36,295 research outputs found
MSAT network architecture
The Mobile Satellite (MSAT) communications system will support mobile voice and data services using circuit switched and packet switched facilities with interconnection to the public switched telephone network and private networks. Control of the satellite network will reside in a Network Control System (NCS) which is being designed to be extremely flexible to provide for the operation of the system initially with one multi-beam satellite, but with capability to add additional satellites which may have other beam configurations. The architecture of the NCS is described. The signalling system must be capable of supporting the protocols for the assignment of circuits for mobile public telephone and private network calls as well as identifying packet data networks. The structure of a straw-man signalling system is discussed
Close encounters involving free-floating planets in star clusters
Instabilities in planetary systems can result in the ejection of planets from
their host system, resulting in free-floating planets (FFPs). If this occurs in
a star cluster, the FFP may remain bound to the star cluster for some time and
interact with the other cluster members until it is ejected. Here, we use
-body simulations to characterise close star-planet and planet-planet
encounters and the dynamical fate of the FFP population in star clusters
containing single or binary star members. We find that FFPs ejected
from their planetary system at low velocities typically leave the star cluster
40% earlier than their host stars, and experience tens of close ( AU)
encounters with other stars and planets before they escape. The fraction of
FFPs that experiences a close encounter depends on both the stellar density and
the initial velocity distribution of the FFPs. Approximately half of the close
encounters occur within the first 30 Myr, and only 10% occur after 100 Myr. The
periastron velocity distribution for all encounters is well-described by a
modified Maxwell-Bolzmann distribution, and the periastron distance
distribution is linear over almost the entire range of distances considered,
and flattens off for very close encounters due to strong gravitational
focusing. Close encounters with FFPs can perturb existing planetary systems and
their debris structures, and they can result in re-capture of FFPs. In
addition, these FFP populations may be observed in young star clusters in
imaging surveys; a comparison between observations and dynamical predictions
may provide clues to the early phases of stellar and planetary dynamics in star
clusters.Comment: Accepted for publication in MNRAS; 18 pages, 12 figure
Dust properties of external galaxies; NGC 891 revisited
We compare 850um SCUBA images of NGC 891 with the corresponding V-band
optical depth predicted from radiation transfer simulations. These two tracers
of dust show a very similar distribution along the minor axis and a reasonable
agreement along the major axis. Assuming that the grains responsible for
optical extinction are also the source of 850um emission we derive a
submillimeter emissivity (emission efficiency) for dust in the NGC 891 disk.
This quantity is found to be a factor of 2-3 higher than the generally-accepted
(but highly uncertain) values adopted for the Milky Way. It should be stated,
however, that if a substantial fraction of dust in NGC 891 is clumped, the
emissivity in the two galaxies may be quite similar. We use our newly-acquired
emissivity to convert our 850um images into detailed maps of dust mass and,
utilizing 21cm and CO-emission data for NGC 891, derive the gas-to-dust ratio
along the disk. We compute an average ratio of 260 -- a value consistent with
the Milky Way and external spirals within the uncertainties in deriving both
the dust mass and the quantity of molecular gas. The bulk of dust in NGC 891
appears to be closely associated with the molecular gas phase although it may
start to follow the distribution of atomic hydrogen at radii >9 kpc (i.e. >0.5
R_25). Using the optical depth of the NGC 891 disk, we quantify how light
emitted at high redshift is attenuated by dust residing in foreground spirals.
