597 research outputs found
On a class of 2-surface observables in general relativity
The boundary conditions for canonical vacuum general relativity is
investigated at the quasi-local level. It is shown that fixing the area element
on the 2- surface S (rather than the induced 2-metric) is enough to have a well
defined constraint algebra, and a well defined Poisson algebra of basic
Hamiltonians parameterized by shifts that are tangent to and divergence-free on
$. The evolution equations preserve these boundary conditions and the value of
the basic Hamiltonian gives 2+2 covariant, gauge-invariant 2-surface
observables. The meaning of these observables is also discussed.Comment: 11 pages, a discussion of the observables in stationary spacetimes is
included, new references are added, typos correcte
Discovery of blue companions to two southern Cepheids: WW Car and FN Vel
A large number of high-dispersion spectra of classical Cepheids were obtained
in the region of the CaII H+K spectral lines. The analysis of these spectra
allowed us to detect the presence of a strong Balmer line, H, for
several Cepheids, interpreted as the signature of a blue companion: the
presence of a sufficiently bright blue companion to the Cepheid results in a
discernible strengthening of the CaII H + Hepsilon line relative to the CaII K
line. We investigated 103 Cepheids, including those with known hot companions
(B5-B6 main-sequence stars) in order to test the method. We could confirm the
presence of a companion to WW Car and FN Vel (the existence of the former was
only suspected before) and we found that these companions are blue hot stars.
The method remains efficient when the orbital velocity changes in a binary
system cannot be revealed and other methods of binarity detection are not
efficient.Comment: 6 pages, 5 figures, 4 tables, published on MNRAS in March 201
Approximate Killing Vectors on S^2
We present a new method for computing the best approximation to a Killing
vector on closed 2-surfaces that are topologically S^2. When solutions of
Killing's equation do not exist, this method is shown to yield results superior
to those produced by existing methods. In addition, this method appears to
provide a new tool for studying the horizon geometry of distorted black holes.Comment: 4 pages, 3 figures, submitted to Physical Review D, revtex
The kernel of the edth operators on higher-genus spacelike two-surfaces
The dimension of the kernels of the edth and edth-prime operators on closed,
orientable spacelike 2-surfaces with arbitrary genus is calculated, and some of
its mathematical and physical consequences are discussed.Comment: 12 page
Negative Komar Mass of Single Objects in Regular, Asymptotically Flat Spacetimes
We study two types of axially symmetric, stationary and asymptotically flat
spacetimes using highly accurate numerical methods. The one type contains a
black hole surrounded by a perfect fluid ring and the other a rigidly rotating
disc of dust surrounded by such a ring. Both types of spacetime are regular
everywhere (outside of the horizon in the case of the black hole) and fulfil
the requirements of the positive energy theorem. However, it is shown that both
the black hole and the disc can have negative Komar mass. Furthermore, there
exists a continuous transition from discs to black holes even when their Komar
masses are negative.Comment: 7 pages, 2 figures, document class iopart. v2: changes made
(including title) to coincide with published versio
A variational principle for stationary, axisymmetric solutions of Einstein's equations
Stationary, axisymmetric, vacuum, solutions of Einstein's equations are
obtained as critical points of the total mass among all axisymmetric and
symmetric initial data with fixed angular momentum. In this
variational principle the mass is written as a positive definite integral over
a spacelike hypersurface. It is also proved that if absolute minimum exists
then it is equal to the absolute minimum of the mass among all maximal,
axisymmetric, vacuum, initial data with fixed angular momentum. Arguments are
given to support the conjecture that this minimum exists and is the extreme
Kerr initial data.Comment: 21 page
Observations of Cepheids with the MOST satellite: Contrast between Pulsation Modes
The quantity and quality of satellite photometric data strings is revealing
details in Cepheid variation at very low levels. Specifically, we observed a
Cepheid pulsating in the fundamental mode and one pulsating in the first
overtone with the Canadian MOST satellite. The 3.7-d period fundamental mode
pulsator (RT Aur) has a light curve that repeats precisely, and can be modeled
by a Fourier series very accurately. The overtone pulsator (SZ Tau, 3.1 d
period) on the other hand shows light curve variation from cycle to cycle which
we characterize by the variations in the Fourier parameters. We present
arguments that we are seeing instability in the pulsation cycle of the overtone
pulsator, and that this is also a characteristic of the O-C curves of overtone
pulsators. On the other hand, deviations from cycle to cycle as a function of
pulsation phase follow a similar pattern in both stars, increasing after
minimum radius. In summary, pulsation in the overtone pulsator is less stable
than that of the fundamental mode pulsator at both long and short timescales.Comment: accepted in MNRAS, 11 pages, 10 figure
Fundamental parameters of Cepheids. V. Additional photometry and radial velocity for southern Cepheids
I present photometric and radial velocity data for Galactic Cepheids, most of
them being in the southern hemisphere. There are 1250 Geneva 7-color
photometric measurements for 62 Cepheids, the average uncertainty per
measurement is better than 0.01 mag. A total of 832 velocity measurements have
been obtained with the CORAVEL radial velocity spectrograph for 46 Cepheids.
The average accuracy of the radial velocity data is 0.38 km/s. There are 33
stars with both photometry and radial velocity data. I discuss the possible
binarity or period change that these new data reveal. I also present reddenings
for all Cepheids with photometry. The data are available electronically.Comment: To appear in ApJS. Data available electronically at
ftp://cfa-ftp.harvard.edu/pub/dbersier
Conservation laws for vacuum tetrad gravity
Ten conservation laws in useful polynomial form are derived from a Cartan
form and Exterior Differential System (EDS) for the tetrad equations of vacuum
relativity. The Noether construction of conservation laws for well posed EDS is
introduced first, and an illustration given, deriving 15 conservation laws of
the free field Maxwell Equations from symmetries of its EDS. The Maxwell EDS
and tetrad gravity EDS have parallel structures, with their numbers of
dependent variables, numbers of generating 2-forms and generating 3-forms, and
Cartan character tables all in the ratio of 1 to 4. They have 10 corresponding
symmetries with the same Lorentz algebra, and 10 corresponding conservation
laws.Comment: Final version with additional reference
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