5,342 research outputs found
Binary evolution using the theory of osculating orbits: conservative Algol evolution
Our aim is to calculate the evolution of Algol binaries within the framework
of the osculating orbital theory, which considers the perturbing forces acting
on the orbit of each star arising from mass exchange via Roche lobe overflow
(RLOF). The scheme is compared to results calculated from a `classical'
prescription. Using our stellar binary evolution code BINSTAR, we calculate the
orbital evolution of Algol binaries undergoing case A and case B mass transfer,
by applying the osculating scheme. The velocities of the ejected and accreted
material are evaluated by solving the restricted three-body equations of
motion, within the ballistic approximation. This allows us to determine the
change of linear momentum of each star, and the gravitational force applied by
the mass transfer stream. Torques applied on the stellar spins by tides and
mass transfer are also considered. Using the osculating formalism gives shorter
post-mass transfer orbital periods typically by a factor of 4 compared to the
classical scheme, owing to the gravitational force applied onto the stars by
the mass transfer stream. Additionally, during the rapid phase of mass
exchange, the donor star is spun down on a timescale shorter than the tidal
synchronization timescale, leading to sub-synchronous rotation. Consequently,
between 15 and 20 per cent of the material leaving the inner-Lagrangian point
is accreted back onto the donor (so-called `self-accretion'), further enhancing
orbital shrinkage. Self-accretion, and the sink of orbital angular momentum
which mass transfer provides, may potentially lead to more contact binaries.
Even though Algols are mainly considered, the osculating prescription is
applicable to all types of interacting binaries, including those with eccentric
orbits.Comment: A&A in press. Minor typos correcte
Characteristic spatial scale of vesicle pair interactions in a plane linear flow
We report the experimental studies on interaction of two vesicles trapped in
a microfluidic analog of four-roll mill, where a plane linear flow is realized.
We found that the dynamics of a single vesicle is significantly altered by the
presence of another vesicle at separation distances up to about 3.2 \div 3.7
times of effective radius of the vesicles. This is supported by direct
measurements of a single vesicle back-reaction on the velocity field. Thus, the
experiment provides the lower bound for the interaction scale of vesicles and
so the corresponding upper bound for the volume fraction \phi=0.08 \div 0.13 of
non-interacting vesicle suspensions.Comment: 5 pages, 8 figures, PRE accepted for publicatio
Model-en data-analyse ten behoeve van betere tij-verwachtingen: deelrapport 2. GOG-werking langs de Zeeschelde
Hydrologie en getij bevaarbare waterlopen in de eerste maanden van 2016: van neerslag… naar afvoer… bij een stevige wind in het getijgebied… geeft werk
Sea surface temperature of the coastal zones of France
Thermal gradients in French coastal zones for the period of one year were mapped in order to enable a coherent study of certain oceanic features detectable by the variations in the sea surface temperature field and their evolution in time. The phenomena examined were mesoscale thermal features in the English Channel, the Bay of Biscay, and the northwestern Mediterranean; thermal gradients generated by French estuary systems; and diurnal heating in the sea surface layer. The investigation was based on Heat Capacity Mapping Mission imagery
Sea surface temperature of the coastal zones of France
The results of an investigation to map the various thermal gradients in the coastal zones of France are presented. Paricular emphasis is given to the natural phenomena and man made thermal effluents. It is shown that a close correlation exist between wind speed direction and the offshore width of the effluent
BINSTAR: a new binary stellar evolution code
We provide a detailed description of a new stellar evolution code, BINSTAR, which has been developed to study interacting binaries. Based on the stellar evolution code STAREVOL, it is specifically designed to study low- and intermediate-mass binaries. We describe the state-of-the-art input physics, which includes treatments of tidal interactions, mass transfer and angular momentum exchange within the system. A generalised Henyey method is used to solve simultaneously the stellar structure equations of each component as well as the separation and eccentricity of the orbit. Test simulations for cases A and B mass transfer are presented and compared with available models. The results of the evolution of Algol systems are in remarkable agreement with the calculations of the Vrije Universiteit Brussel (VUB) group, thus validating our code. We also computed a large grid of models for various masses (2 ≤ M/M⊙ ≤ 20) and seven metallicities (Z = 0.0001, 0.001, 0.004, 0.008, 0.01, 0.02, 0.03) to provide a useful analytical parameterisation of the tidal torque constant E2, which allows the determination of the circularisation and synchronisation timescales for stars with a radiative envelope and convective core. The evolution of E2 during the main sequence shows noticeable differences compared to available models. In particular, our new calculations indicate that the circularisation timescale is constant during core hydrogen burning. We also show that E2 weakly depends on core overshooting but is substantially increased when the metallicity becomes lower
Attitude Determination from Single-Antenna Carrier-Phase Measurements
A model of carrier phase measurement (as carried out by a satellite
navigation receiver) is formulated based on electromagnetic theory. The model
shows that the phase of the open-circuit voltage induced in the receiver
antenna with respect to a local oscillator (in the receiver) depends on the
relative orientation of the receiving and transmitting antennas. The model
shows that using a {\it single} receiving antenna, and making carrier phase
measurements to seven satellites, the 3-axis attitude of a user platform (in
addition to its position and time) can be computed relative to an initial
point. This measurement model can also be used to create high-fidelity
satellite signal simulators that take into account the effect of platform
rotation as well as translation.Comment: 12 pages, and one figure. Published in J. Appl. Phys. vol. 91, No. 7,
April 1, 200
A comparison of the biogeographical and geomorphological characteristics of gullied and non-gullied valley head mires in eastern Lesotho
Environmental degradation is a serious problem in Lesotho, Africa. The majority of
studies dealing with soil loss and gully erosion have mainly focused on the mechanics of
the erosion. However, mountain watersheds are sensitive and vulnerable to degradation
and in so being, have large inherent environmental implications. Gully erosion diversifies
the micro-topographical and hydrological environment. The severe changes and eventual
system degradation incurred as a result of gullying has rarely been quantified. This paper
specifically highlights mire phytogeographic responses to valley head degradation at
selected sites in eastern Lesotho by comparing patterns observed in gullied mires to those
seen in other non-gullied mires. Soil and vegetation belt transects are set up across five
mires (2 gullied) in the highlands of eastern Lesotho. The response of many of the plant
species to the overall environmental gradient was asymmetrical and unimodal in pattern.
The spatial distribution of the soil’s physical properties, topography and vegetation
community patterns were found to reflect the spatial mosaic of the soil moisture gradient.
The negative impact that gullying has on the ecohydrological regime of the valley heads
is evident and is allowing for shrub encroachment. Both the wetland and dryland
vegetation communities correlate strongly with the changes in the surface soil moisture
gradient. Gully erosion is clearly a threshold phenomenon. Continued grassland
degradation, accelerated soil erosion and subsequent gullying of the wetlands will lead to
plant and animal diversity loss, decreased livestock productivity, sediment-laden water
and shortened dam life-span
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