801 research outputs found
The Capture of Centaurs as Trojans
Large scale simulations of Centaurs have yielded vast amounts of data, the
analysis of which allows interesting but uncommon scenarios to be studied. One
such rare phenomenon is the temporary capture of Centaurs as Trojans of the
giant planets. Such captures are generally short (10 kyr to 100 kyr), but occur
with sufficient frequency (about 40 objects larger than 1 km in diameter every
Myr) that they may well contribute to the present-day populations. Uranus and
Neptune seem to have great difficulty capturing Centaurs into the 1:1
resonance, while Jupiter captures some, and Saturn the most (80 %). We
conjecture that such temporary capture from the Centaur population may be the
dominant delivery route into the Saturnian Trojans. Photometric studies of the
Jovian Trojans may reveal outliers with Centaur-like as opposed to asteroidal
characteristics, and these would be prime candidates for captured Centaurs.Comment: 5 pages, 2 figures, submitted to MNRAS (Letters
A Dark Matter Hurricane: Measuring the S1 Stream with Dark Matter Detectors
The recently discovered S1 stream passes through the Solar neighbourhood on a
low inclination, counter-rotating orbit. The progenitor of S1 is a dwarf galaxy
with a total mass comparable to the present-day Fornax dwarf spheroidal, so the
stream is expected to have a significant DM component. We compute the effects
of the S1 stream on WIMP and axion detectors as a function of the density of
its unmeasured dark component. In WIMP detectors the S1 stream supplies more
high energy nuclear recoils so will marginally improve DM detection prospects.
We find that even if S1 comprises less than 10% of the local density, multi-ton
xenon WIMP detectors can distinguish the S1 stream from the bulk halo in the
relatively narrow mass range between 5 and 25 GeV. In directional WIMP
detectors such as CYGNUS, S1 increases DM detection prospects more
substantially since it enhances the anisotropy of the WIMP signal. Finally, we
show that axion haloscopes possess by far the greatest potential sensitivity to
the S1 stream. Once the axion mass has been discovered, the distinctive
velocity distribution of S1 can easily be extracted from the axion power
spectrum.Comment: 21 pages, 11 figure
First Clear Signature of an Extended Dark Matter Halo in the Draco Dwarf Spheroidal
We present the first clear evidence for an extended dark matter halo in the
Draco dwarf spheroidal galaxy based on a sample of new radial velocities for
159 giant stars out to large projected radii. Using a two parameter family of
halo models spanning a range of density profiles and velocity anisotropies, we
are able to rule out (at about the 2.5 sigma confidence level) haloes in which
mass follows light. The data strongly favor models in which the dark matter is
significantly more extended than the visible dwarf. However, haloes with
harmonic cores larger than the light distribution are also excluded. When
combined with existing measurements of the proper motion of Draco, our data
strongly suggest that Draco has not been tidally truncated within ~1 kpc. We
also show that the rising velocity dispersion at large radii represents a
serious problem for modified gravity (MOND).Comment: to be published in ApJL; 5 pages, 4 figure
Are there sextuplet and octuplet image systems?
We study gravitational lensing by the family of scale-free galaxies with flat
rotation curves. The models are defined by a shape function, which prescribes
the radius of the isophote as a function of position angle from the major axis.
The critical curves are analytic, while the caustic network is reducible to a
simple quadrature. The cusps are always located at the turning points of the
shape function. We show that the models with exactly elliptic isophotes never
admit butterfly or swallowtail cusps and so have at most 4 (or 5) images.
