562 research outputs found
An artificial neural network model for rainfall forecasting in Bangkok, Thailand
This paper presents a new approach using an Artificial Neural Network technique to improve rainfall forecast performance. A real world case study was set up in Bangkok; 4 years of hourly data from 75 rain gauge stations in the area were used to develop the ANN model. The developed ANN model is being applied for real time rainfall forecasting and flood management in Bangkok, Thailand. Aimed at providing forecasts in a near real time schedule, different network types were tested with different kinds of input information. Preliminary tests showed that a generalized feedforward ANN model using hyperbolic tangent transfer function achieved the best generalization of rainfall. Especially, the use of a combination of meteorological parameters (relative humidity, air pressure, wet bulb temperature and cloudiness), the rainfall at the point of forecasting and rainfall at the surrounding stations, as an input data, advanced ANN model to apply with continuous data containing rainy and non-rainy period, allowed model to issue forecast at any moment. Additionally, forecasts by ANN model were compared to the convenient approach namely simple persistent method. Results show that ANN forecasts have superiority over the ones obtained by the persistent model. Rainfall forecasts for Bangkok from 1 to 3 h ahead were highly satisfactory. Sensitivity analysis indicated that the most important input parameter besides rainfall itself is the wet bulb temperature in forecasting rainfall
The Rigidly Rotating Magnetosphere of Sigma Ori E
We attempt to characterize the observed variability of the magnetic
helium-strong star sigma Ori E in terms of a recently developed rigidly
rotating magnetosphere model. This model predicts the accumulation of
circumstellar plasma in two co-rotating clouds, situated in magnetohydrostatic
equilibrium at the intersection between magnetic and rotational equators. We
find that the model can reproduce well the periodic modulations observed in the
star's light curve, H alpha emission-line profile, and longitudinal field
strength, confirming that it furnishes an essentially correct, quantitative
description of the star's magnetically controlled circumstellar environment.Comment: 4 pages, 3 figures, accepted by Ap
Focusing by blocking: repeatedly generating central density peaks in self-propelled particle systems by exploiting diffusive processes
Over the past few years the displacement statistics of self-propelled
particles has been intensely studied, revealing their long-time diffusive
behavior. Here, we demonstrate that a concerted combination of boundary
conditions and switching on and off the self-propelling drive can generate and
afterwards arbitrarily often restore a non-stationary centered peak in their
spatial distribution. This corresponds to a partial reversibility of their
statistical behavior, in opposition to the above-mentioned long-time diffusive
nature. Interestingly, it is a diffusive process that mediates and makes
possible this procedure. It should be straightforward to verify our predictions
in a real experimental system.Comment: 6 pages, 6 figure
Testing the companion hypothesis for the origin of the X-ray emission from intermediate-mass main-sequence stars
There is no straightforward explanation for intrinsic X-ray emission from
intermediate-mass main-sequence stars. Therefore the observed emission is often
interpreted in terms of (hypothesized) late-type magnetically active companion
stars. We use Chandra imaging observations to spatially resolve in X-rays a
sample of main-sequence B-type stars with recently discovered companions at
arcsecond separation. We find that all spatially resolved companions are X-ray
emitters, but seven out of eleven intermediate-mass stars are also X-ray
sources. If this emission is interpreted in terms of additional sub-arcsecond
or spectroscopic companions, this implies a high multiplicity of B-type stars.
