19,470 research outputs found
An Optimal Incentive System For Real Estate Agents
This article presents an alternative system for selling real estate. It overcomes the well-known deficiencies of the percentage commission system. In our system, the agent purchases the property from the seller and simultaneously receives a put option. The put option gives the agent the right to put the property back to the original owner. It is shown that this system has many of the desirable properties of a dealer system, while avoiding some of the problems that are inherent in that system.
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Distributed tuplespace and location management - an integrated perspective using Bluetooth
Location based or "context aware" computing is becoming increasingly recognized as a vital part of a mobile computing environment. As a consequence, the need for location-management middleware is widely recognized and actively researched. Location management is frequently offered to the application through an API where the location is given in the form of coordinates. It is the opinion of the authors that a localization API should offer localized data (e.g. direction to the nearest pharmacy) directly through a transparent and integrated API. Our proposed middleware for location and context management is built on top of Mobispace. Mobispace is a distributed tuplespace made for J2me units where replication between local replicas takes place with a central server (over GPRS) or with other mobile units (using Bluetooth). Since a Bluetooth connection indicates physical proximity to another node, a set of stationary nodes may distribute locality information over Bluetooth connections, and this information may be retrieved through the ordinary tuplespace AP
Reduction of lattice thermal conductivity from planar faults in the layered Zintl compound SrZnSb_2
The layered Zintl compound SrZnSb_2 is investigated using transmission electron microscopy (TEM) to understand the low lattice thermal conductivity. The material displays out-of-phase boundaries with a spacing from 100 down to 2 nm. Density functional theory calculations confirm that the TEM-derived defect structure is energetically reasonable. The impact of these defects on phonon scattering is analyzed within the DebyeâCallaway model, which reveals a significant reduction in the acoustic phonon mean free path. This enhancement in phonon scattering leads to an ~30% reduction in lattice thermal conductivity at 300 K
First Starbursts at high redshift: Formation of globular clusters
Numerical simulations of a Milky Way-size galaxy demonstrate that globular
clusters with the properties similar to observed can form naturally at z > 3 in
the concordance Lambda-CDM cosmology. The clusters in our model form in the
strongly baryon-dominated cores of supergiant molecular clouds. The first
clusters form at z = 12, while the peak formation appears to be at z = 3-5. The
zero-age mass function of globular clusters can be approximated by a power-law
dN/dM ~ M^-2, in agreement with observations of young massive star clusters.Comment: 4 pages, proceedings of the "Multi-Wavelength Cosmology" meeting,
June 200
The Snapshot Hubble U-Band Cluster Survey (SHUCS) II. Star Cluster Population of NGC 2997
We study the star cluster population of NGC 2997, a giant spiral galaxy
located at 9.5 Mpc and targeted by the Snapshot Hubble U-band Cluster Survey
(SHUCS). Combining our U-band imaging from SHUCS with archival BVI imaging from
HST, we select a high confidence sample of clusters in the circumnuclear ring
and disk through a combination of automatic detection procedures and visual
inspection. The cluster luminosity functions in all four filters can be
approximated by power-laws with indices of to . Some deviations
from pure power-law shape are observed, hinting at the presence of a high-mass
truncation in the cluster mass function. However, upon inspection of the
cluster mass function, we find it is consistent with a pure power-law of index
despite a slight bend at M. No
statistically significant truncation is observed. From the cluster age
distributions, we find a low rate of disruption () in both the
disk and circumnuclear ring. Finally, we estimate the cluster formation
efficiency () over the last 100 Myr in each region, finding %
for the disk, % for the circumnuclear ring, and % for the
entire UBVI footprint. This study highlights the need for wide-field UBVI
coverage of galaxies to study cluster populations in detail, though a small
sample of clusters can provide significant insight into the characteristics of
the population.Comment: 31 pages, 9 figures, accepted to the A
The star cluster - field star connection in nearby spiral galaxies I. Data analysis techniques and application to NGC 4395
It is generally assumed that a large fraction of stars are initially born in
clusters. However, a large fraction of these disrupt on short timescales and
the stars end up belonging to the field. Understanding this process is of
paramount importance if we wish to constrain the star formation histories of
external galaxies using star clusters. We attempt to understand the relation
between field stars and star clusters by simultaneously studying both in a
number of nearby galaxies. As a pilot study, we present results for the
late-type spiral NGC 4395 using HST/ACS and HST/WFPC2 images. Different
detection criteria were used to distinguish point sources (star candidates) and
extended objects (star cluster candidates). Using a synthetic CMD method, we
estimated the star formation history. Using simple stellar population model
fitting, we calculated the mass and age of the cluster candidates. The field
star formation rate appears to have been roughly constant, or to have possibly
increased by up to about a factor of two, for ages younger than 300 Myr
within the fields covered by our data. Our data do not allow us to constrain
the star formation histories at older ages. We identify a small number of
clusters in both fields. Neither massive ( M) clusters nor
clusters with ages Gyr were found in the galaxy and we found few
clusters older than 100 Myr. Based on our direct comparison of field stars and
clusters in NGC 4395, we estimate the ratio of star formation rate in clusters
that survive for to years to the total star formation to be
. We suggest that this relatively low value is caused
by the low star formation rate of NGC 4395.Comment: 16 pages, 20 figures, accepted for publication in A&
HST Survey of Clusters in Nearby Galaxies. II. Statistical Analysis of Cluster Populations
We present a statistical system that can be used in the study of cluster
populations. The basis of our approach is the construction of synthetic cluster
color-magnitude-radius diagrams (CMRDs), which we compare with the observed
data using a maximum likelihood calculation. This approach permits a relatively
easy incorporation of incompleteness (a function of not only magnitude and
color, but also radius), photometry errors and biases, and a variety of other
complex effects into the calculation, instead of the more common procedure of
attempting to correct for those effects.
