197 research outputs found
Spinor classification of the Weyl tensor in five dimensions
We investigate the spinor classification of the Weyl tensor in five
dimensions due to De Smet. We show that a previously overlooked reality
condition reduces the number of possible types in the classification. We
classify all vacuum solutions belonging to the most special algebraic type. The
connection between this spinor and the tensor classification due to Coley,
Milson, Pravda and Pravdov\'a is investigated and the relation between most of
the types in each of the classifications is given. We show that the black ring
is algebraically general in the spinor classification.Comment: 40 page
Kerr-Schild spacetimes with (A)dS background
General properties of Kerr-Schild spacetimes with (A)dS background in
arbitrary dimension are studied. It is shown that the geodetic Kerr-Schild
vector k is a multiple WAND of the spacetime. Einstein Kerr-Schild spacetimes
with non-expanding k are shown to be of Weyl type N, while the expanding
spacetimes are of type II or D. It is shown that this class of spacetimes obeys
the optical constraint. This allows us to solve Sachs equation, determine
r-dependence of boost weight zero components of the Weyl tensor and discuss
curvature singularities.Comment: 17 pages, minor change
An anisotropic distribution of spin vectors in asteroid families
Current amount of ~500 asteroid models derived from the disk-integrated
photometry by the lightcurve inversion method allows us to study not only the
spin-vector properties of the whole population of MBAs, but also of several
individual collisional families. We create a data set of 152 asteroids that
were identified by the HCM method as members of ten collisional families, among
them are 31 newly derived unique models and 24 new models with well-constrained
pole-ecliptic latitudes of the spin axes. The remaining models are adopted from
the DAMIT database or the literature. We revise the preliminary family
membership identification by the HCM method according to several additional
criteria - taxonomic type, color, albedo, maximum Yarkovsky semi-major axis
drift and the consistency with the size-frequency distribution of each family,
and consequently we remove interlopers. We then present the spin-vector
distributions for eight asteroidal families. We use a combined orbital- and
spin-evolution model to explain the observed spin-vector properties of objects
among collisional families. In general, we observe for studied families similar
trends in the (a_p, \beta) space: (i) larger asteroids are situated in the
proximity of the center of the family; (ii) asteroids with \beta>0{\deg} are
usually found to the right from the family center; (iii) on the other hand,
asteroids with \beta<0{\deg} to the left from the center; (iv) majority of
asteroids have large pole-ecliptic latitudes (|\beta|\gtrsim 30{\deg}); and
finally (v) some families have a statistically significant excess of asteroids
with \beta>0{\deg} or \beta<0{\deg}. Our numerical simulation of the long-term
evolution of a collisional family is capable of reproducing well the observed
spin-vector properties. Using this simulation, we also independently constrain
the age of families Flora (1.0\pm0.5 Gyr) and Koronis (2.5-4 Gyr).Comment: Accepted for publication in A&A (September 16, 2013
The Appearance and Disappearance of Ship Tracks on Large Spatial Scales
The 1-km advanced very high resolution radiometer observations from the morning, NOAA-12, and afternoon,
NOAA-11, satellite passes over the coast of California during June 1994 are used to determine the altitudes,
visible optical depths, and cloud droplet effective radii for low-level clouds. Comparisons are made between
the properties of clouds within 50 km of ship tracks and those farther than 200 km from the tracks in order to
deduce the conditions that are conducive to the appearance of ship tracks in satellite images. The results indicate
that the low-level clouds must be sufficiently close to the surface for ship tracks to form. Ship tracks rarely
appear in low-level clouds having altitudes greater than 1 km. The distributions of visible optical depths and
cloud droplet effective radii for ambient clouds in which ship tracks are embedded are the same as those for
clouds without ship tracks. Cloud droplet sizes and liquid water paths for low-level clouds do not constrain the
appearance of ship tracks in the imagery. The sensitivity of ship tracks to cloud altitude appears to explain why
the majority of ship tracks observed from satellites off the coast of California are found south of 358N. A small
rise in the height of low-level clouds appears to explain why numerous ship tracks appeared on one day in a
particular region but disappeared on the next, even though the altitudes of the low-level clouds were generally
less than 1 km and the cloud cover was the same for both days. In addition, ship tracks are frequent when lowlevel
clouds at altitudes below 1 km are extensive and completely cover large areas. The frequency of imagery
pixels overcast by clouds with altitudes below 1 km is greater in the morning than in the afternoon and explains
why more ship tracks are observed in the morning than in the afternoon. If the occurrence of ship tracks in
satellite imagery data depends on the coupling of the clouds to the underlying boundary layer, then cloud-top
altitude and the area of complete cloud cover by low-level clouds may be useful indices for this coupling.This work was supported in part by the Office of Naval Research and by the National Science Foundation through the Center for Clouds, Chemistry and Climate at the Scripps Institution of Oceanography, an NSF Science and Technology Center
On higher dimensional Einstein spacetimes with a warped extra dimension
We study a class of higher dimensional warped Einstein spacetimes with one
extra dimension. These were originally identified by Brinkmann as those
Einstein spacetimes that can be mapped conformally on other Einstein
spacetimes, and have subsequently appeared in various contexts to describe,
e.g., different braneworld models or warped black strings. After clarifying the
relation between the general Brinkmann metric and other more specific
coordinate systems, we analyze the algebraic type of the Weyl tensor of the
solutions. In particular, we describe the relation between Weyl aligned null
directions (WANDs) of the lower dimensional Einstein slices and of the full
spacetime, which in some cases can be algebraically more special. Possible
spacetime singularities introduced by the warp factor are determined via a
study of scalar curvature invariants and of Weyl components measured by
geodetic observers. Finally, we illustrate how Brinkmann's metric can be
employed to generate new solutions by presenting the metric of spinning and
accelerating black strings in five dimensional anti-de Sitter space.Comment: 14 pages, minor changes in the text, mainly in Section 2.
