1,205 research outputs found
The Cow Boy Schottische
https://digitalcommons.library.umaine.edu/mmb-ps/1017/thumbnail.jp
The Discovery of a Strong Magnetic Field and Co-rotating Magnetosphere in the Helium-weak Star HD 176582
We report the detection of a strong, reversing magnetic field and variable
H-alpha emission in the bright helium-weak star HD 176582 (HR 7185). Spectrum,
magnetic and photometric variability of the star are all consistent with a
precisely determined period of 1.5819840 +/- 0.0000030 days which we assume to
be the rotation period of the star. From the magnetic field curve, and assuming
a simple dipolar field geometry, we derive a polar field strength of
approximately 7 kG and a lower limit of 52 degrees for the inclination of the
rotation axis. However, based on the behaviour of the H-alpha emission we adopt
a large inclination angle of 85 degrees and this leads to a large magnetic
obliquity of 77 degrees. The H-alpha emission arises from two distinct regions
located at the intersections of the magnetic and rotation equators and which
corotate with the star at a distance of about 3.5 R* above its surface. We
estimate that the emitting regions have radial and meridional sizes on the
order of 2 R* and azimuthal extents (perpendicular to the magnetic equator) of
less than approximately 0.6 R*. HD 176582 therefore appears to show many of the
cool magnetospheric phenomena as that displayed by other magnetic helium-weak
and helium-strong stars such as the prototypical helium-strong star sigma Ori
E. The observations are consistent with current models of magnetically confined
winds and rigidly-rotating magnetospheres for magnetic Bp stars.Comment: 16 pages, 6 figure
Centrifugal Breakout of Magnetically Confined Line-Driven Stellar Winds
We present 2D MHD simulations of the radiatively driven outflow from a
rotating hot star with a dipole magnetic field aligned with the star's rotation
axis. We focus primarily on a model with moderately rapid rotation (half the
critical value), and also a large magnetic confinement parameter, . The magnetic field
channels and torques the wind outflow into an equatorial, rigidly rotating disk
extending from near the Kepler corotation radius outwards. Even with
fine-tuning at lower magnetic confinement, none of the MHD models produce a
stable Keplerian disk. Instead, material below the Kepler radius falls back on
to the stellar surface, while the strong centrifugal force on material beyond
the corotation escape radius stretches the magnetic loops outwards, leading to
episodic breakout of mass when the field reconnects. The associated dissipation
of magnetic energy heats material to temperatures of nearly K, high
enough to emit hard (several keV) X-rays. Such \emph{centrifugal mass ejection}
represents a novel mechanism for driving magnetic reconnection, and seems a
very promising basis for modeling X-ray flares recently observed in rotating
magnetic Bp stars like Ori E.Comment: 5 pages, 3 figures, accepted by ApJ
Development of a window correlation matching method for improved radar rainfall estimation
International audienceThe present study develops a method called window correlation matching method (WCMM) to reduce collocation and timing errors in matching pairs of radar measured reflectivity, Ze, and gauge measured rainfall intensity, R, for improving the accuracy of the estimation of Ze?R relationships. This method was compared with the traditional matching method (TMM), the probability matching method (PMM) and the window probability matching method (WPMM). The calibrated relationship Ze=18.05 R1.45 obtained from 7×7 km of space window and both present and 5 min previous time of radar observation for time window (S77T5) produces the best results for radar rainfall estimates for orographic rain over the Mae Chaem Watershed in the north of Thailand. The comparison shows that the Ze?R relationship obtained from WCMM provide more accuracy in radar rainfall estimates as compared with the other three methods. The Ze?R relationships estimated using TMM and PMM provide large overestimation and underestimation, respectively, of mean areal rainfall whereas WPMM slightly underestimated the mean areal rainfall. Based on the overall results, it can be concluded that WCMM can reduce collocation and timing errors in Ze?R pairs matching and improve the estimation of Ze?R relationships for radar rainfall. WCMM is therefore a promising method for improved radar-measured rainfall, which is an important input for hydrological and environmental modeling and water resources management
Signaling alterations and cytokine profiles in late metastatic colorectal cancer promote an antiinflammatory profile and proliferation enhacement
Comunicaciones a congreso
Stellar magnetic field parameters from a Bayesian analysis of high-resolution spectropolarimetric observations
In this paper we describe a Bayesian statistical method designed to infer the
magnetic properties of stars observed using high-resolution circular
spectropolarimetry in the context of large surveys. This approach is well
suited for analysing stars for which the stellar rotation period is not known,
and therefore the rotational phases of the observations are ambiguous. The
model assumes that the magnetic observations correspond to a dipole oblique
rotator, a situation commonly encountered in intermediate and high-mass stars.
Using reasonable assumptions regarding the model parameter prior probability
density distributions, the Bayesian algorithm determines the posterior
probability densities corresponding to the surface magnetic field geometry and
strength by performing a comparison between the observed and computed Stokes V
profiles.