For B-band observations of galaxies typically found in the Hubble Deep Field,
the amount of light lost is expected to be small (~ 5%). This value depends
critically on the maximum radial extent of cold dust in spiral disks (which is
poorly known). It may also represent a lower limit if galaxies expel dust over
time into the intergalactic medium.Comment: 22 pages, 7 figures, A&A accepte
The World Distribution of Household Wealth
There has been much recent research on the world distribution of income, but also growing recognition of the importance of other contributions to well-being, including those of household wealth. Wealth is important in providing security and opportunity, particularly in poorer countries that lack full social safety nets and adequate facilities for borrowing and lending. We find, however, that it is precisely in the latter countries where household wealth is the lowest, both in absolute and relative terms. Globally, wealth is more concentrated than income both on an individual and national basis. Roughly 30 per cent of world wealth is found in each of North America, Europe, and the rich Asian-Pacific countries. These areas account for virtually all of the world?s top 1 per cent of wealth holders. On an official exchange rate basis India accounts for about a quarter of the adults in the bottom three global wealth deciles while China provides about a third of those in the fourth to eighth deciles. If current growth trends continue, India, China and the transition countries will move up in the global distribution, and the lower deciles will be increasingly dominated by countries in Africa, Latin American and poor parts of the Asian-Pacific region. Thus wealth may continue to be lowest in areas where it is needed the most.wealth, net worth, personal assets, wealth inequality, households, balance sheets, portfolios
Age Dating Stellar Populations in the Near Infrared: An absolute age indicator from the presence/absence of red supergiants
The determination of age is a critical component in the study of a population
of stellar clusters. In this letter we present a new absolute age indicator for
young massive star clusters based on J-H colour. This novel method identifies
clusters as older or younger than 5.7 +/- 0.8 Myr based on the appearance of
the first population of red supergiant stars. We test the technique on the
stellar cluster population of the nearby spiral galaxy, M83, finding good
agreement with the theoretical predictions. The localisation of this technique
to the near-IR promises that it may be used well into the future with space--
and ground--based missions optimised for near-IR observations.Comment: 5 pages, 5 figures. Accepted to MNRAS, November 201
A Robust Age Indicator for Old Stellar Populations
We derive new spectral H_gamma index definitions which are robust age
indicators for old and relatively old stellar populations and thus have great
potential for solving the age-metallicity degeneracy of galaxy spectra. To
study H_gamma as a function of age, metallicity and resolution, we used a new
spectral synthesis model which predicts SEDs of single-age, single-metallicity
stellar populations at resolution FWHM=1.8A (which can be smoothed to different
resolutions), allowing direct measurements of the equivalent widths of
particular absorption features. We find that the H_gamma strong age
disentangling power strongly depends strongly on the adopted resolution and
galaxy velocity dispersion. We propose a system of indices which are completely
insensitive to metallicity and stable against resolution, allowing the study of
galaxies up to ~300 km/s. Observational spectra of very high S/N and relatively
high dispersion, are required to gain this unprecedented age discriminating
power. Once such spectra are obtained, accurate and reliable estimates for the
luminosity-weighted average stellar ages of these galaxies will become possible
for the first time, without assessing their metallicities. We measured this
index for two globular clusters, a number of low-luminosity elliptical galaxies
and a standard S0 galaxy. We find a large spread in the average stellar ages of
a sample of low-luminosity ellipticals. In particular these indices yield 4 Gyr
for M32, in agreement with the age provided by an extraordinary fit to the full
spectrum of this galaxy that we achieve here.Comment: 22 pages, 4 figures. ApJ, in press. Models and details can be found
at http://www.ioa.s.u-tokyo.ac.jp/~vazdekis
Organizing information on the next generation web - Design and implementation of a new bookmark structure
The next-generation Web will increase the need for a highly organized and ever evolving method to store references to Web objects. These requirements could be realized by the development of a new bookmark structure. This paper endeavors to identify the key requirements of such a bookmark, specifically in relation to Web documents, and sets out a suggested design through which these needs may be accomplished. A prototype developed offers such features as the sharing of bookmarks between users and groups of users. Bookmarks for Web documents in this prototype allow more specific information to be stored such as: URL, the document type, the document title, keywords, a summary, user annotations, date added, date last visited and date last modified. Individuals may access the service from anywhere on the Internet, as long as they have a Java-enabled Web browser
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