7,601 research outputs found
A parallel multigrid solver for multi-patch Isogeometric Analysis
Isogeometric Analysis (IgA) is a framework for setting up spline-based
discretizations of partial differential equations, which has been introduced
around a decade ago and has gained much attention since then. If large spline
degrees are considered, one obtains the approximation power of a high-order
method, but the number of degrees of freedom behaves like for a low-order
method. One important ingredient to use a discretization with large spline
degree, is a robust and preferably parallelizable solver. While numerical
evidence shows that multigrid solvers with standard smoothers (like Gauss
Seidel) does not perform well if the spline degree is increased, the multigrid
solvers proposed by the authors and their co-workers proved to behave optimal
both in the grid size and the spline degree. In the present paper, the authors
want to show that those solvers are parallelizable and that they scale well in
a parallel environment.Comment: The first author would like to thank the Austrian Science Fund (FWF)
for the financial support through the DK W1214-04, while the second author
was supported by the FWF grant NFN S117-0
On the accuracy of retrieved wind information from Doppler lidar observations
A single pulsed Doppler lidar was successfully deployed to measure air flow and turbulence over the Malvern hills, Worcester, UK. The DERA Malvern lidar used was a CO2 ”m pulsed Doppler lidar. The lidar pulse repetition rate was 120 Hz and had a pulse duration of 0.6 ”s The system was set up to have 41 range gates with range resolution of 112 m. This gave a theoretical maximum range of approximately 4.6 km. The lidar site was 2 km east of the Malvern hill ridge which runs in a north-south direction and is approximately 6 km long. The maximum height of the ridge is 430 m. Two elevation scans (Range-Height Indicators) were carried out parallel and perpendicular to the mean surface flow. Since the surface wind was primarily westerly the scans were carried out perpendicular and parallel to the ridge of the Malvern hills.
The data were analysed and horizontal winds, vertical winds and turbulent fluxes were calculated for profiles throughout the boundary layer. As an aid to evaluating the errors associated with the derivation of velocity and turbulence profiles, data from a simple idealized profile was also analysed using the same method. The error analysis shows that wind velocity profiles can be derived to an accuracy of 0.24 m s-1 in the horizontal and 0.3 m s-1 in the vertical up to a height of 2500 m. The potential for lidars to make turbulence measurements, over a wide area, through the whole depth of the planetary boundary layer and over durations from seconds to hours is discussed
The first magnetic maps of a pre-main sequence binary star system - HD 155555
We present the first maps of the surface magnetic fields of a pre-main
sequence binary system. Spectropolarimetric observations of the young, 18 Myr,
HD 155555 (V824 Ara, G5IV + K0IV) system were obtained at the Anglo-Australian
Telescope in 2004 and 2007. Both datasets are analysed using a new binary
Zeeman Doppler imaging (ZDI) code. This allows us to simultaneously model the
contribution of each component to the observed circularly polarised spectra.
Stellar brightness maps are also produced for HD 155555 and compared to
previous Doppler images. Our radial magnetic maps reveal a complex surface
magnetic topology with mixed polarities at all latitudes. We find rings of
azimuthal field on both stars, most of which are found to be non-axisymmetric
with the stellar rotational axis. We also examine the field strength and the
relative fraction of magnetic energy stored in the radial and azimuthal field
components at both epochs. A marked weakening of the field strength of the
secondary star is observed between the 2004 and 2007 epochs. This is
accompanied by an apparent shift in the location of magnetic energy from the
azimuthal to radial field. We suggest that this could be indicative of a
magnetic activity cycle. We use the radial magnetic maps to extrapolate the
coronal field (by assuming a potential field) for each star individually - at
present ignoring any possible interaction. The secondary star is found to
exhibit an extreme tilt (~75 deg) of its large scale magnetic field to that of
its rotation axis for both epochs. The field complexity that is apparent in the
surface maps persists out to a significant fraction of the binary separation.
Any interaction between the fields of the two stars is therefore likely to be
complex also. Modelling this would require a full binary field extrapolation.Comment: 17 pages, 12 figures, accepted for publication in MNRA
The Angular Momentum Evolution of Very Low Mass Stars
We present theoretical models of the angular momentum evolution of very low
mass stars (0.1 - 0.5 M_sun) and solar analogues (0.6 - 1.1 M_sun). We
investigate the effect of rotation on the effective temperature and luminosity
of these stars. We find that the decrease in T_eff and L can be significant at
the higher end of our mass range, but becomes negligible below 0.4 M_sun.
Formulae for relating T_eff to mass and v_rot are presented.
