9,150 research outputs found
Predicting the occurrence of sand banks in the North Sea
Sand banks have a wavelength between 1 and 10 km, and they are up to several tens of meters high. Also, sand banks may have an impact on large-scale human activities that take place in the North Sea like sand mining, shipping, offshore wind farms, etc. Therefore, it is important to know where sand banks occur and what their natural behavior is. Here, we use an idealized model to predict the occurrence of sand banks in the North Sea. The aim of the paper is to research to what extent the model is able to predict the occurrence of sand banks in the North Sea. We apply a sensitivity analysis to optimize the model results for a North Sea environment. The results show that the model correctly predicts whether or not sand banks occur for two thirds of the North Sea area
An Attempt to Detect the Galactic Bulge at 12 microns with IRAS
Surface brightness maps at 12 microns, derived from observations with the
Infrared Astronomical Satellite (IRAS), are used to estimate the integrated
flux at this wavelength from the Galactic bulge as a function of galactic
latitude along the minor axis. A simple model was used to remove Galactic disk
emission (e.g. unresolved stars and dust) from the IRAS measurements. The
resulting estimates are compared with predictions for the 12 micron bulge
surface brightness based on observations of complete samples of optically
identified M giants in several minor axis bulge fields. No evidence is found
for any significant component of 12m emission in the bulge other than that
expected from the optically identified M star sample plus normal, lower
luminosity stars. Known large amplitude variables and point sources from the
IRAS catalogue contribute only a small fraction to the total 12 micron flux.Comment: Accepted for publication in ApJ; 13 pages of text including tables in
MS WORD97 generated postscript; 3 figures in postscript by Sigma Plo
Valuation of functions of the Wadden Area
The rationale of this position paper is to explore ways to employ valuation methods to assess the impacts of alternative policy decisions on the functions of the Wadden Area. Based on this rationale, this paper aims to bring an ecological-economic perspective on the benefits of the Wadden Area. It highlights key issues involved in the notion and application of monetary valuation methods for valuing the (ecological) functions of the area. It also pays attention to the development of policy handles on the basis of the outcome of the valuation of ecological functions. Particularly, the paper discuss Payment for Environmental Services (PES), which aims at preservation and protection by purchasing conservation. The use of a valuation instrument within a Payment for Environmental Services Scheme is new. Valuation is of course an established technique within a Cost-Benefit framework. However, Payment for Environmental Services is an innovative way of designing policy instruments in the collaboration between ecologists and economist
'Calving laws', 'sliding laws' and the stability of tidewater glaciers
A new calving criterion is introduced, which predicts calving where the depth of surface crevasses equals ice height above sea level. Crevasse depth is calculated from strain rates, and terminus position and calving rate are therefore functions of ice velocity, strain rate, ice thickness and water depth. We couple the calving criterion with three 'sliding laws', in which velocity is controlled by (1) basal drag, (2) lateral drag and (3) a combination of the two. In model 1, velocities and strain rates are dependent on effective pressure, and hence ice thickness relative to water depth. Imposed thinning can lead to acceleration and terminus retreat, and ice shelves cannot form. In model 2, ice velocity is independent of changes in ice thickness unless accompanied by changes in surface gradient. Velocities are strongly dependent on channel width, and calving margins tend to stabilize at flow-unit widenings. Model 3 exhibits the combined characteristics of the other two models, and suggests that calving glaciers are sensitive to imposed thickness changes if basal drag provides most resistance to flow, but stable if most resistance is from lateral drag. ice shelves can form if reduction of basal drag occurs over a sufficiently long spatial scale. In combination, the new calving criterion and the basal-lateral drag sliding function (model 3) can be used to simulate much of the observed spectrum of behaviour of calving glaciers, and present new opportunities to model ice-sheet response to climate change.</p
OH-selected AGB and post-AGB objects I.Infrared and maser properties
Using 766 compact objects from a survey of the galactic Plane in the 1612-MHz
OH line, new light is cast on the infrared properties of evolved stars on the
TP-AGB and beyond. The usual mid-infrared selection criteria, based on IRAS
colours, largely fail to distinguish early post-AGB stages. A two-colour
diagram from narrower-band MSX flux densities, with bimodal distributions,
provides a better tool to do the latter. Four mutually consistent selection
criteria for OH-masing red PPNe are given, as well as two for early post-AGB
masers and one for all post--AGB masers, including the earliest ones. All these
criteria miss a group of blue, high-outflow post-AGB sources with 60-mum
excess; these will be discussed in detail in Paper II. The majority of post-AGB
sources show regular double-peaked spectra in the OH 1612-MHz line, with fairly
low outflow velocities, although the fractions of single peaks and irregular
spectra may vary with age and mass. The OH flux density shows a fairly regular
relation with the stellar flux and the envelope optical depth, with the maser
efficiency increasing with IRAS colour R21. The OH flux density is linearly
correlated with the 60-mum flux density.Comment: 16 pages, LaTex, 22 figures, AJ (accepted
Reicht optischer FluĂ wirklich nicht zum Heimfinden?
