consider the problem of random uniform generation of traces (the elements
of a free partially commutative monoid) in light of the uniform measure on the
boundary at infinity of the associated monoid. We obtain a product
decomposition of the uniform measure at infinity if the trace monoid has
several irreducible components-a case where other notions such as Parry
measures, are not defined. Random generation algorithms are then examined.Comment: Full version of the paper in MFCS 2015 with the same titl
The gravitational wave rocket
Einstein's equations admit solutions corresponding to photon rockets. In
these a massive particle recoils because of the anisotropic emission of
photons. In this paper we ask whether rocket motion can be powered only by the
emission of gravitational waves. We use the double series approximation method
and show that this is possible. A loss of mass and gain in momentum arise in
the second approximation because of the emission of quadrupole and octupole
waves.Comment: 10 pages LaTe
Anti-Nirvana: crime, culture and instrumentalism in the age of insecurity
‘Anti-Nirvana’ explores the relationship between consumer culture, media and criminal motivations. It has appeared consistently on the list of the top-ten most-read articles in this award-winning international journal, and it mounts a serious neo-Freudian challenge to the predominant naturalistic notion of ‘resistance’ at the heart of liberal criminology and media studies. It is also cited in the Oxford Handbook of Criminology and other criminology texts as a persuasive argument in support of the theory that criminality amongst young people is strongly linked to the acquisitive values of consumerism and the images of possessive individualism that dominate mass media
New Discrete Basis for Nuclear Structure Studies
A complete discrete set of spherical single-particle wave functions for
studies of weakly-bound many-body systems is proposed. The new basis is
obtained by means of a local-scale point transformation of the spherical
harmonic oscillator wave functions. Unlike the harmonic oscillator states, the
new wave functions decay exponentially at large distances. Using the new basis,
characteristics of weakly-bound orbitals are analyzed and the ground state
properties of some spherical doubly-magic nuclei are studied. The basis of the
transformed harmonic oscillator is a significant improvement over the harmonic
oscillator basis, especially in studies of exotic nuclei where the coupling to
the particle continuum is important.Comment: 13 pages, RevTex, 6 p.s. figures, submitted to Phys. Rev.
Titanium abundances in late-type stars I. 1D non-LTE modelling in benchmark dwarfs and giants
The titanium abundances of late-type stars are important tracers of Galactic
formation history. However, abundances inferred from Ti I and Ti II lines can
be in stark disagreement in very metal-poor giants. Departures from local
thermodynamic equilibrium (LTE) have a large impact on the minority neutral
species and thus influences the ionisation imbalance, but satisfactory non-LTE
modelling for both dwarfs and giants has not been achieved in previous
literature. The reliability of titanium abundances is reassessed in benchmark
dwarfs and giants using a new non-LTE model and one-dimensional (1D) model
atmospheres. A comprehensive model atom was compiled with a more extended level
structure and newly published data for inelastic collisions between Ti I and
neutral hydrogen. In 1D LTE, the Ti I and Ti II lines agree to within
dex for the Sun, Arcturus, and the very metal-poor stars HD84937 and HD140283.
For the very metal-poor giant HD122563, the Ti I lines give an abundance that
is dex lower than that from Ti II. The 1D non-LTE corrections can reach
dex for individual Ti I lines and dex for individual Ti II lines,
and reduce the overall ionisation imbalance to dex for HD122563.
However, it also increases the imbalance for the very metal-poor dwarf and
sub-giant to around dex. Using 1D non-LTE reduces the ionisation
imbalance in very metal-poor giants but breaks the balance of other very
metal-poor stars, consistent with the conclusions in earlier literature. To
make further progress, consistent 3D non-LTE models are needed.Comment: 9 pages plus appendix, 6 figures; accepted for publication in
Astronomy & Astrophysic
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