Firm results on B star multiplicity pending, the alternative, that B stars
produce intrinsic X-rays, can not be discarded. The appropriate scenario in
this vein is might be a magnetically confined wind, as suggested for the X-ray
emission of the magnetic Ap star IQ Aur. However, the only Ap star in the
Chandra sample is not detected in X-rays, and therefore does not support this
picture.Comment: 12 pages; accepted for publication in Astronomy & Astrophysic
Predictions for mass-loss rates and terminal wind velocities of massive O-type stars
Mass loss forms an important aspect of the evolution of massive stars, as
well as for the enrichment of the surrounding ISM. Our goal is to predict
accurate mass-loss rates and terminal wind velocities. These quantities can be
compared to empirical values, thereby testing radiation-driven wind models. One
specific issue is that of the "weak-wind problem", where empirically derived
mass-loss rates fall orders of magnitude short of predicted values. We employ
an established Monte Carlo model and a recently suggested new line acceleration
formalism to solve the wind dynamics consistently. We provide a new grid of
mass-loss rates and terminal wind velocities of O stars, and compare the values
to empirical results. Our models fail to provide mass-loss rates for
main-sequence stars below a luminosity of log(L/Lsun) = 5.2, where we run into
a fundamental limit. At luminosities below this critical value there is
insufficient momentum transferred in the region below the sonic point to
kick-start the acceleration. This problem occurs at the location of the onset
of the weak-wind problem. For O dwarfs, the boundary between being able to
start a wind, and failing to do so, is at spectral type O6/O6.5. The direct
cause of this failure is a combination of the lower luminosity and a lack of Fe
V lines at the wind base. This might indicate that another mechanism is
required to provide the necessary driving to initiate the wind. For stars more
luminous than log(L/Lsun) = 5.2, our new mass-loss rates are in excellent
agreement with the mass-loss prescription by Vink et al. 2000. This implies
that the main assumption entering the method of the Vink et al. prescriptions -
i.e. that the momentum equation is not explicitly solved for - does not
compromise the reliability of the Vink et al. results for this part of
parameter space (Abridged).Comment: 10 pages, 10 figures, Astronomy & Astrophysics (in press
X-ray Variability in the Young Massive Triple theta2 Ori A
Massive stars rarely show intrinsic X-ray variability. The only O-stars
credited to be intrinsically variable are theta1 Ori C due to effects from
magnetic confinement of its wind, and theta2 Ori A suspected of similar
activity. Early Chandra observations have shown that the most massive star
system in the Orion Trapezium Cluster, theta2 Ori A, shows rapid variability on
time scales of hours. We determine X-ray fluxes and find that the star shows
very strong variability over the last 5 years. We observed a second large X-ray
outburst in November 2004 with the high resolution transmission grating
spectrometer on-board Chandra. In the low state X-ray emissivities indicate
temperatures well above 25 MK. In the high state we find an extended emissivity
distribution with high emissivities in the range from 3 MK to over 100 MK. The
outburst event in stellar terms is one of the most powerful ever observed and
the most energetic one in the ONC with a lower total energy limit of 1.5x10^37
ergs. The line diagnostics show that under the assumption that the line
emitting regions in the low states are as close as within 1 -- 2 stellar radii
from the O-star's photosphere, whereas the hard states suggest a distance of 3
-- 5 stellar radii. The two outbursts are very close to the periastron passage
of the stars. We argue that the high X-ray states are possibly the result of
reconnection events from magnetic interactions of the primary and secondary
stars of the spectroscopic binary. Effects from wind collisions seem unlikely
for this system. The low state emissivity and R-ratios strengthen the
predicament that the X-ray emission is enhanced by magnetic confinement of the
primary wind. We also detect Fe fluorescence indicative of the existence of
substantial amounts of neutral Fe in the vicinity of the X-ray emission.Comment: 11 pages, 8 figures, accepted for publication in The Astrophysical
Main Journa
The effect of magnetic dipolar interactions on the interchain spin wave dispersion in CsNiF_3
Inelastic neutron scattering measurements were performed on the ferromagnetic
chain system CsNiF_3 in the collinear antiferromagnetic ordered state below T_N
= 2.67K. The measured spin wave dispersion was found to be in good agreement
with linear spin wave theory including dipolar interactions. The additional
dipole tensor in the Hamiltonian was essential to explain some striking
phenomena in the measured spin wave spectrum: a peculiar feature of the
dispersion relation is a jump at the zone center, caused by strong dipolar
interactions in this system. The interchain exchange coupling constant and the
planar anisotropy energy were determined within the present model to be J'/k_B
= -0.0247(12)K and A/k_B = 3.3(1)K. This gives a ratio J/J' \approx 500, using
the previously determined intrachain coupling constant J/k_B = 11.8$. The small
exchange energy J' is of the same order as the dipolar energy, which implies a
strong competition between the both interactions.Comment: 18 pages, TeX type, 7 Postscript figures included. To be published in
Phys. Rev.