Die Literatur legt nahe, daĂ selbst fĂŒr einfache Orientierungs- und Heimfindeaufgaben die durch optischen FluĂ gegebene Information unzureichend ist und vestibulĂ€re und kinĂ€sthetische Reize benötigt werden. Um diese Behauptung zu testen, fĂŒhrten wir DreiecksvervollstĂ€ndigungsexperimente in einer virtuellen Umgebung durch, die als einzige Informationsquelle optischen FluĂ anbot. Die simulierte Eigenbewegung wurden visuell auf einer halbzylindrischen 180° Projektionsleinwand (7m Durchmesser) dargeboten und ĂŒber Maus-Tasten gesteuert. Damit die Versuchspersonen zur Navigation nur Pfadintegration und keine Landmarkeninformation verwenden konnten, bestand die simulierte Welt lediglich aus einer 3D Punktewolke. Diese enthielt keinerlei hilfreiche Orientierungspunkte (Landmarken), vermittelte jedoch ein ĂŒberzeugendes GefĂŒhl von Eigenbewegung (Vektion). In Exp 1 sollten die Versuchspersonen Drehungen um bestimmte Winkel ausfĂŒhren sowie Distanzen reproduzieren, wobei die Geschwindigkeiten randomisiert wurden. Exp 2 3 waren DreiecksvervollstĂ€ndigungsexperimente: Versuchspersonen folgten zwei Schenkeln eines Dreiecks und sollten dann selbststĂ€ndig zum nicht markierten Ausgangspunkt zurĂŒckfinden. In Exp 2 wurden fĂŒnf verschiedene gleichschenklige Dreiecke fĂŒr Links- und Rechtsdrehungen verwendet, in Exp 3 hingegen 60 verschiedene Dreiecke mit randomisierten SchenkellĂ€ngen und Winkeln. UnabhĂ€ngig von der Bewegungsgeschwindigkeit konnten untrainierte Versuchspersonen in Exp 1 Drehungen und Distanzen mit nur geringfĂŒgigem systematischen Fehler ausfĂŒhren. Wir fanden in Exp 2 3 generell eine lineare Korrelation zwischen ausgefĂŒhrten und korrekten Werten fĂŒr die MeĂgröĂen Drehwinkel und zurĂŒckgelegte Distanz. FĂŒr die weitere Analyse verwendeten wir deshalb fĂŒr beide MeĂgröĂen die Steigungen der Regressionsgeraden (âKompressionsrateâ) und die Abweichungen vom korrekten Wert (signed error). Exp 2 zeigte keine signifikanten Fehler (d.h. generelle Ăber- oder UnterschĂ€tzung) fĂŒr Drehungen oder Distanzen. Distanzantworten waren stark in Richtung Mittelwert verschoben (Kompressionsrate 0.58), Winkelantworten jedoch kaum (0.91). FĂŒr randomisierte Dreiecksgeometrien in Exp 3 reduzierte sich diese Tendenz zu mittleren Antworten fĂŒr Distanzen (0.86), verstĂ€rkte sich jedoch fĂŒr Drehungen (0.77). In Ă€hnlichen Experimenten zur DreiecksvervollstĂ€ndigung unter BeschrĂ€nkung auf visuelle Information (Virtual Reality: PĂ©ruch et al., Perc. â97; Duchon et al., Psychonomics â99) und auf propriozeptive Reize (blindes gehen: Loomis et al., JEP â93) zeigte sich eine starke Tendenz zu mittleren Drehwinkeln (Kompressionsrate < 0.5), die wir nicht fanden. Die Tendenz, bei reinen Drehaufgaben in visuellen virtuellen Umgebungen nicht weit genug zu drehen (PĂ©ruch â97; Bakker, Presence â99) konnte ebenfalls nicht beobachtet werden (Exp 1). Pfadintegration aufgrund optischen Flusses erwies sich in unseren Experimenten als ausreichend und verlĂ€Ălich fĂŒr Orientierungs- und Heimfindeaufgaben. VestibulĂ€re und kinĂ€sthetische Information waren hierfĂŒr nicht erforderlich
