38,039 research outputs found
Computing Functions of Random Variables via Reproducing Kernel Hilbert Space Representations
We describe a method to perform functional operations on probability
distributions of random variables. The method uses reproducing kernel Hilbert
space representations of probability distributions, and it is applicable to all
operations which can be applied to points drawn from the respective
distributions. We refer to our approach as {\em kernel probabilistic
programming}. We illustrate it on synthetic data, and show how it can be used
for nonparametric structural equation models, with an application to causal
inference
Spectroscopy of Four Cataclysmic Variables with Periods above 7 Hours
We present spectroscopy of four cataclysmic variables. Using radial velocity
measurements, we find orbital periods for the first time. The stars and their
periods are GY Hya, 0.347230(9) d; SDSS J204448-045929, 1.68(1) d; V392 Hya,
0.324952(5) d; and RX J1951.7+3716, 0.492(1) d. We also detect the spectra of
the secondary stars, estimate their spectral types, and derive distances based
on surface brightness and Roche lobe constraints.Comment: 22 pages, 3 figures, 5 tables, to be published in December 2006 PAS
Testing Lorentz Invariance by Comparing Light Propagation in Vacuum and Matter
We present a Michelson-Morley type experiment for testing the isotropy of the
speed of light in vacuum and matter. The experiment compares the resonance
frequency of a monolithic optical sapphire resonator with the resonance
frequency of an orthogonal evacuated optical cavity made of fused silica while
the whole setup is rotated on an air bearing turntable once every 45 s.
Preliminary results yield an upper limit for the anisotropy of the speed of
light in matter (sapphire) of \Delta c/c < 4x10^(-15), limited by the frequency
stability of the sapphire resonator operated at room temperature. Work to
increase the measurement sensitivity by more than one order of magnitude by
cooling down the sapphire resonator to liquid helium temperatures (LHe) is
currently under way.Comment: Presented at the Fifth Meeting on CPT and Lorentz Symmetry,
Bloomington, Indiana, June 28-July 2, 201
Magnetic phases in the correlated Kondo-lattice model
We study magnetic ordering of an extended Kondo-lattice model including an
additional on-site Coulomb interaction between the itinerant states. The model
is solved in the dynamical mean-field theory using Wilson's numerical
renormalization group approach as impurity solver. For a bipartite lattice we
find at half filling the expected antiferromagnetic phase. Upon doping this
phase is gradually suppressed and hints towards phase separation are observed.
For large doping the model exhibits ferromagnetism, the appearance of which can
at first sight be explained by Rudermann-Kittel-Kasuya-Yosida interaction.
However, for large values of the Kondo coupling significant differences to
a simple Rudermann-Kittel-Kasuya-Yosida picture can be found. We furthermore
observe signs of quantum critical points for antiferromagnetic Kondo coupling
between the local spins and band states
Unconventional carrier-mediated ferromagnetism above room temperature in ion-implanted (Ga, Mn)P:C
Ion implantation of Mn ions into hole-doped GaP has been used to induce
ferromagnetic behavior above room temperature for optimized Mn concentrations
near 3 at.%. The magnetism is suppressed when the Mn dose is increased or
decreased away from the 3 at.% value, or when n-type GaP substrates are used.
At low temperatures the saturated moment is on the order of one Bohr magneton,
and the spin wave stiffness inferred from the Bloch-law T^3/2 dependence of the
magnetization provides an estimate Tc = 385K of the Curie temperature that
exceeds the experimental value, Tc = 270K. The presence of ferromagnetic
clusters and hysteresis to temperatures of at least 330K is attributed to
disorder and proximity to a metal-insulating transition.Comment: 4 pages, 4 figures (RevTex4
Lifetime Adherence to Physical Activity Recommendations and Fall Occurrence in Community-dwelling Older Adults: a Retrospective Cohort Study
Falling is a major health concern for community-dwelling older adults. Regular physical activity has been proposed to prevent falls. The aim of this study was to assess whether the achievement of the 2004 UK Department of Health physical activity recommendations over a lifetime had a protective effect against falling in older people. 313 community-dwelling older adults completed a questionnaire about lifetime physical activity and fall occurrence. There were significantly fewer falls in those who had led an active lifestyle compared to those who had not (χ2Yates=4.568, p=0.033), with a lower relative risk of fall occurrence for the active respondents (RR=0.671) compared to the inactive (RR=1.210). Of those who were sufficiently active in their early adulthood, the decade where there was the biggest decrease in remaining active enough was in the 60s. It is concluded that an active lifestyle may have decreased the likelihood of having a fall in older ag
Recurrent shell infall events in a B0.5e star: HD 58978 1979-1988
Infall from the circumstellar envelope onto the bright B0.5 IVe star, HD 58978 was studied. The IUE data indicate that the star was surrounded by a low and moderately ionized circumstellar shell at least 12 times between 1979 and 1988. During 6 of these episodes, the signatures of cool circumstellar material were redshifted with respect to the photosphere by 20 to 80 km/sec. The data indicate that the transition from infall to minimal shell absorption can occur in under 10 days, and are consistent either with infall phases lasting up to 6 months, or with infall episodes shorter than 10 to 15 days. The long term behavior of the shell episodes is compared with variability in the stellar wind
The Shape of Dark Matter Haloes IV. The Structure of Stellar Discs in Edge-on Galaxies
We present optical and near-infrared archival observations of eight edge-on
galaxies. These observations are used to model the stellar content of each
galaxy using the FitSKIRT software package. Using FitSKIRT, we can
self-consistently model a galaxy in each band simultaneously while treating for
dust. This allows us to accurately measure both the scale length and scale
height of the stellar disc, plus the shape parameters of the bulge. By
combining this data with the previously reported integrated magnitudes of each
galaxy, we can infer their true luminosities. We have successfully modelled
seven out of the eight galaxies in our sample. We find that stellar discs can
be modelled correctly, but have not been able to model the stellar bulge
reliably. Our sample consists for the most part of slow rotating galaxies, and
we find that the average dust layer is much thicker than what is reported for
faster rotating galaxies.Comment: Accepted for publication by Monthly Notices RAS. Hi-res. version
available at www.astro.rug.nl/~vdkruit/Petersetal-IV.pd
The Shape of Dark Matter Haloes II. The Galactus HI Modelling & Fitting Tool
We present a new HI modelling tool called \textsc{Galactus}. The program has
been designed to perform automated fits of disc-galaxy models to observations.
It includes a treatment for the self-absorption of the gas. The software has
been released into the public domain. We describe the design philosophy and
inner workings of the program. After this, we model the face-on galaxy NGC2403,
using both self-absorption and optically thin models, showing that
self-absorption occurs even in face-on galaxies. It is shown that the maximum
surface brightness plateaus seen in Paper I of this series are indeed signs of
self-absorption. The apparent HI mass of an edge-on galaxy can be drastically
lower compared to that same galaxy seen face-on. The Tully-Fisher relation is
found to be relatively free from self-absorption issues.Comment: Accepted for publication by Monthly Notices RAS. Hi-res. version
available at www.astro.rug.nl/~vdkruit/Petersetal-II.pd
- …