We then apply this procedure to our NGC 3627 data from Paper I. We find that
we are able to successfully model the observed CMRD and constrain a number of
parameters of the cluster population. We measure a power law mass function
slope of alpha = -1.50 +/- 0.07, and a distribution of core radii centered at
r_c = 1.53 +/- 0.15 pc. Although the extinction distribution is less
constrained, we measured a value for the mean extinction consistent with that
determined in Paper I from the Cepheids.Comment: 21 pages, 3 figures accepted for publication by A
Instantaneous Pair Theory for High-Frequency Vibrational Energy Relaxation in Fluids
Notwithstanding the long and distinguished history of studies of vibrational
energy relaxation, exactly how it is that high frequency vibrations manage to
relax in a liquid remains somewhat of a mystery. Both experimental and
theoretical approaches seem to say that there is a natural frequency range
associated with intermolecular motions in liquids, typically spanning no more
than a few hundred cm^{-1}. Landau-Teller-like theories explain how a solvent
can absorb any vibrational energy within this "band", but how is it that
molecules can rid themselves of superfluous vibrational energies significantly
in excess of these values? We develop a theory for such processes based on the
idea that the crucial liquid motions are those that most rapidly modulate the
force on the vibrating coordinate -- and that by far the most important of
these motions are those involving what we have called the mutual nearest
neighbors of the vibrating solute. Specifically, we suggest that whenever there
is a single solvent molecule sufficiently close to the solute that the solvent
and solute are each other's nearest neighbors, then the instantaneous
scattering dynamics of the solute-solvent pair alone suffices to explain the
high frequency relaxation. The many-body features of the liquid only appear in
the guise of a purely equilibrium problem, that of finding the likelihood of
particularly effective solvent arrangements around the solute. These results
are tested numerically on model diatomic solutes dissolved in atomic fluids
(including the experimentally and theoretically interesting case of I_2 in Xe).
The instantaneous pair theory leads to results in quantitative agreement with
those obtained from far more laborious exact molecular dynamics simulations.Comment: 55 pages, 6 figures Scheduled to appear in J. Chem. Phys., Jan, 199
A dynamical and kinematical model of the Galactic stellar halo and possible implications for galaxy formation scenarios
We re-analyse the kinematics of the system of blue horizontal branch field
(BHBF) stars in the Galactic halo (in particular the outer halo), fitting the
kinematics with the model of radial and tangential velocity dispersions in the
halo as a function of galactocentric distance r proposed by Sommer-Larsen,
Flynn & Christensen (1994), using a much larger sample (almost 700) of BHBF
stars. The basic result is that the character of the stellar halo velocity
ellipsoid changes markedly from radial anisotropy at the sun to tangential
anisotropy in the outer parts of the Galactic halo (r greater than approx 20
kpc). Specifically, the radial component of the stellar halo's velocity
ellipsoid decreases fairly rapidly beyond the solar circle, from approx 140 +/-
10 km/s at the sun, to an asymptotic value of 89 +/- 19 km/s at large r. The
rapid decrease in the radial velocity dispersion is matched by an increase in
the tangential velocity dispersion, with increasing r.
Our results may indicate that the Galaxy formed hierarchically (partly or
fully) through merging of smaller subsystems - the 'bottom-up' galaxy formation
scenario, which for quite a while has been favoured by most theorists and
recently also has been given some observational credibility by HST observations
of a potential group of small galaxies, at high redshift, possibly in the
process of merging to a larger galaxy (Pascarelle et al 1996).Comment: Latex, 16 pages. 2 postscript figures. Submitted to the Astrophysical
Journal. also available at http://astro.utu.fi/~cflynn/outerhalo.htm
Evidence of an interaction from resolved stellar populations: The curious case of NGC1313
The galaxy NGC1313 has attracted the attention of various studies due to the
peculiar morphology observed in optical bands, although it is classified as a
barred, late-type galaxy with no apparent close-by companions. However, the
velocity field suggests an interaction with a satellite companion. Using
resolved stellar populations, we study different parts of the galaxy to
understand further its morphology. Based on HST/ACS images, we estimated star
formation histories by means of the synthetic CMD method in different areas in
the galaxy. Incompleteness limits our analysis to ages younger than ~100Myr.
Stars in the red and blue He burning phases are used to trace the distribution
of recent star formation. Star formation histories suggest a burst in the
southern-west region. We support the idea that NGC1313 is experiencing an
interaction with a satellite companion, observed as a tidally disrupted
satellite galaxy in the south-west of NGC1313. However, we do not observe any
indication of a perturbation due to the interaction with the satellite galaxy
at other locations across the galaxy, suggesting that only a modest-sized
companion that did not trigger a global starburst was involved.Comment: 10 pages, 4 figures, accepted for publication in MNRA
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