Lower bounds for several online variants of bin packing
We consider several previously studied online variants of bin packing and
prove new and improved lower bounds on the asymptotic competitive ratios for
them. For that, we use a method of fully adaptive constructions. In particular,
we improve the lower bound for the asymptotic competitive ratio of online
square packing significantly, raising it from roughly 1.68 to above 1.75.Comment: WAOA 201
High-Resolution Chandra X-Ray Imaging And Spectroscopy Of The Sigma Orionis Cluster
We present results of a 90 ks Chandra X-ray observation of the young sigma Orionis cluster ( age similar to 3 Myr) obtained with the HETGS. We use the high-resolution grating spectrum and moderate-resolution CCD spectrum of the massive central star sigma Ori AB (O9.5 V + B0.5 V) to test wind shock theories of X-ray emission and also analyze the high spatial resolution zero-order ACIS-S image of the central cluster region. Chandra detected 42 X-ray sources on the primary CCD (ACIS-S3). All but five have near-IR or optical counterparts and about one-fourth are variable. Notable high-mass stellar detections are sigma Ori AB, the magnetic B star sigma Ori E, and the B5 V binary HD 37525. Most of the other detections have properties consistent with lower mass K- or M-type stars. We present the first X-ray spectrum of the unusual infrared source IRS 1, located approximate to 3 \u27\u27 north of sigma Ori AB. Its X-ray properties and elongated mid-IR morphology suggest that it is an embedded low-mass T Tauri star whose disk/envelope is being photoevaporated by sigma Ori AB. We focus on the radiative wind shock interpretation of the soft luminous X-ray emission from sigma Ori AB, but also consider possible alternatives including magnetically confined wind shocks and colliding wind shocks. Its emission lines show no significant asymmetries or centroid shifts and are moderately broadened to HWHM approximate to 264 km s(-1), or one-fourth the terminal wind speed. Forbidden lines in He-like ions are formally undetected, implying strong UV suppression. The Mg XI triplet forms in the wind acceleration zone within one stellar radius above the surface. These X-ray properties are consistent in several respects with the predictions of radiative wind shock theory for an optically thin wind, but explaining the narrow line widths presents a challenge to the theory
NGC 1624-2: A slowly rotating, X-ray luminous Of?cp star with an extraordinarily strong magnetic field
This paper presents a first observational investigation of the faint Of?cp
star NGC 1624-2, yielding important new constraints on its spectral and
physical characteristics, rotation, magnetic field strength, X-ray emission and
magnetospheric properties. Modeling the spectrum and spectral energy
distribution, we conclude that NGC 1624-2 is a main sequence star of mass M
{\simeq} 30 M{\odot}, and infer an effective temperature of 35 {\pm} 2 kK and
log g = 4.0 {\pm} 0.2. Based on an extensive time series of optical spectral
observations we report significant variability of a large number of spectral
lines, and infer a unique period of 157.99 {\pm} 0.94 d which we interpret as
the rotational period of the star. We report the detection of a very strong -
5.35 {\pm} 0.5 kG - longitudinal magnetic field , coupled with probable
Zeeman splitting of Stokes I profiles of metal lines confirming a surface field
modulus of 14 {\pm} 1 kG, consistent with a surface dipole of polar
strength >~ 20 kG. This is the largest magnetic field ever detected in an
O-type star, and the first report of Zeeman splitting of Stokes I profiles in
such an object. We also report the detection of reversed Stokes V profiles
associated with weak, high-excitation emission lines of O iii, which we propose
may form in the close magnetosphere of the star. We analyze archival Chandra
ACIS-I X-ray data, inferring a very hard spectrum with an X-ray efficiency log
Lx/Lbol = -6.4, a factor of 4 larger than the canonical value for O-type stars
and comparable to that of the young magnetic O-type star {\theta}1 Ori C and
other Of?p stars. Finally, we examine the probable magnetospheric properties of
the star, reporting in particular very strong magnetic confinement of the
stellar wind, with {\eta}* {\simeq} 1.5 {\times} 10^4, and a very large Alfven
radius, RAlf = 11.4 R*.Comment: 17 pages, MNRAS accepted and in pres
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