6,858 research outputs found
Axial rotation and turbulence of RR ab stars: the Peterson Conundrum revisited
We calibrate and then use the relation between equivalent width (EW) and
full-width-half-maximum (FWHM) of metallic absorption lines in the spectra of
RR Lyrae stars to estimate a new upper limit of Vrot sini less than or equal to
6 km/s on their axial equatorial rotational velocities, and to derive the
variations of macroturbulent velocities in their atmospheres during pulsation
cycles. Finally, we present a simple way to estimate macroturbulent/rotational
velocity from FWHM of the cross-correlation function.Comment: 15 pages, 7 figures, 1 table. EAS Publications Series.: "New advances
in stellar physics: from microscopic to macroscopic processes", 27-31 May
2013, Roscoff, Franc
A light-cone gauge for black-hole perturbation theory
The geometrical meaning of the Eddington-Finkelstein coordinates of
Schwarzschild spacetime is well understood: (i) the advanced-time coordinate v
is constant on incoming light cones that converge toward r=0, (ii) the angles
theta and phi are constant on the null generators of each light cone, (iii) the
radial coordinate r is an affine-parameter distance along each generator, and
(iv) r is an areal radius, in the sense that 4 pi r^2 is the area of each
two-surface (v,r) = constant. The light-cone gauge of black-hole perturbation
theory, which is formulated in this paper, places conditions on a perturbation
of the Schwarzschild metric that ensure that properties (i)--(iii) of the
coordinates are preserved in the perturbed spacetime. Property (iv) is lost in
general, but it is retained in exceptional situations that are identified in
this paper. Unlike other popular choices of gauge, the light-cone gauge
produces a perturbed metric that is expressed in a meaningful coordinate
system; this is a considerable asset that greatly facilitates the task of
extracting physical consequences. We illustrate the use of the light-cone gauge
by calculating the metric of a black hole immersed in a uniform magnetic field.
We construct a three-parameter family of solutions to the perturbative
Einstein-Maxwell equations and argue that it is applicable to a broader range
of physical situations than the exact, two-parameter Schwarzschild-Melvin
family.Comment: 12 page
Light-cone coordinates based at a geodesic world line
Continuing work initiated in an earlier publication [Phys. Rev. D 69, 084007
(2004)], we construct a system of light-cone coordinates based at a geodesic
world line of an arbitrary curved spacetime. The construction involves (i) an
advanced-time or a retarded-time coordinate that labels past or future light
cones centered on the world line, (ii) a radial coordinate that is an affine
parameter on the null generators of these light cones, and (iii) angular
coordinates that are constant on each generator. The spacetime metric is
calculated in the light-cone coordinates, and it is expressed as an expansion
in powers of the radial coordinate in terms of the irreducible components of
the Riemann tensor evaluated on the world line. The formalism is illustrated in
two simple applications, the first involving a comoving world line of a
spatially-flat cosmology, the other featuring an observer placed on the axis of
symmetry of Melvin's magnetic universe.Comment: 11 pages, 1 figur
A History of Brooks in transportation and warehousing
Flexibility, versatility ,and attention to service have been the keynotes in the rise of truck transportation to a position of vital importance in the economic life of our nation.
In this picture and playing a vital role are the Brooks Transportation Company, Incorporated, Brooks Warehouse Corporation, and Brooks Transfer and Storage Company, Incorporated. Since its beginning after the Civil War, Brooks has been a leader in the field of truck transportation in tho South-east. With branch offices in most of the major cities along the East Coast, Brooks serves individuals and firms, not only in the State of Virginia, but in a majority of the states east of the Mississippi River
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