760 research outputs found
A note on a third order curvature invariant in static spacetimes
We consider here the third order curvature invariant
in static spacetimes
for which is conformally flat. We evaluate
explicitly the invariant for the -dimensional Majumdar-Papapetrou multi
black-holes solution, confirming that does indeed vanish on the event
horizons of such black-holes. Our calculations show, however, that solely the
vanishing of is not sufficient to locate an event horizon in
non-spherically symmetric spacetimes. We discuss also some tidal effects
associated to the invariant .Comment: 5 pages, 3 figures. Extra material available at
http://vigo.ime.unicamp.br/in
Unveiling extremely veiled T Tauri stars
Photospheric absorption lines in classical T Tauri stars (CTTS) are weak
compared to normal stars. This so-called veiling is normally identified with an
excess continuous emission formed in shock-heated gas at the stellar surface
below the accretion streams. We have selected four stars (RW Aur A, RU Lup, S
CrA NW and S CrA SE) with unusually strong veiling to make a detailed
investigation of veiling versus stellar brightness and emission line strengths
for comparisons to standard accretion models. We have monitored the stars
photometrically and spectroscopically at several epochs. In standard accretion
models a variable accretion rate will lead to a variable excess emission.
Consequently, the stellar brightness should vary accordingly. We find that the
veiling of absorption lines in these stars is strongly variable and usually so
large that it would require the release of several stellar luminosities of
potential energy. At states of very large line dilution, the correspondingly
large veiling factors derived correlate only weakly with brightness. Moreover,
the emission line strengths violate the expected trend of veiling versus line
strength. The veiling can change dramatically in one night, and is not
correlated with the phase of the rotation periods found for two stars. We show
that in at least three of the stars, when the veiling becomes high, the
photospheric lines become filled-in by line emission, which produces large
veiling factors unrelated to changes in any continuous emission from shocked
regions. We also consider to what extent extinction by dust and electron
scattering in the accretion stream may affect veiling measures in CTTS. We
conclude that the degree of veiling cannot be used as a measure of accretion
rates in CTTS with rich emission line spectra.Comment: Accepted for publication in A&A Letters. New language-edited version.
(4 pages, 3 figures
The Spectral Signature of Dust Scattering and Polarization in the Near IR to Far UV. I. Optical Depth and Geometry Effects
Spectropolarimetry from the near IR to the far UV of light scattered by dust
provides a valuable diagnostic of the dust composition, grain size distribution
and spatial distribution. To facilitate the use of this diagnostic, we present
detailed calculations of the intensity and polarization spectral signature of
light scattered by optically thin and optically thick dust in various
geometries. The polarized light radiative transfer calculations are carried out
using the adding-doubling method for a plane-parallel slab, and are extended to
an optically thick sphere by integrating over its surface. The calculations are
for the Mathis, Rumple & Nordsieck Galactic dust model, and cover the range
from 1 to 500 \AA. We find that the wavelength dependence of the
scattered light intensity provides a sensitive probe of the optical depth of
the scattering medium, while the polarization wavelength dependence provides a
probe of the grain scattering properties, which is practically independent of
optical depth. We provide a detailed set of predictions, including polarization
maps, which can be used to probe the properties of dust through imaging
spectropolarimetry in the near IR to far UV of various Galactic and
extragalactic objects. In a following paper we use the codes developed here to
provide predictions for the dependence of the intensity and polarization on
grain size distribution and composition.Comment: 29 pages + 21 figures, accepted for the Astrophysical Journal
Supplement February 2000 issue. Some revision, mostly in the introduction and
the conclusions, and a couple of correction
Assessment of Gas-Surface Interaction Models for Computation of Rarefied Hypersonic Flow
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76727/1/AIAA-36375-113.pd
Implementation of Flow Tripping Capability in the USM3D Unstructured Flow Solver
A flow tripping capability is added to an established NASA tetrahedral unstructured parallel Navier-Stokes flow solver, USM3D. The capability is based on prescribing an appropriate profile of turbulence model variables to energize the boundary layer in a plane normal to a specified trip region on the body surface. We demonstrate this approach using the k-e two-equation turbulence model of USM3D. Modification to the solution procedure primarily consists of developing a data structure to identify all unstructured tetrahedral grid cells located in the plane normal to a specified surface trip region and computing a function based on the mean flow solution to specify the modified profile of the turbulence model variables. We leverage this data structure and also show an adjunct approach that is based on enforcing a laminar flow condition on the otherwise fully turbulent flow solution in user specified region. The latter approach is applied for the solutions obtained using other one- and two-equation turbulence models of USM3D. A key ingredient of the present capability is the use of a graphical user-interface tool PREDISC to define a trip region on the body surface in an existing grid. Verification of the present modifications is demonstrated on three cases, namely, a flat plate, the RAE2822 airfoil, and the DLR F6 wing-fuselage configuration
Esophageal motor abnormalities in scleroderma and related diseases
Esophageal motor activity was measured by intra-esophageal pressure recordings in 53 patients with scleroderma and 29 patients with other collagen diseases. The purpose of the study was to determine the relationship of motor abnormalities to esophageal symptoms, to compare the abnormalities in scleroderma with those in other collagen diseases, and to try to increase understanding of the responsible mechanism. Methacholine was given to 36 of the 53 patients with scleroderma to confirm that the Mecholyl test is negative in scleroderma and to see whether intraluminal pressure changes accompany the resulting improvement in esophageal emptying.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44360/1/10620_2005_Article_BF02233564.pd
Mass-loss rates of Very Massive Stars
We discuss the basic physics of hot-star winds and we provide mass-loss rates
for (very) massive stars. Whilst the emphasis is on theoretical concepts and
line-force modelling, we also discuss the current state of observations and
empirical modelling, and address the issue of wind clumping.Comment: 36 pages, 15 figures, Book Chapter in "Very Massive Stars in the
Local Universe", Springer, Ed. Jorick S. Vin
Commitment of independent and institutional women directors to corporate social responsibility reporting
This paper examines how independent and institutional women directors on boards affect corporate social responsibility (hereafter CSR) reporting. Most of the previous empirical evidence has shown a linear association between female directors and CSR disclosure, but to the best of our knowledge, no research has investigated the individual effect of independent and institutional female directors on CSR reporting. Therefore, the analysis of how the disclosure of CSR information is affected by independent and institutional women directors in a separate way merits our attention. Thus, we posit that there is a nonlinear association, concretely quadratic, between independent and institutional female directors on boards and CSR reporting. Our results demonstrate that, in line with the monitoring hypothesis, as the presence of independent and institutional women directors on boards increases, the CSR disclosure improves, but when their presence on boards reaches a tipping point (20.47% and 13.32%, respectively), CSR reporting decreases, which is consistent with the collusion hypothesis. This research contributes to the existing literature on the relationship between board gender diversity and CSR disclosure by suggesting that board structures formed by institutional and independent female directors have an effect on CSR reporting. Hence, female directors play a relevant role on boards since they may influence the CSR disclosure
The deuteron: structure and form factors
A brief review of the history of the discovery of the deuteron in provided.
The current status of both experiment and theory for the elastic electron
scattering is then presented.Comment: 80 pages, 33 figures, submited to Advances in Nuclear Physic
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