Exact Black Holes and Universality in the Backreaction of non-linear Sigma Models with a potential in (A)dS4
The aim of this paper is to construct accelerated, stationary and
axisymmetric exact solutions of the Einstein theory with self interacting
scalar fields in (A)dS4. To warm up, the backreaction of the (non)-minimally
coupled scalar field is solved, the scalar field equations are integrated and
all the potentials compatible with the metric ansatz and Einstein gravity are
found. With these results at hand the non-linear sigma model is tackled. The
scalar field Lagrangian is generic; neither the coupling to the curvature,
neither the metric in the scalar manifold nor the potential, are fixed ab
initio. The unique assumption in the analysis is the metric ansatz: it has the
form of the most general Petrov type D vacuum solution of general relativity;
it is a a cohomogeneity two Weyl rescaling of the Carter metric and therefore
it has the typical Plebanski-Demianski form with two arbitrary functions of one
variable and one arbitrary functions of two variables. It is shown, by an
straightforward manipulation of the field equations, that the metric is
completely integrable without necessity of specifiying anything in the scalar
Lagrangian. This results in that the backreaction of the scalar fields, within
this class of metrics, is universal. The metric functions generically show an
explicit dependence on a dynamical exponent that allows to smoothly connect
this new family of solutions with the actual Plebanski-Demianski spacetime. The
remaining field equations imply that the scalar fields follow geodesics in the
scalar manifold with an affine parameter given by a non-linear function of the
spacetime coordinates and define the on-shell form of the potential plus a
functional equation that it has to satisfy. Finally, a general family of (A)dS4
static hairy black holes is explicitly constructed and its properties are
outlined.Comment: Several typos correcte
Geodesics and Symmetries of Doubly-Spinning Black Rings
This paper studies various properties of the Pomeransky-Sen'kov
doubly-spinning black ring spacetime. I discuss the structure of the
ergoregion, and then go on to demonstrate the separability of the
Hamilton-Jacobi equation for null, zero energy geodesics, which exist in the
ergoregion. These geodesics are used to construct geometrically motivated
coordinates that cover the black hole horizon. Finally, I relate this weak form
of separability to the existence of a conformal Killing tensor in a particular
4-dimensional spacetime obtained by Kaluza-Klein reduction, and show that a
related conformal Killing-Yano tensor only exists in the singly-spinning case.Comment: Minor corrections/clarifications and references added, results of
paper unchanged. Accepted for publication by Class. Quant. Grav. (26 pages, 5
figures
Planetary Transits of the Trans-Atlantic Exoplanet Survey- Candidate TrES-1b
The AAVSO compiled 10,560 CCD observations of the suspected exoplanet transit object TrES-1b covering seven complete transit windows, three windows of partial coverage, and coverage of baseline non-transit periods. Visual inspection of the light curves reveals the presence of slight humps at the egress points of some transits. A boot strap Monte Carlo simulation was applied to the data to confirm that the humps exist to a statistically significant degree. However, it does not rule out systemic effects which will be tested with campaigns in the 2005 observing season
The Recently-Discovered Dwarf Nova System ASAS J002511+1217.2: A New WZ Sagittae Star
The cataclysmic variable ASAS J002511+1217.2 was discovered in outburst by
the All-Sky Automated Survey in September 2004, and intensively monitored by
AAVSO observers through the following two months. Both photometry and
spectroscopy indicate that this is a very short-period system. Clearly defined
superhumps with a period of 0.05687 +/- 0.00001 days (1-sigma) are present
during the superoutburst, 5 to 18 days following the ASAS detection. We observe
a change in superhump profile similar to the transition to ``late superhumps''
observed in other short-period systems; the superhump period appears to
increase slightly for a time before returning to the original value, with the
resulting superhump phase offset by approximately half a period. We detect
variations with a period of 0.05666 +/- 0.00003 days (1-sigma) during the
four-day quiescent phase between the end of the main outburst and the single
echo outburst. Weak variations having the original superhump period reappear
during the echo and its rapid decline. Time-resolved spectroscopy conducted
nearly 30 days after detection and well into the decline yields an orbital
period measurement of 82 +/- 5 minutes. Both narrow and broad components are
present in the emission line spectra, indicating the presence of multiple
emission regions. The weight of the observational evidence suggests that ASAS
J002511+1217.2 is a WZ Sge-type dwarf nova, and we discuss how this system fits
into the WZ classification scheme.Comment: 24 pages, 11 figures, accepted to PASP; minor revision to add two
authors and adjust text to match that of the published version. No
adjustments to results or conclusion
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