Higher order imaging is brought about by deviations of the isophotes from pure
ellipses -- such as pointedness caused by embedded disks or boxiness caused by
recent merging. The criteria for the onset of sextuple and octuple imaging can
be calculated analytically in terms of the ellipticity and the fourth-order
Fourier coefficients (a_4 and b_4) used by observers to parametrise the
isophote shapes. The 6 or 8 images are arranged roughly in a circle, which
appears as an incomplete Einstein ring if inadequately resolved. Using data on
the shapes of elliptical galaxies and merger remnants, we estimate that 1% of
all multiply imaged quasars may be sextuplet systems or higher. Forthcoming
satellites like the Global Astrometric Interferometer for Astrophysics (GAIA)
will provide datasets of roughly 4000 multiply imaged systems, and so about 40
will show sextuple imaging or higher.Comment: 10 pages, 9 figures, MNRAS (in press
Market Power in UK Food Retailing: Theory and Evidence from Seven Product Groups
Establishing the presence of market power in food chains has become an increasingly pertinent line of enquiry given the trend towards increasing concentration that has been observed in many parts of the world. This paper presents a theoretical model of price transmission in vertically related markets under imperfect competition. The model delivers a quasi-reduced form representation that is empirically tractable using readily available market data to test for the presence of market power. In particular, we show that the hypothesis of perfect competition can be rejected if shocks to the demand and supply function are significant and correctly signed in price transmission equations. Using a cointegrated vector autoregression, we find empirical results that are consistent with downstream market power in six out of seven food products investigated, supporting both the findings of the UK competition authority's recent investigation in to supermarkets and renewed calls for further scrutiny of supermarket behaviour by the UK's Office of Trading.imperfect competition, Cointegrated VARs, UK food industry, Marketing, D4, L81,
A Dynamical Fossil in the Ursa Minor Dwarf Spheroidal Galaxy
The nearby Ursa Minor dwarf spheroidal (UMi dSph) is one of the most dark
matter dominated galaxies known, with a central mass to light ratio roughly
equal to 70. Somewhat anomalously, it appears to contain morphological
substructure in the form of a second peak in the stellar number density. It is
often argued that this substructure must be transient because it could not
survive for the > 10 Gyr age of the system, given the crossing time implied by
UMi's 8.8 km/s internal velocity dispersion. In this paper, however, we present
evidence that the substructure has a cold kinematical signature, and argue that
UMi's clumpiness could indeed be a primordial artefact. Using numerical
simulations, we demonstrate that substructure is incompatible with the cusped
dark matter haloes predicted by the prevailing Cold Dark Matter (CDM) paradigm,
but is consistent with an unbound stellar cluster sloshing back and forth
within the nearly harmonic potential of a cored dark matter halo. Thus CDM
appears to disagree with observation at the least massive, most dark matter
dominated end of the galaxy mass spectrum.Comment: Astrophysical Journal (Letters), in pres
Reclaiming the political : emancipation and critique in security studies
The critical security studies literature has been marked by a shared commitment towards the politicization of security – that is, the analysis of its assumptions, implications and the practices through which it is (re)produced. In recent years, however, politicization has been accompanied by a tendency to conceive security as connected with a logic of exclusion, totalization and even violence. This has resulted in an imbalanced politicization that weakens critique. Seeking to tackle this situation, the present article engages with contributions that have advanced emancipatory versions of security. Starting with, but going beyond, the so-called Aberystwyth School of security studies, the argument reconsiders the meaning of security as emancipation by making the case for a systematic engagement with the notions of reality and power. This revised version of security as emancipation strengthens critique by addressing political dimensions that have been underplayed in the critical security literature
Contending cultures of counterterrorism: transatlantic divergence or convergence?
Terrorist attacks on the United States, Spain and the United Kingdom have underlined the differing responses of Europe and the United States to the 'new terrorism'. This article analyses these responses through the prism of historically determined strategic cultures. For the last four years the United States has directed the full resources of a 'national security' approach towards this threat and has emphasized unilateralism. Europe, based on its own past experience of terrorism, has adopted a regulatory approach pursued through multilateralism. These divergences in transatlantic approaches, with potentially major implications for the future of the relationship, have appeared to be mitigated by a revised American strategy of counterterrorism that has emerged during 2005. However, this article contends that while strategic doctrines may change, the more immutable nature of strategic culture will make convergence difficult. This problem will be compounded by the fact that neither Europe nor America have yet addressed the deeper connections between terrorism and the process of globalization
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