Based on the results of numerical simulations, we conclude that this method
yields a useful estimate of the surface dipole field strength based on a small
number (i.e. 1 or 2) of observations. On the other hand, the method provides
only weak constraints on the dipole geometry. The odds ratio, a parameter
computed by the algorithm that quantifies the relative appropriateness of the
magnetic dipole model versus the non-magnetic model, provides a more sensitive
diagnostic of the presence of weak magnetic signals embedded in noise than
traditional techniques.
To illustrate the application of the technique to real data, we analyse seven
ESPaDOnS and Narval observations of the early B-type magnetic star LP Ori.
Insufficient information is available to determine the rotational period of the
star and therefore the phase of the data; hence traditional modelling
techniques fail to infer the dipole strength. In contrast, the Bayesian method
allows a robust determination of the dipole polar strength,
G.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Societ
Wind Circulation in Selected Rotating Magnetic Early-B Stars
The rotating magnetic B stars have oblique dipolar magnetic fields and often
anomalous helium and metallic compositions. These stars develop co-rotating
torus-shaped clouds by channelling winds from their magnetic poles to an
anchored planar disk over the magnetic equator. The line absorptions from the
cloud can be studied as the complex rotates and periodically occults the star.
We describe an analysis of the clouds of four stars (HD184927, beta Cep, sigma
Ori E, and HR6684). From line synthesis models, we find that the metallic
compositions are spatially uniform over the stars' surfaces. Next, using the
Hubeny CIRCUS code, we demonstate that periodic UV continuum fluxes can be
explained by the absorption of low-excitation lines. The analysis also
quantifies the cloud temperatures, densities, and turbulences, which appear to
increase inward toward the stars. The temperatures range from about 12,000K for
the weak Fe lines up to temperatures of 33,000K for N V absorptions, which is
in excess of temperatures expected from radiative equilibrium.
The spectroscopic hallmark of this stellar class is the presence of strong C
IV and N V resonance line absorptions at occultation phases and of redshifted
emissions at magnetic pole-on phases. The emissions have characteristics which
seem most compatible with the generation of high-energy shocks at the
wind-cloud interface, as predicted by Babel.Comment: 19 pages, Latex plus 6 figures A&A single-spaced, accepted by
Astronomy & Astrophysics. Files available by ftp at
nobel.stsci.edu/pub/aapaper
Stable, Thermally Conductive Fillers for Bolted Joints
A commercial structural epoxy [Super Koropon (or equivalent)] has been found to be a suitable filler material for bolted joints that are required to have large thermal conductances. The contact area of such a joint can be less than 1 percent of the apparent joint area, the exact value depending on the roughnesses of the mating surfaces. By occupying the valleys between contact peaks, the filler widens the effective cross section for thermal conduction. In comparison with prior thermal joint-filler materials, the present epoxy offers advantages of stability, ease of application, and -- as a byproduct of its stability -- lasting protection against corrosion. Moreover, unlike silicone greases that have been used previously, this epoxy does not migrate to contaminate adjacent surfaces. Because this epoxy in its uncured state wets metal joint surfaces and has low viscosity, it readily flows to fill the gaps between the mating surfaces: these characteristics affect the overall thermal conductance of the joint more than does the bulk thermal conductivity of the epoxy, which is not exceptional. The thermal conductances of metal-to-metal joints containing this epoxy were found to range between 5 and 8 times those of unfilled joints
Analyzing Femorotibial Cartilage Thickness Using Anatomically Standardized Maps: Reproducibility and Reference Data.
Alterations in cartilage thickness (CTh) are a hallmark of knee osteoarthritis, which remain difficult to characterize at high resolution, even with modern magnetic resonance imaging (MRI), due to a paucity of standardization tools. This study aimed to assess a computational anatomy method producing standardized two-dimensional femorotibial CTh maps. The method was assessed with twenty knees, processed following three common experimental scenarios. Cartilage thickness maps were obtained for the femorotibial cartilages by reconstructing bone and cartilage mesh models in tree-dimension, calculating three-dimensional CTh maps, and anatomically standardizing the maps. The intra-operator accuracy (median (interquartile range, IQR) of -0.006 (0.045) mm), precision (0.152 (0.070) mm), entropy (7.02 (0.71) and agreement (0.975 (0.020))) results suggested that the method is adequate to capture the spatial variations in CTh and compare knees at varying osteoarthritis stages. The lower inter-operator precision (0.496 (0.132) mm) and agreement (0.808 (0.108)) indicate a possible loss of sensitivity to detect differences in a setting with multiple operators. The results confirmed the promising potential of anatomically standardized maps, with the lower inter-operator reproducibility stressing the need to coordinate operators. This study also provided essential reference data and indications for future research using CTh maps
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