2,707 research outputs found
Magnetic Structure of Rapidly Rotating FK Comae-Type Coronae
We present a three-dimensional simulation of the corona of an FK Com-type
rapidly rotating G giant using a magnetohydrodynamic model that was originally
developed for the solar corona in order to capture the more realistic,
non-potential coronal structure. We drive the simulation with surface maps for
the radial magnetic field obtained from a stellar dynamo model of the FK Com
system. This enables us to obtain the coronal structure for different field
topologies representing different periods of time. We find that the corona of
such an FK Com-like star, including the large scale coronal loops, is dominated
by a strong toroidal component of the magnetic field. This is a result of part
of the field being dragged by the radial outflow, while the other part remains
attached to the rapidly rotating stellar surface. This tangling of the magnetic
field,in addition to a reduction in the radial flow component, leads to a
flattening of the gas density profile with distance in the inner part of the
corona. The three-dimensional simulation provides a global view of the coronal
structure. Some aspects of the results, such as the toroidal wrapping of the
magnetic field, should also be applicable to coronae on fast rotators in
general, which our study shows can be considerably different from the
well-studied and well-observed solar corona. Studying the global structure of
such coronae should also lead to a better understanding of their related
stellar processes, such as flares and coronal mass ejections, and in
particular, should lead to an improved understanding of mass and angular
momentum loss from such systems.Comment: Accepted to ApJ, 10 pages, 6 figure
Automatic, fast and robust characterization of noise distributions for diffusion MRI
Knowledge of the noise distribution in magnitude diffusion MRI images is the
centerpiece to quantify uncertainties arising from the acquisition process. The
use of parallel imaging methods, the number of receiver coils and imaging
filters applied by the scanner, amongst other factors, dictate the resulting
signal distribution. Accurate estimation beyond textbook Rician or noncentral
chi distributions often requires information about the acquisition process
(e.g. coils sensitivity maps or reconstruction coefficients), which is not
usually available. We introduce a new method where a change of variable
naturally gives rise to a particular form of the gamma distribution for
background signals. The first moments and maximum likelihood estimators of this
gamma distribution explicitly depend on the number of coils, making it possible
to estimate all unknown parameters using only the magnitude data. A rejection
step is used to make the method automatic and robust to artifacts. Experiments
on synthetic datasets show that the proposed method can reliably estimate both
the degrees of freedom and the standard deviation. The worst case errors range
from below 2% (spatially uniform noise) to approximately 10% (spatially
variable noise). Repeated acquisitions of in vivo datasets show that the
estimated parameters are stable and have lower variances than compared methods.Comment: v2: added publisher DOI statement, fixed text typo in appendix A
Detecting chaos in particle accelerators through the frequency map analysis method
The motion of beams in particle accelerators is dominated by a plethora of
non-linear effects which can enhance chaotic motion and limit their
performance. The application of advanced non-linear dynamics methods for
detecting and correcting these effects and thereby increasing the region of
beam stability plays an essential role during the accelerator design phase but
also their operation. After describing the nature of non-linear effects and
their impact on performance parameters of different particle accelerator
categories, the theory of non-linear particle motion is outlined. The recent
developments on the methods employed for the analysis of chaotic beam motion
are detailed. In particular, the ability of the frequency map analysis method
to detect chaotic motion and guide the correction of non-linear effects is
demonstrated in particle tracking simulations but also experimental data.Comment: Submitted for publication in Chaos, Focus Issue: Chaos Detection
Methods and Predictabilit
The well-aligned orbit of WASP-84b: evidence for disc migration
We report the sky-projected orbital obliquity (spin-orbit angle) of WASP-84b,
a 0.70- planet in a 8.52-day orbit around a G9V/K0V star, to be
. We obtain a true obliquity of from a measurement of the inclination of the stellar spin axis with
respect to the sky plane. Due to the young age and the weak tidal forcing of
the system, we suggest that the orbit of WASP-84b is unlikely to have both
realigned and circularised from the misaligned and/or eccentric orbit likely to
have arisen from high-eccentricity migration. Therefore we conclude that the
planet probably migrated via interaction with the protoplanetary disc. This
would make it the first short-orbit, giant planet to have been shown to have
migrated via this pathway. Further, we argue that the distribution of
obliquities for planets orbiting cool stars ( < 6250 K) suggests
that high-eccentricity migration is an important pathway for the formation of
short-orbit, giant planets.Comment: 6 pages, 3 figures, 1 table, submitted to ApJ
Aid to conflict-affected countries : lessons for donors
