19,501 research outputs found
Dynamics of social queues
Queues formed by social wasps to inherit the dominant position in the nest are analyzed by using a transient quasi-birth-and-death (QBD) process. We show that the extended nest lifespan due to division of labor between queen and helpers has a big impact on nest productivity
Microlensing
Microlensing observations have now become a useful tool in searching for
non--luminous astrophysical compact objects (brown dwarfs, faint stars, neutron
stars, black holes and even planets). Originally conceived for establishing
whether the halo of the Galaxy is composed of this type of objects, the ongoing
searches are actually also sensitive to the dark constituents of other Galactic
components (thin and thick disks, outer spheroid, bulge). We discuss here the
present searches for microlensing of stars in the Magellanic Clouds and in the
Galactic bulge (EROS, MACHO, OGLE and DUO collaborations). We analyse the
information which can be obtained regarding the spatial distribution and motion
of the lensing objects as well as about their mass function and their overall
contribution to the mass of the Galaxy. We also discuss the additional signals,
such as the parallax due to the motion of the Earth, the effects due to the
finite source size and the lensing events involving binary objects, which can
further constrain the lens properties.
We describe the future prospects for these searches and the further proposed
observations which could help to elucidate these issues, such as microlensing
of stars in the Andromeda galaxy, satellite parallax measurements and infrared
observations.Comment: Latex file, 59 p., 13 ps figures, to appear in Physics Reports
(postcript file replaced by latex file
The Microlensing Properties of a Sample of 87 Lensed Quasars
Gravitational microlensing is a powerful tool for probing the physical
properties of quasar accretion disks and properties of the lens galaxy such as
its dark matter fraction and mean stellar mass. Unfortunately the number of
lensed quasars () exceeds our monitoring capabilities. Thus,
estimating their microlensing properties is important for identifying good
microlensing candidates as well as for the expectations of future surveys. In
this work we estimate the microlensing properties of a sample of 87 lensed
quasars. While the median Einstein radius crossing time scale is 20.6 years,
the median source crossing time scale is 7.3 months. Broadly speaking, this
means that on year timescales roughly half the lenses will be
quiescent, with the source in a broad demagnified valley, and roughly half will
be active with the source lying in the caustic ridges. We also found that the
location of the lens system relative to the CMB dipole has a modest effect on
microlensing timescales, and in theory microlensing could be used to confirm
the kinematic origin of the dipole. As a corollary of our study we analyzed the
accretion rate parameters in a sub-sample of 32 lensed quasars. At fixed black
hole mass, it is possible to sample a broad range of luminosities (i.e.,
Eddington factors) if it becomes feasible to monitor fainter lenses.Comment: 31 pages, 7 figures, 2 tables, corrected typos in Table 2, revised
version accepted for publication in Ap
Constraining the Cosmological Parameters using Strong Lensing
We investigate the potentiality of using strong lensing clusters to constrain
the cosmological parameters Omega_m and Omega_lambda. The existence of a
multiple image system with known redshift allows, for a given (Omega_m,
Omega_lambda) cosmology, absolute calibration of the total mass deduced from
lens modelling. Recent Hubble Space Telescope (HST) observations of galaxy
clusters reveal a large number of multiple images, which are predicted to be at
different redshifts. If it is possible to measure spectroscopically the
redshifts of many multiple images then one can in principle constrain
(Omega_m,Omega_lambda) through ratios of angular diameter distances,
independently of any external assumptions. For a regular/relaxed cluster
observed by HST with 3 multiple image systems, each with different
spectroscopic redshifts, we show by analytic calculation that the following
uncertainties can be expected: Omega_m=0.30 +/- 0.11, Omega_lambda=0.70 +/-
0.23 or Omega_m=1.00 +/- 0.17, Omega_lambda=0.00 +/- 0.48 for the two most
popular world models. Numerical tests on simulated data confirm these good
constraints, even in the case of more realistic cluster potentials, such as
bimodal clusters, or when including perturbations by galaxies. To investigate
the sensitivity of the method to different mass profiles, we also use an
analytic ``pseudo-elliptical'' Navarro, Frenk & White profile in the
simulations. These constraints can be improved if more than 3 multiple images
with spectroscopic redshifts are observed, or by combining the results from
different clusters. Some prospects on the determination of the cosmological
parameters with gravitational lensing are given.Comment: Revised Version. 15 pages, 18 figures and 6 table
Evolutionary multiplayer games on graphs with edge diversity
Evolutionary game dynamics in structured populations has been extensively
explored in past decades. However, most previous studies assume that payoffs of
individuals are fully determined by the strategic behaviors of interacting
parties and social ties between them only serve as the indicator of the
existence of interactions. This assumption neglects important information
carried by inter-personal social ties such as genetic similarity, geographic
proximity, and social closeness, which may crucially affect the outcome of
interactions. To model these situations, we present a framework of evolutionary
multiplayer games on graphs with edge diversity, where different types of edges
describe diverse social ties. Strategic behaviors together with social ties
determine the resulting payoffs of interactants. Under weak selection, we
provide a general formula to predict the success of one behavior over the
other. We apply this formula to various examples which cannot be dealt with
using previous models, including the division of labor and relationship- or
edge-dependent games. We find that labor division facilitates collective
cooperation by decomposing a many-player game into several games of smaller
sizes. The evolutionary process based on relationship-dependent games can be
approximated by interactions under a transformed and unified game. Our work
stresses the importance of social ties and provides effective methods to reduce
the calculating complexity in analyzing the evolution of realistic systems.Comment: 50 pages, 7 figure
Properties of Planet-induced Deviations in the Astrometric Microlensing Centroid Shift Trajectory
In this paper, we investigate the properties of the planet-induced deviations
in the trajectory of the microlensed source star centroid motion (astrometric
curve) and the correlations between the astrometric and photometric deviations.
For this, we construct vector field maps of excess centroid shifts. Fromthe
investigation of the maps, we find that the astrometric deviation is closely
correlated with the photometric one. The astrometric deviation increases as the
photometric deviation increases and \Delta\deltavec is directed towards the
planet when the light curve has positive deviation and vice versa. We also
present excess centroid shift maps for lens systems with various values of the
planetary separation, planet/primary mass ratio, and source size to show the
changes in the pattern of excess centroid shifts with these parameters.Comment: total 22 pages, including 14 figures and no table, MNRAS, submitted,
better quality pdf file is avalilable at
http://astroph.chungbuk.ac.kr/~cheongho/publication.htm
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