399 research outputs found
Outline of a Decision Support System for Area-Wide Water Quality Planning
This working paper outlines requirements for an implementation of a computerized decision support system which addresses the technical aspects of area-wide water quality planning. The framework for this work is in the context of the environmental law adopted in the United States during 1972. This law, known as the Federal Water Pollution Control Act Amendments of 1972, specifies various requirements that both municipal and industrial discharges must eventually conform. By 1977 municipal waste treatment plants must have in place secondary treatment facilities and for industry it is necessary to utilize what is referred to as "best practical technology" for waste treatment. Under certain circumstances as described in section 303 of the law further treatment may be required to meet water quality standards. Section 208 of the Federal Water Pollution Control Act Amendments of 1972 calls for area-wide implementation of technical and management planning, with the objectives of meeting 1983 water quality goals and establishing a plan for municipal and industrial facilities construction over a twenty year period. Emphasis is placed on locally controlled planning, on dealing with non-point sources as well as point sources, and on consideration of both structural and nonstructural control methods. The scope of present examination is limited to those aspects of technical planning which are amenable to implementation within the framework of a computerized decision support system
Impactor flux and cratering on Ceres and Vesta: Implications for the early Solar System
We study the impactor flux and cratering on Ceres and Vesta caused by the
collisional and dynamical evolution of the asteroid Main Belt. We develop a
statistical code based on a well-tested model for the simultaneous evolution of
the Main Belt and NEA size distributions. This code includes catastrophic
collisions and noncollisional removal processes such as the Yarkovsky effect
and the orbital resonances. The model assumes that the dynamical depletion of
the early Main Belt was very strong, and owing to that, most Main Belt
comminution occurred when its dynamical structure was similar to the present
one. Our results indicate that the number of D > 1 km Main Belt asteroids
striking Ceres and Vesta over the Solar System history are approximately 4 600
and 1 100 respectively. The largest Main Belt asteroids expected to have
impacted Ceres and Vesta had diameters of 71.7 km and 21.1 km. The number of D
> 0.1 km craters on Ceres is \sim 3.4 \times 10^8 and 6.2 \times 10^7 on Vesta.
The number of craters with D > 100 km are 47 on Ceres and 8 on Vesta. Our study
indicates that the D = 460 km crater observed on Vesta had to be formed by the
impact of a D \sim 66.2 km projectile, which has a probability of occurr \sim
30% over the Solar System history. If significant discrepancies between our
results about the cratering on Ceres and Vesta and data obtained from the Dawn
Mission were found, they should be linked to a higher degree of collisional
evolution during the early Main Belt and/or the existence of the late heavy
bombardment. An increase in the collisional activity in the early phase may be
provided for an initial configuration of the giant planets consistent with, for
example, the Nice model. From this, the Dawn Mission would be able to give us
clues about the initial configuration of the early Solar System and its
subsequent dynamical evolution.Comment: Accepted for publication in Astronomy and Astrophysic
The Cratering History of Asteroid (2867) Steins
The cratering history of main belt asteroid (2867) Steins has been
investigated using OSIRIS imagery acquired during the Rosetta flyby that took
place on the 5th of September 2008. For this purpose, we applied current models
describing the formation and evolution of main belt asteroids, that provide the
rate and velocity distributions of impactors. These models coupled with
appropriate crater scaling laws, allow the cratering history to be estimated.