We compare our models to rotational data from young open clusters of
different ages to infer the rotational history of low mass stars, and the
dependence of initial conditions and rotational evolution on mass. We find that
the qualitative conclusions for stars below 0.6 M_sun do not depend on the
assumptions about internal angular momentum transport, which makes these low
mass stars ideal candidates for the study of the angular momentum loss law and
distribution of initial conditions. We find that neither models with solid body
nor differential rotation can simultaneously reproduce the observed stellar
spin down in the 0.6 to 1.1 M_sun mass range and for stars between 0.1 and 0.6
M_sun. The most likely explanation is that the saturation threshold drops more
steeply at low masses than would be predicted with a simple Rossby scaling. In
young clusters there is a systematic increase in the mean rotation rate with
decreased temperature below 3500 K (0.4 M_sun). This suggests either
inefficient angular momentum loss or mass-dependent initial conditions for
stars near the fully convective boundary. (abridged)Comment: To appear in the May 10, 2000 Ap
The Doppler Shadow of WASP-3b: A tomographic analysis of Rossiter-McLaughlin observations
Hot-Jupiter planets must form at large separations from their host stars
where the temperatures are cool enough for their cores to condense. They then
migrate inwards to their current observed orbital separations. Different
theories of how this migration occurs lead to varying distributions of orbital
eccentricity and the alignment between the rotation axis of the star and the
orbital axis of the planet. The spin-orbit alignment of a transiting system is
revealed via the Rossiter-McLaughlin effect, which is the anomaly present in
the radial velocity measurements of the rotating star during transit due to the
planet blocking some of the starlight. In this paper we aim to measure the
spin-orbit alignment of the WASP-3 system via a new way of analysing the
Rossiter-McLaughlin observations. We apply a new tomographic method for
analysing the time variable asymmetry of stellar line profiles caused by the
Rossiter-McLaughlin effect. This new method eliminates the systematic error
inherent in previous methods used to analyse the effect. We find a value for
the projected stellar spin rate of v sin i = 13.9 \pm 0.03 km/s which is in
agreement with previous measurements but has a much higher precision. The
system is found to be well aligned which favours an evolutionary history for
WASP-3b involving migration through tidal interactions with a protoplanetary
disc. Using gyrochronology we estimate the age of the star to be ~300 Myr with
an upper limit of 2 Gyr from comparison with isochrones.Comment: Accepted for publication in A&A, 8 pages, 4 figures, 2 table
The first WASP public data release
The WASP (wide angle search for planets) project is an exoplanet transit survey that has been automatically taking wide field images since 2004. Two instruments, one in La Palma and the other in South Africa, continually monitor the night sky, building up light curves of millions of unique objects. These light curves are used to search for the characteristics of exoplanetary transits. This first public data release (DR1) of the WASP archive makes available all the light curve data and images from 2004 up to 2008 in both the Northern and Southern hemispheres. A web interface () to the data allows easy access over the Internet. The data set contains 3â631â972 raw images and 17â970â937 light curves. In total the light curves have 119â930â299â362 data points available between them
Nanoelectrode Scanning Probes from Fluorocarbon-Coated Single-Walled Carbon Nanotubes
We have developed a method to coat single-walled carbon nanotubes attached to AFM tips with conformal fluorocarbon polymer films formed in an inductively coupled plasma reactor. The polymer provides a chemically inert and electrically insulating outer layer and mechanically stabilizes the attached nanotube sufficiently to enable imaging in liquids without the need for an intervening adhesive. Electrical pulse etching of the insulating coating exclusively at the nanotube tip end results in well-defined highly conductive nanoelectrodes. For these probes, the conductive properties of the nanotubes are not affected by the coating. Some nanoelectrodes behave as rectifying diodes, which may be developed into novel molecular devices integrated onto scanning probes
Qatar-1b: a hot Jupiter orbiting a metal-rich K dwarf star
We report the discovery and initial characterisation of Qatar-1b, a hot
Jupiter orbiting a metal-rich K dwarf star, the first planet discovered by the
Alsubai Project exoplanet transit survey. We describe the strategy used to
select candidate transiting planets from photometry generated by the Alsubai
Project instrument. We examine the rate of astrophysical and other false
positives found during the spectroscopic reconnaissance of the initial batch of
candidates. A simultaneous fit to the follow-up radial velocities and
photometry of Qatar-1b yield a planetary mass of 1.09+/-0.08 Mjup and a radius
of 1.16+/-0.05 Rjup. The orbital period and separation are 1.420033 days and
0.0234 AU for an orbit assumed to be circular. The stellar density, effective
temperature and rotation rate indicate an age greater than 4 Gyr for the
system.Comment: 8 pages, 5 figures, submitted to Monthly Notices of the Royal
Astronomical Societ
The color of smiling: computational synaesthesia of facial expressions
This note gives a preliminary account of the transcoding or rechanneling
problem between different stimuli as it is of interest for the natural
interaction or affective computing fields. By the consideration of a simple
example, namely the color response of an affective lamp to a sensed facial
expression, we frame the problem within an information- theoretic perspective.
A full justification in terms of the Information Bottleneck principle promotes
a latent affective space, hitherto surmised as an appealing and intuitive
solution, as a suitable mediator between the different stimuli.Comment: Submitted to: 18th International Conference on Image Analysis and
Processing (ICIAP 2015), 7-11 September 2015, Genova, Ital
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