50,058 research outputs found
Boundary Conditions for Kerr-AdS Perturbations
The Teukolsky master equation and its associated spin-weighted spheroidal
harmonic decomposition simplify considerably the study of linear gravitational
perturbations of the Kerr(-AdS) black hole. However, the formulation of the
problem is not complete before we assign the physically relevant boundary
conditions. We find a set of two Robin boundary conditions (BCs) that must be
imposed on the Teukolsky master variables to get perturbations that are
asymptotically global AdS, i.e. that asymptotes to the Einstein Static
Universe. In the context of the AdS/CFT correspondence, these BCs allow a
non-zero expectation value for the CFT stress-energy tensor while keeping fixed
the boundary metric. When the rotation vanishes, we also find the gauge
invariant differential map between the Teukolsky and the Kodama-Ishisbashi
(Regge-Wheeler-Zerilli) formalisms. One of our Robin BCs maps to the scalar
sector and the other to the vector sector of the Kodama-Ishisbashi
decomposition. The Robin BCs on the Teukolsky variables will allow for a
quantitative study of instability timescales and quasinormal mode spectrum of
the Kerr-AdS black hole. As a warm-up for this programme, we use the Teukolsky
formalism to recover the quasinormal mode spectrum of global AdS-Schwarzschild,
complementing previous analysis in the literature.Comment: 33 pages, 6 figure
AdS nonlinear instability: moving beyond spherical symmetry
Anti-de Sitter (AdS) is conjectured to be nonlinear unstable to a weakly
turbulent mechanism that develops a cascade towards high frequencies, leading
to black hole formation [1,2]. We give evidence that the gravitational sector
of perturbations behaves differently from the scalar one studied in [2]. In
contrast with [2], we find that not all gravitational normal modes of AdS can
be nonlinearly extended into periodic horizonless smooth solutions of the
Einstein equation. In particular, we show that even seeds with a single normal
mode can develop secular resonances, unlike the spherically symmetric scalar
field collapse studied in [2]. Moreover, if the seed has two normal modes, more
than one resonance can be generated at third order, unlike the spherical
collapse of [2]. We also show that weak turbulent perturbative theory predicts
the existence of direct and inverse cascades, with the former dominating the
latter for equal energy two-mode seeds.Comment: 7 pages, no figures, 2 table
Ages and metallicities of star clusters: new calibrations and diagnostic diagrams from visible integrated spectra
We present homogeneous scales of ages and metallicities for star clusters
from very young objects, through intermediate-age ones up to the oldest known
clusters. All the selected clusters have integrated spectra in the visible
range, as well as reliable determinations of their ages and metallicities. From
these spectra equivalent widths (EWs) of KCaII, Gband(CH) and MgI metallic, and
Hdelta, Hgamma and Hbeta Balmer lines have been measured homogeneously. The
analysis of these EWs shows that the EW sums of the metallic and Balmer H
lines, separately, are good indicators of cluster age for objects younger than
10 Gyr, and that the former is also sensitive to cluster metallicity for ages
greater than 10 Gyr. We propose an iterative procedure for estimating cluster
ages by employing two new diagnostic diagrams and age calibrations based on the
above EW sums. For clusters older than 10 Gyr, we also provide a calibration to
derive their overall metal contents.Comment: 9 pages, 4 figures, accepted by A&
Localised Black Holes
We numerically construct asymptotically global black holes that are localised on the . These are
solutions to type IIB supergravity with horizon topology that
dominate the theory in the microcanonical ensemble at small energies. At higher
energies, there is a first-order phase transition to
-Schwarzschild. By the AdS/CFT
correspondence, this transition is dual to spontaneously breaking the
R-symmetry of super Yang-Mills down to . We extrapolate
the location of this phase transition and compute the expectation value of the
resulting scalar operators in the low energy phase.Comment: 11 pages, 6 figure
Lumpy AdS S Black Holes and Black Belts
Sufficiently small Schwarzschild black holes in global AdSS are
Gregory-Laflamme unstable. We construct new families of black hole solutions
that bifurcate from the onset of this instability and break the full SO
symmetry group of the S down to SO. These new "lumpy" solutions are
labelled by the harmonics . We find evidence that the branch
never dominates the microcanonical/canonical ensembles and connects through a
topology-changing merger to a localised black hole solution with S
topology. We argue that these S black holes should become the dominant
phase in the microcanonical ensemble for small enough energies, and that the
transition to Schwarzschild black holes is first order. Furthermore, we find
two branches of solutions with . We expect one of these branches to
connect to a solution containing two localised black holes, while the other
branch connects to a black hole solution with horizon topology which we call a "black belt".Comment: 20 pages (plus 17 pages for Appendix on Kaluza-Klein Holography), 14
figure
Tracking Vector Magnetograms with the Magnetic Induction Equation