Hence, we derive Steins' cratering retention age, namely the time lapsed since
its formation or global surface reset. We also investigate the influence of
various factors -like bulk structure and crater erasing- on the estimated age,
which spans from a few hundred Myrs to more than 1Gyr, depending on the adopted
scaling law and asteroid physical parameters. Moreover, a marked lack of
craters smaller than about 0.6km has been found and interpreted as a result of
a peculiar evolution of Steins cratering record, possibly related either to the
formation of the 2.1km wide impact crater near the south pole or to YORP
reshaping.Comment: Accepted by Planetary and Space Scienc
Fragment properties at the catastrophic disruption threshold: The effect of the parent body's internal structure
Numerical simulations of asteroid break-ups, including both the fragmentation
of the parent body and the gravitational interactions between the fragments,
have allowed us to reproduce successfully the main properties of asteroid
families formed in different regimes of impact energy, starting from a
non-porous parent body. In this paper, using the same approach, we concentrate
on a single regime of impact energy, the so-called catastrophic threshold
usually designated by Q*D, which results in the escape of half of the target's
mass. Thanks to our recent implementation of a model of fragmentation of porous
materials, we can characterize Q*D for both porous and non-porous targets with
a wide range of diameters. We can then analyze the potential influence of
porosity on the value of Q*D, and by computing the gravitational phase of the
collision in the gravity regime, we can characterize the collisional outcome in
terms of the fragment size and ejection speed distributions, which are the main
outcome properties used by collisional models to study the evolutions of the
different populations of small bodies. We also check the dependency of Q*D on
the impact speed of the projectile. In the strength regime, which corresponds
to target sizes below a few hundreds of meters, we find that porous targets are
more difficult to disrupt than non-porous ones. In the gravity regime, the
outcome is controlled purely by gravity and porosity in the case of porous
targets. In the case of non-porous targets, the outcome also depends on
strength. We then propose some power-law relationships between Q*D and both
target's size and impact speed that can be used in collisional evolution
models.Comment: 18 pages, 19 Figures. Accepted for publication in Icaru
Cratering Experiments on the Self Armoring of Coarse-Grained Granular Targets
Recently published crater statistics on the small asteroids 25143 Itokawa and
433 Eros show a significant depletion of craters below approx. 100 m in
diameter. Possible mechanisms that were brought up to explain this lack of
craters were seismic crater erasure and self armoring of a coarse, boulder
covered asteroid surface. While seismic shaking has been studied in this
context, the concept of armoring lacks a deeper inspection and an experimental
ground truth. We therefore present cratering experiments of glass bead
projectiles impacting into granular glass bead targets, where the grain sizes
of projectile and target are in a similar range. The impact velocities are in
the range of 200 to 300 m/s. We find that craters become fainter and irregular
shaped as soon as the target grains are larger than the projectile sizes and
that granular craters rarely form when the size ratio between projectile and
target grain is around 1:10 or smaller. In that case, we observe a formation of
a strength determined crater in the first struck target grain instead. We
present a simple model based on the transfer of momentum from the projectile to
this first target grain, which is capable to explain our results with only a
single free parameter, which is moreover well determined by previous
experiments. Based on estimates of typical projectile size and boulder size on
Itokawa and Eros, given that our results are representative also for km/s
impact velocities, armoring should play an important role for their evolution.Comment: accepted for publication in Icaur
A critique of the World Resources Institute's report "Pesticides and the immune system: the public health risks".
A recent World Resources Institute (WRI) report concluded that pesticides are a likely cause of immune suppression for millions of people throughout the world. The gravity of this conclusion motivated us to review the scientific evidence cited in the report. The predominant human evidence came from cross-sectional studies conducted in the former Soviet Union.These studies were difficult to evaluate due to incomplete reporting and had obvious limitations in terms of subject selection, exposure assessment,lack of quality control, statistical analysis, adequacy of the comparison group, and confounding. The toxicologic evidence was comprised mainly of acute high-dose studies in which the exposure conditions resulted in systemic toxicity. The relevance of these studies to effects at typical human exposure levels is questionable. We did not find consistent, credible evidence to support the conclusion of widespread pesticide-related immune suppression. Nonetheless, the WRI report is an important document because it focuses attention on a potentially important issue for future research and brings a substantial literature of foreign language studies to the attention of Western scientists
Dust Production from collisions in extrasolar planetary systems The inner Beta-Pictoris disc
Dust particles observed in extrasolar planetary discs originate from
undetectable km-sized bodies but this valuable information remains
uninteresting if the theoretical link between grains and planetesimals is not
properly known. We outline in this paper a numerical approach we developed in
order to address this issue for the case of dust producing collisional
cascades. The model is based on a particle-in-a-box method. We follow the size
distribution of particles over eight orders of magnitude in radius taking into
account fragmentation and cratering according to different prescriptions. A
very particular attention is paid to the smallest particles, close to the
radiation pressure induced cut-off size , which are placed on highly
eccentric orbits by the stellar radiation pressure....(abstract continued in
the uploaded paper)Comment: A&A accepted (in press
Collisional processes and size distribution in spatially extended debris discs
We present a new multi-annulus code for the study of collisionally evolving
extended debris discs. We first aim to confirm results obtained for a
single-annulus system, namely that the size distribution in "real" debris discs
always departs from the theoretical collisional equilibrium
dN\proptoR^{-3.5}dR power law, especially in the crucial size range of
observable particles (<1cm), where it displays a characteristic wavy pattern.
