507 research outputs found
Visualization and spectral synthesis of rotationally distorted stars
Simple spherical, non-rotating stellar models are inadequate when describing
real stars in the limit of very fast rotation: Both the observable spectrum and
the geometrical shape of the star deviate strongly from simple models. We
attempt to approach the problem of modeling geometrically distorted, rapidly
rotating stars from a new angle: By constructing distorted geometrical models
and integrating standard stellar models with varying temperature, gravity, and
abundances, over the entire surface, we attempt a semi-empirical approach to
modeling. Here we present our methodology, and present simple examples of
applications.Comment: 7 pages, 5 figures. Contribution to GREAT-ESF Workshop on "Stellar
Atmospheres in the Gaia Era", Brussels 23-24 June 201
Probing Dust in the Atmosphere of Brown Dwarfs Through Polarization
Theoretical analysis and observational evidences indicate that a brown dwarf
with effective temperature greater than 1400 K would have dust cloud in its
atmosphere. In this letter, we show that dust scattering should yield polarized
continuum radiation from the relatively warm brown dwarfs and the polarized
flux profile could be a potential diagnosis tool for the optical and the
physical properties of dust grains. The degree of polarization due to multiple
scattering will be more in the optical region if the particle size is small
while significant polarization should be detected in the infra-red region if
the particle size is large. It is pointed out that the departure from
sphericity in the shape of the object due to rapid rotation and due to tidal
effect by the companion in a binary system ensures the disc integrated
polarization to be non-zero.Comment: 9 pages (Latex AAS v4.0), 2 postscript figures, Accepted by The
Astrophysical Journal Letter
(In)finite extent of stationary perfect fluids in Newtonian theory
For stationary, barotropic fluids in Newtonian gravity we give simple
criteria on the equation of state and the "law of motion" which guarantee
finite or infinite extent of the fluid region (providing a priori estimates for
the corresponding stationary Newton-Euler system). Under more restrictive
conditions, we can also exclude the presence of "hollow" configurations. Our
main result, which does not assume axial symmetry, uses the virial theorem as
the key ingredient and generalises a known result in the static case. In the
axially symmetric case stronger results are obtained and examples are
discussed.Comment: Corrections according to the version accepted by Ann. Henri Poincar
The average kinetic energy density of Cooper pairs above in , , and
We have obtained isofield curves for the square root of the average kinetic
energy density of the superconducting state for three single crystals of
underdoped , an optimally doped single crystal of
, and Nb. These curves, determined from isofield
magnetization versus temperature measurements and the virial theorem of
superconductivity, probe the order parameter amplitude near the upper critical
field. The striking differences between the Nb and the high- curves
clearly indicate for the latter cases the presence of a unique superconducting
condensate below and above
Hard Interactions of Quarks and Gluons: a Primer for LHC Physics
In this review article, we develop the perturbative framework for the
calculation of hard scattering processes. We undertake to provide both a
reasonably rigorous development of the formalism of hard scattering of quarks
and gluons as well as an intuitive understanding of the physics behind the
scattering. We emphasize the importance of logarithmic corrections as well as
power counting of the strong coupling constant in order to understand the
behavior of hard scattering processes. We include "rules of thumb" as well as
"official recommendations", and where possible seek to dispel some myths.
Experiences that have been gained at the Fermilab Tevatron are recounted and,
where appropriate, extrapolated to the LHC.Comment: 118 pages, 107 figures; to be published in Reports on Progress in
Physic
Supermassive Binaries and Extragalactic Jets
Some quasars show Doppler shifted broad emission line peaks. I give new
statistics of the occurrence of these peaks and show that, while the most
spectacular cases are in quasars with strong radio jets inclined to the line of
sight, they are also almost as common in radio-quiet quasars. Theories of the
origin of the peaks are reviewed and it is argued that the displaced peaks are
most likely produced by the supermassive binary model. The separations of the
peaks in the 3C 390.3-type objects are consistent with orientation-dependent
"unified models" of quasar activity. If the supermassive binary model is
correct, all members of "the jet set" (astrophysical objects showing jets)
could be binaries.Comment: 31 pages, PostScript, missing figure is in ApJ 464, L105 (see
http://www.aas.org/ApJ/v464n2/5736/5736.html
Rotation of planet-harbouring stars
The rotation rate of a star has important implications for the detectability,
characterisation and stability of any planets that may be orbiting it. This
chapter gives a brief overview of stellar rotation before describing the
methods used to measure the rotation periods of planet host stars, the factors
affecting the evolution of a star's rotation rate, stellar age estimates based
on rotation, and an overview of the observed trends in the rotation properties
of stars with planets.Comment: 16 pages, 4 figures: Invited review to appear in 'Handbook of
Exoplanets', Springer Reference Works, edited by Hans J. Deeg and Juan
Antonio Belmont
The Spin Structure of the Nucleon
We present an overview of recent experimental and theoretical advances in our
understanding of the spin structure of protons and neutrons.Comment: 84 pages, 29 figure
Differential cross sections and spin density matrix elements for the reaction gamma p -> p omega
High-statistics differential cross sections and spin density matrix elements
for the reaction gamma p -> p omega have been measured using the CLAS at
Jefferson Lab for center-of-mass (CM) energies from threshold up to 2.84 GeV.
Results are reported in 112 10-MeV wide CM energy bins, each subdivided into
cos(theta_CM) bins of width 0.1. These are the most precise and extensive omega
photoproduction measurements to date. A number of prominent structures are
clearly present in the data. Many of these have not previously been observed
due to limited statistics in earlier measurements
photoproduction on the proton for photon energies from 0.725 to 2.875 GeV
Differential cross sections for the reaction have been
measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged
photon beam with energies from 0.725 to 2.875 GeV. Where available, the results
obtained here compare well with previously published results for the reaction.
Agreement with the SAID and MAID analyses is found below 1 GeV. The present set
of cross sections has been incorporated into the SAID database, and exploratory
fits have been made up to 2.7 GeV. Resonance couplings have been extracted and
compared to previous determinations. With the addition of these cross sections
to the world data set, significant changes have occurred in the high-energy
behavior of the SAID cross-section predictions and amplitudes.Comment: 18 pages, 10 figure
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