8,805 research outputs found
APPLICATION OF A BIO-ECONOMIC-ENGINEERING MODEL FOR SHRIMP MARICULTURE SYSTEMS
Resource /Energy Economics and Policy,
BIOECONOMIC MODELING OF THE GULF SHRIMP FISHERY: AN APPLICATION TO GALVESTON BAY AND ADJACENT OFFSHORE AREAS
Resource /Energy Economics and Policy,
The prediction and management of aquatic nitrogen pollution across Europe: an introduction to the Integrated Nitrogen in European Catchments project (INCA)
Excess nitrogen in soils, fresh water, estuarine and marine systems contributes to nutrient enrichment in key ecosystems throughout Europe, often leading to detrimental environmental impacts, such as soil acidification or the eutrophication of water bodies. The Integrated Nitrogenmodel for European Catchments (INCA) project aims to develop a generic version of the Integrated Nitrogen in Catchments (INCA) model to simulate the retention and transport of nitrogen within river systems, thereby providing a tool to aid the understanding of nitrogen dynamics and for river-basin management/policy-making. To facilitate the development of the model, 10 partners have tested the INCA model with data collected in study sites located in eight European countries as part of the INCA project. This paper summarises the key nitrogen issues within Europe, describes the main aims and methodology of the INCA project, and sets the project in the context of the current major research initiatives at a European level.</p> <p style='line-height: 20px;'><b>Keywords: </b>Europe, European Union, nitrogen, nitrate, ammonium, river basin management, modelling, water chemistry, acidification, eutrophication, Water Framework Directive, INCA
The prediction and management of aquatic nitrogen pollution across Europe: an introduction to the Integrated Nitrogen in European Catchments project (INCA)
International audienceExcess nitrogen in soils, fresh water, estuarine and marine systems contributes to nutrient enrichment in key ecosystems throughout Europe, often leading to detrimental environmental impacts, such as soil acidification or the eutrophication of water bodies. The Integrated Nitrogenmodel for European Catchments (INCA) project aims to develop a generic version of the Integrated Nitrogen in Catchments (INCA) model to simulate the retention and transport of nitrogen within river systems, thereby providing a tool to aid the understanding of nitrogen dynamics and for river-basin management/policy-making. To facilitate the development of the model, 10 partners have tested the INCA model with data collected in study sites located in eight European countries as part of the INCA project. This paper summarises the key nitrogen issues within Europe, describes the main aims and methodology of the INCA project, and sets the project in the context of the current major research initiatives at a European level. Keywords: Europe, European Union, nitrogen, nitrate, ammonium, river basin management, modelling, water chemistry, acidification, eutrophication, Water Framework Directive, INCA
Detection of ultra-weak magnetic fields in Am stars: beta UMa and theta Leo
An extremely weak circularly polarized signature was recently discovered in
spectral lines of the chemically peculiar Am star Sirius A. A weak surface
magnetic field was proposed to account for the observed polarized signal, but
the shape of the phase-averaged signature, dominated by a prominent positive
lobe, is not expected in the standard theory of the Zeeman effect. We aim at
verifying the presence of weak circularly polarized signatures in two other
bright Am stars, beta UMa and theta Leo, and investigating the physical origin
of Sirius-like polarized signals further. We present here a set of deep
spectropolarimetric observations of beta UMa and theta Leo, observed with the
NARVAL spectropolarimeter. We analyzed all spectra with the Least Squares
Deconvolution multiline procedure. To improve the signal-to-noise ratio and
detect extremely weak signatures in Stokes V profiles, we co-added all
available spectra of each star (around 150 observations each time). Finally, we
ran several tests to evaluate whether the detected signatures are consistent
with the behavior expected from the Zeeman effect. The line profiles of the two
stars display circularly polarized signatures similar in shape and amplitude to
the observations previously gathered for Sirius A. Our series of tests brings
further evidence of a magnetic origin of the recorded signal. These new
detections suggest that very weak magnetic fields may well be present in the
photospheres of a significant fraction of intermediate-mass stars. The strongly
asymmetric Zeeman signatures measured so far in Am stars (featuring a dominant
single-sign lobe) are not expected in the standard theory of the Zeeman effect
and may be linked to sharp vertical gradients in photospheric velocities and
magnetic field strengths
Investigating the Magnetospheres of Rapidly Rotating B-type Stars
Recent spectropolarimetric surveys of bright, hot stars have found that ~10%
of OB-type stars contain strong (mostly dipolar) surface magnetic fields (~kG).
The prominent paradigm describing the interaction between the stellar winds and
the surface magnetic field is the magnetically confined wind shock (MCWS)
model. In this model, the stellar wind plasma is forced to move along the
closed field loops of the magnetic field, colliding at the magnetic equator,
and creating a shock. As the shocked material cools radiatively it will emit
X-rays. Therefore, X-ray spectroscopy is a key tool in detecting and
characterizing the hot wind material confined by the magnetic fields of these
stars. Some B-type stars are found to have very short rotational periods. The
effects of the rapid rotation on the X-ray production within the magnetosphere
have yet to be explored in detail. The added centrifugal force due to rapid
rotation is predicted to cause faster wind outflows along the field lines,
leading to higher shock temperatures and harder X-rays. However, this is not
observed in all rapidly rotating magnetic B-type stars. In order to address
this from a theoretical point of view, we use the X-ray Analytical Dynamical
Magnetosphere (XADM) model, originally developed for slow rotators, with an
implementation of new rapid rotational physics. Using X-ray spectroscopy from
ESA's XMM-Newton space telescope, we observed 5 rapidly rotating B-type stars
to add to the previous list of observations. Comparing the observed X-ray
luminosity and hardness ratio to that predicted by the XADM allows us to
determine the role the added centrifugal force plays in the magnetospheric
X-ray emission of these stars.Comment: IAUS Conference Proceeding
PG 1018â047 : the longest period subdwarf B binary
About 50 per cent of all known hot subdwarf B stars (sdBs) reside in close (short-period) binaries, for which common-envelope ejection is the most likely formation mechanism. However, Han et al. predict that the majority of sdBs should form through stable mass transfer leading to long-period binaries. Determining orbital periods for these systems is challenging and while the orbital periods of âŒ100 short-period systems have been measured, there are no periods measured above 30 d. As part of a large programme to characterize the orbital periods of sdB binaries and their formation history, we have found that PG 1018â047 has an orbital period of 759.8 ± 5.8 d, easily making it the longest period ever detected for a sdB binary. Exploiting the Balmer lines of the subdwarf primary and the narrow absorption lines of the companion present in the spectra, we derive the radial velocity amplitudes of both stars, and estimate the mass ratio MMS/MsdB= 1.6 ± 0.2. From the combination of visual and infrared photometry, the spectral type of the companion star is determined to be mid-K
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