The footprint of cometary dust analogues: II. Morphology as a tracer of tensile strength and application to dust collection by the Rosetta spacecraft
The structure of cometary dust is a tracer of growth processes in the
formation of planetesimals. Instrumentation on board the Rosetta mission to
comet 67P/Churyumov- Gerasimenko captured dust particles and analysed them in
situ. However, these deposits are a product of a collision within the
instrument. We conducted laboratory experiments with cometary dust analogues,
simulating the collection process by Rosetta instruments (specifically COSIMA,
MIDAS). In Paper I we reported that velocity is a key driver in determining the
appearance of deposits. Here in Paper II we use materials with different
monomer sizes, and study the effect of tensile strength on the appearance of
deposits. We find that mass transfer efficiency increases from 1 up to
10% with increasing monomer diameter from 0.3 m to 1.5 m (i.e.
tensile strength decreasing from 12 to 3 kPa), and velocities
increasing from 0.5 to 6 m/s. Also, the relative abundance of small fragments
after impact is higher for material with higher tensile strength. The
degeneracy between the effects of velocity and material strength may be lifted
by performing a closer study of the deposits. This experimental method makes it
possible to estimate the mass transfer efficiency in the COSIMA instrument.
Extrapolating these results implies that more than half of the dust collected
during the Rosetta mission has not been imaged. We analysed two COSIMA targets
containing deposits from single collisions. The collision that occurred closest
to perihelion passage led to more small fragments on the target.Comment: 13 pages, 11 figures, accepted for publication in MNRA
Thermal stability of ultrasoft FeâZrâN films
The thermal stability of nanocrystalline ultrasoft magnetic (Fe98Zr2)1âxNx films with x = 0.10â0.25 was studied using thermal desorption spectrometry, positron beam analysis and high resolution transmission electron microscopy. The results demonstrate that grain growth during the heat treatment is accompanied by an increase of the free volume and nitrogen relocation and desorption. All these phenomena can drastically degrade the ultrasoft magnetic properties. The nitrogen desorption has already started at temperatures around 400 K. Nevertheless, most of the nitrogen leaves the sample at a temperature above 800 K. We found that nitrogen out-diffusion is significantly retarded compared with the prediction of the diffusion in bulk α-Fe. A qualitative model is proposed in which the nitrogen out-diffusion in nanocrystalline material is retarded by trapping at immobile defects, namely Zr atoms, and also by voids at grain boundaries. From a certain temperature, nitrogen migrates from the interior of the nanograins to the nanovoids at the grain boundaries and the out-diffusion to the outer surface is controlled by transport between the voids.
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