The first section looks at the implications of conflict for aid effectiveness and selectivity. We argue that, while aid is generally effective in promoting growth and by implication reducing poverty, it is more effective in promoting growth in post-conflict countries. We then consider the implications of these findings for donor selectivity models and for assessment of donor performance in allocating development aid among recipient countries. We argue that, while further research on aid effectiveness in post-conflict scenarios is needed, existing selectivity models should be augmented with, inter alia, post-conflict variables, and donors should be evaluated on the basis, inter alia, of the share of their aid budgets allocated to countries experiencing post-conflict episodes. We also argue for aid delivered in the form of projects to countries with weak institutions in early post-conflict years. The second section focuses on policies for donors operating in conflict-affected countries. We set out five of the most important principles: (1) focus on broad-based recovery from war; (2) to achieve a broad-based recovery, get involved before the conflict ends; (3) focus on poverty, but avoid ‘wish lists’; (4) help to reduce insecurity so aid can contribute more effectively to growth and poverty reduction; and (5) in economic reform, focus on improving public expenditure management and revenue mobilisation. The third section concludes by emphasising the fact that there is no hard or fast dividing line between ‘war’ and ‘peace’ and that it may take many years for a society to become truly ‘post’-conflict’. Donors, therefore, need to prepare for the long haul.<br /
WASP-189b: an ultra-hot Jupiter transiting the bright A star HR 5599 in a polar orbit
We report the discovery of WASP-189b: an ultra-hot Jupiter in a 2.72-d
transiting orbit around the A star WASP-189 (HR 5599). We detected
periodic dimmings in the star's lightcurve, first with the WASP-South survey
facility then with the TRAPPIST-South telescope. We confirmed that a planet is
the cause of those dimmings via line-profile tomography and radial-velocity
measurements using the HARPS and CORALIE spectrographs. Those reveal WASP-189b
to be an ultra-hot Jupiter ( = 2.13 0.28 ;
= 1.374 0.082 ) in a polar orbit (; ) around a rapidly rotating
A6IV-V star ( = 8000 100 K; 100
km\, s). We calculate a predicted equilibrium temperature of = 2641 34 K, assuming zero albedo and efficient redistribution,
which is the third hottest for the known exoplanets. WASP-189 is the brightest
known host of a transiting hot Jupiter and the third-brightest known host of
any transiting exoplanet. We note that of the eight hot-Jupiter systems with
7000 K, seven have strongly misaligned orbits, and two of the
three systems with 8000 K have polar orbits (the third is
aligned).Comment: Submitted to MNRAS. 10 pages, 9 figures, 3 table
Reddening, Emission-Line, and Intrinsic Absorption Properties in the Narrow-Line Seyfert 1 Galaxy Akn 564
We use Hubble Space Telescope UV and optical spectra of the narrow-line
Seyfert 1 (NLS1) galaxy Akn 564 to investigate its internal reddening and
properties of its emission-line and intrinsic UV absorption gas. We find that
the extinction curve of Akn 564, derived from a comparison of its UV/optical
continuum to that of an unreddened NLS1, lacks a 2200 A bump and turns up
towards the UV at a longer wavelength (4000 A) than the standard Galactic, LMC,
and SMC curves. However, it does not show the extremely steep rise to 1200 A
that characterizes the extinction curve of the Seyfert 1 galaxy NGC 3227. The
emission-lines and continuum experience the same amount of reddening,
indicating the presence of a dust screen that is external to the narrow-line
region (NLR). Echelle spectra from the Space Telescope Imaging Spectrograph
show intrinsic UV absorption lines due to Ly-alpha, N V, C IV, Si IV, and Si
III, centered at a radial velocity of -190 km/s (relative to the host galaxy).
Photoionization models of the UV absorber indicate that it has a sufficient
columnand is at a sufficient distance from the nucleus (D > 95 pc) to be the
source of the dust screen. Thus, Akn 564 contains a dusty ``lukewarm absorber''
similar to that seen in NGC 3227.Comment: 25 pages, 4 figures. LaTeX with encapsulated postscript figures.
Accepted for publication in the Astrophysical Journa
Ion production by lasers using high-power densities in a near infrared region
Results are presented of experiments on ion production from Ta targets using a short pulse (350-600 ps in focus) illumination with focal power densities exceeding 1014 Wcm-2 at the wavelength of an iodine photodissociation laser (1.315 ÎŒm) and its harmonics. Strong evidence of the existence of tantalum ions with the charge state +45 near the target surface was obtained by X-ray spectroscopy methods. The particle diagnostics point to the existence of frozen high charge states (4 MeV) for the highest observed charge states. A tentative theoretical explanation of the observed anomalous charge state freezing phenomenon in the expanding plasma produced by a subnanosecond laser pulse is give
Actors and networks or agents and structures: towards a realist view of information systems
Actor-network theory (ANT) has achieved a measure of popularity in the analysis of information systems. This paper looks at ANT from the perspective of the social realism of Margaret Archer. It argues that the main issue with ANT from a realist perspective is its adoption of a `flat' ontology, particularly with regard to human beings. It explores the value of incorporating concepts from ANT into a social realist approach, but argues that the latter offers a more productive way of approaching information systems
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