We also aim at studying how debris discs density distributions, scattered light
luminosity profiles, and SEDs are affected by the coupled effect of collisions
and radial mixing due to radiation pressure affected small grains. The size
distribution evolution is modeled from micron-sized grains to 50km-sized
bodies. The model takes into account the crucial influence of radiation
pressure-affected small grains. We consider the collisional evolution of a
fiducial a=120AU radius disc with an initial surface density in
. We show that the system's radial extension plays
a crucial role: in most regions the collisional and size evolution of the dust
is imposed by small particles on eccentric or unbound orbits produced further
inside the disc. The spatial distribution of small grains strongly departs from
the initial profile, while the bigger objects, containing most of the system's
mass, still follow the initial distribution. This has consequences on the
scattered--light radial profiles which get significantly flatter, and we
propose an empirical law to trace back the distribution of large unseen parent
bodies from the observed profiles. We finally provide empirical formula for the
collisional size distribution and collision timescale that can be used for
future debris disc modeling.Comment: Accepted for publication in Astronomy and Astrophysics (with better
figures) (note: full abstract in the *.pdf file
Generalized h-index for Disclosing Latent Facts in Citation Networks
What is the value of a scientist and its impact upon the scientific thinking?
How can we measure the prestige of a journal or of a conference? The evaluation
of the scientific work of a scientist and the estimation of the quality of a
journal or conference has long attracted significant interest, due to the
benefits from obtaining an unbiased and fair criterion. Although it appears to
be simple, defining a quality metric is not an easy task. To overcome the
disadvantages of the present metrics used for ranking scientists and journals,
J.E. Hirsch proposed a pioneering metric, the now famous h-index. In this
article, we demonstrate several inefficiencies of this index and develop a pair
of generalizations and effective variants of it to deal with scientist ranking
and with publication forum ranking. The new citation indices are able to
disclose trendsetters in scientific research, as well as researchers that
constantly shape their field with their influential work, no matter how old
they are. We exhibit the effectiveness and the benefits of the new indices to
unfold the full potential of the h-index, with extensive experimental results
obtained from DBLP, a widely known on-line digital library.Comment: 19 pages, 17 tables, 27 figure
E-government adoption: A cultural comparison
This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ Springer Science + Business Media, LLC 2008.E-government diffusion is an international phenomenon. This study compares e-government adoption in the U.K. to adoption in the U.S. In particular, this study seeks to determine if the same factors are salient in both countries. Several studies have explored citizen acceptance of e-government services in the U.S. However, few studies have explored this phenomenon in the U.K. To identify the similarities and differences between the U.K. and the U.S. a survey is conducted in the U.K. and the findings are compared to the literature that investigates diffusion in the U.S. This study proposes a model of e-government adoption in the U.K. based on salient factors in the U.S. A survey is administered to 260 citizens in London to assess the importance of relative advantage, trust and the digital divide on intention to use e-government. The results of binary logistic regression indicate that there are cultural differences in e-government adoption in the U.K. and the U.S. The results indicate that of the prevailing adoption constructs, relative advantage and trust are pertinent in both the U.S. and the U.K., while ICT adoption barriers such as access and skill may vary by culture. Implications for research and practice are discussed
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