The differential affine velocity estimator (DAVE) developed in Schuck (2006)
for estimating velocities from line-of-sight magnetograms is modified to
directly incorporate horizontal magnetic fields to produce a differential
affine velocity estimator for vector magnetograms (DAVE4VM). The DAVE4VM's
performance is demonstrated on the synthetic data from the anelastic
pseudospectral ANMHD simulations that were used in the recent comparison of
velocity inversion techniques by Welsch (2007). The DAVE4VM predicts roughly
95% of the helicity rate and 75% of the power transmitted through the
simulation slice. Inter-comparison between DAVE4VM and DAVE and further
analysis of the DAVE method demonstrates that line-of-sight tracking methods
capture the shearing motion of magnetic footpoints but are insensitive to flux
emergence -- the velocities determined from line-of-sight methods are more
consistent with horizontal plasma velocities than with flux transport
velocities. These results suggest that previous studies that rely on velocities
determined from line-of-sight methods such as the DAVE or local correlation
tracking may substantially misrepresent the total helicity rates and power
through the photosphere.Comment: 30 pages, 13 figure
Spectral evolution of star clusters in the Large Magellanic Cloud: I. Blue concentrated clusters in the age range 40-300 Myr
Integrated spectroscopy of a sample of 17 blue concentrated Large Magellanic
Cloud (LMC) clusters is presented and its spectral evolution studied. The
spectra span the range ~3600-6800A with a resolution of ~14A FWHM, being used
to determine cluster ages and, in connection with their spatial distribution,
to explore the LMC structure and cluster formation history. Cluster reddening
values were estimated by interpolation, using the available extinction maps. We
used two methods to derive cluster ages: (i) template matching, in which line
strengths and continuum distribution of the cluster spectra were compared and
matched to those of template clusters with known astrophysical properties, and
(ii) equivalent width (EW) method, in which new age/metallicity calibrations
were used together with diagnostic diagrams involving the sum of EWs of
selected spectral lines (KCaII, G band (CH), MgI, Hdelta, Hgamma and Hbeta).
The derived cluster ages range from 40Myr (NGC2130 and SL237) to 300Myr
(NGC1932 and SL709), a good agreement between the results of the two methods
being obtained. Combining the present sample with additional ones indicates
that cluster deprojected distances from the LMC center are related to age in
the sense that inner clusters tend to be younger. Spectral libraries of star
clusters are useful datasets for spectral classifications and extraction of
parameter information for target star clusters and galaxies. The present
cluster sample complements previous ones, in an effort to gather a spectral
library with several clusters per age bin.Comment: 13 pages, 22 figures. Accepted for publication in A&
Static, spherically symmetric solutions with a scalar field in Rastall gravity
Rastall's theory belongs to the class of non-conservative theories of
gravity. In vacuum, the only non-trivial static, spherically symmetric solution
is the Schwarzschild one, except in a very special case. When a canonical
scalar field is coupled to the gravity sector in this theory, new exact
solutions appear for some values of the Rastall parameter . Some of these
solutions describe the same space-time geometry as the recently found solutions
in the -essence theory with a power function for the kinetic term of the
scalar field. There is a large class of solutions (in particular, those
describing wormholes and regular black holes) whose geometry coincides with
that of solutions of GR coupled to scalar fields with nontrivial
self-interaction potentials; the form of these potentials, however, depends on
the Rastall parameter . We also note that all solutions of GR with a zero
trace of the energy-momentum tensor, including black-hole and wormhole ones,
may be re-interpreted as solutions of Rastall's theory.Comment: Latex file, 18 pages. To fit published versio
Quasinormal modes of asymptotically flat rotating black holes
We study the main properties of general linear perturbations of rotating
black holes in asymptotically flat higher-dimensional spacetimes. In
particular, we determine the quasinormal mode (QNM) spectrum of singly spinning
and equal angular momenta Myers-Perry black holes (MP BHs). Emphasis is also
given to the timescale of the ultraspinning and bar-mode instabilities in these
two families of MP BHs. For the bar-mode instabilities in the singly spinning
MP BH, we find excellent agreement with our linear analysis and the non-linear
time evolution of Shibata and Yoshino for d=6,7 spacetime dimensions. We find
that d=5 singly spinning BHs are linearly stable. In the context of studying
general relativity in the large dimension limit, we obtain the QNM spectrum of
Schwarzschild BHs and rotating MP BHs for large dimensions. We identify two
classes of modes. For large dimensions, we find that in the limit of zero
rotation, unstable modes of the MP BHs connect to a class of Schwarzschild QNMs
that saturate to finite values.Comment: 52 pages. 25 figure
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