27,200 research outputs found
The steady-state structure of accretion discs in central magnetic fields
We develop a new analytic solution for the steady-state structure of a thin
accretion disc under the influence of a magnetic field that is anchored to the
central star. The solution takes a form similar to that of Shakura and Sunyaev
and tends to their solution as the magnetic moment of the star tends to zero.
As well as the Kramer's law case, we obtain a solution for a general opacity.
The effects of varying the mass transfer rate, spin period and magnetic field
of the star as well as the opacity model applied to the disc are explored for a
range of objects. The solution depends on the position of the magnetic
truncation radius. We propose a new approach for the identification of the
truncation radius and present an analytic expression for its position.Comment: 11 pages, 7 figures, accepted by MNRA
Outbursts of Young Stellar Objects
We argue that the outbursts of the FU Orionis stars occur on timescales which
are much longer than expected from the standard disc instability model with
\alpha_{c} \gtrsim 10^{-3}. The outburst, recurrence, and rise times are
consistent with the idea that the accretion disc in these objects is truncated
at a radius R_{i} \sim 40 \rsun. In agreement with a number of previous authors
we suggest that the inner regions of the accretion discs in FU Ori objects are
evacuated by the action of a magnetic propeller anchored on the central star.
We develop an analytic solution for the steady state structure of an accretion
disc in the presence of a central magnetic torque, and present numerical
calculations to follow its time evolution. These calculations confirm that a
recurrence time that is consistent with observations can be obtained by
selecting appropriate values for viscosity and magnetic field strength.Comment: 13 pages, 7 figures, accepted by MNRA
Effect of topology on dynamics of knots in polymers under tension
We use computer simulations to compare the dynamical behaviour of torus and
even-twist knots in polymers under tension. The knots diffuse through a
mechanism similar to reptation. Their friction coefficients grow linearly with
average knot length for both knot types. For similar complexity, however, the
torus knots diffuse faster than the even twist knots. The knot-length
auto-correlation function exhibits a slow relaxation time that can be linked to
a breathing mode. Its timescale depends on knot type, being typically longer
for torus than for even-twist knots. These differences in dynamical behaviour
are interpreted in terms of topological features of the knots.Comment: 6 pages, 8 figure
Complex dynamics of knotted filaments in shear flow
Coarse-grained simulations are used to demonstrate that knotted filaments in
shear flow at zero Reynolds number exhibit remarkably rich dynamic behaviour.
For stiff filaments that are weakly deformed by the shear forces, the knotted
filaments rotate like rigid objects in the flow. But away from this regime the
interplay between between shear forces and the flexibility of the filament
leads to intricate regular and chaotic modes of motion that can be divided into
distinct families. The set of accessible mode families depends to first order
on a dimensionless number that relates the filament length, the elastic
modulus, the friction per unit length and the shear rate.Comment: 6 pages, 6 figure
Historia, filosofía y enseñanza de las ciencias : la aproximación actual
This paper traces the use of, and arguments for, the history and philosophy of science in school science courses. Specific attention is paid to the British National Curriculum proposals and to the recommendations of the US Project 2061 curriculum guidelines. Some objections to the inclusion of historical material in science courses are outlined and answered. Mention is made of the Piagetian thesis that individual psychological development mirrors the development of concepts in the history of science. This introduces the topic of idealisation in science. Some significant instances are itemised where science education has, at its considerable cost, ignored work in philosophy of science. Arguments for the inclusion of the history and philosophy of science in science teacher etlucation programmes are given. The paper finishes with a list of topical issues in present science education where collaboration between science teachers, historians, philosophers, and sociologists would be of considerable benefit
The bloodstream differentiation - division of Trypanosoma brucei studied using mitochondrial markers
In the bloodstream of its mammalian host, the African trypanosome Trypanosoma brucei undergoes a life cycle stage differentiation from a long, slender form to a short, stumpy form. This involves three known major events: exit from a proliferative cell cycle, morphological change and mitochondrial biogenesis. Previously, models have been proposed accounting for these events (Matthews & Gull 1994a). Refinement of, and discrimination between, these models has been hindered by a lack of stage-regulated antigens useful as markers at the single-cell level. We have now evaluated a variety of cytological markers and applied them to investigate the coordination of phenotypic differentiation and cell cycle arrest. Our studies have focused on the differential expression of the mitochondrial enzyme dihydrolipoamide dehydrogenase relative to the differentiation-division of bloodstream trypanosomes. The results implicate a temporal order of events: commitment, division, phenotypic differentiation
Vino viejo en botellas nuevas : un problema con la epistemología constructivista
This article deals with what the autor regards as a fatal epistemological error in the constructivist doctrine: its theory maintains the Aristotelian-empiricist epistemological paradigm. However, good constructivist pedagogy could be rescued from the deficient theory that parented it if we made a clear distinction between real and theoretical objects; and knowledge is recognised as a process of intellectual production working with real objects that have been described, apprehended, or incorporated by a theoretical object
Orbits and origins of the young stars in the central parsec of the galaxy
We present new proper motions from the 10 m Keck telescopes for a puzzling population of massive, young stars located within a parsec of the supermassive black hole at the Galactic Center. Our proper motion measurements have uncertainties of only 0.07 mas yr^(−1) (3 km s^(−1) ), which is ≳7 times better than previous proper motion measurements for these stars, and enables us to measure accelerations as low as 0.2 mas yr^(−2) (7 km s^(−1) yr^(−1) ). These measurements, along with stellar line-of-sight velocities from the literature, constrain the true orbit of each individual star and allow us to directly test the hypothesis that the massive stars reside in two stellar disks as has been previously proposed. Analysis of the stellar orbits reveals only one disk of young stars using a method that is capable of detecting disks containing at least 7 stars. The detected disk contains 50% (38 of 73) of the young stars, is inclined by ~115° from the plane of the sky, and is oriented at a position angle of ∼100° East of North. The on-disk and off-disk populations have similar K-band luminosity functions and radial distributions that decrease at larger radii as ∝ r^(−2). The disk has an out-of-the-disk velocity dispersion of 28±6 km s^(−1) , which corresponds to a half-opening angle of 7°±2° , and several candidate disk members have eccentricities greater than 0.2. Our findings suggest that the young stars may have formed in situ but in a more complex geometry than a simple thin circular disk
A Luminous Companion to SGR 1806-20
We have obtained infrared spectra of the star suggested to be the counterpart
of the soft gamma-ray repeater (SGR) 1806-20. We found strong emission lines
similar to those seen in the spectra of the rare Luminous Blue Variables and
B[e] stars. A He I absorption line is also seen, from which we infer a spectral
type O9--B2. This classification, in combination with the minimum distance of
\simgt6 kpc inferred from its extinction, makes the star one of the most
luminous in the Galaxy. We infer that it is a companion to SGR 1806-20, and
suggest that the presence of a companion is somehow related to the SGR
phenomenon.Comment: 5 pages, AASTEX text+table and 2 PostScript figures (needs LaTeX
style files aaspptwo.sty, epsf.sty and rotate.sty). In case of problems,
contact [email protected]. Postscript file of complete article available
on request. (Replaced because first version had one wrong reference in it
Limitation of Trypanosoma brucei parasitaemia results from a combination of density-dependent parasite differentiation and parasite killing by the host immune response
In the bloodstream of its mammalian host, the "slender" form of Trypanosoma brucei replicates extracellularly, producing a parasitaemia. At high density, the level of parasitaemia is limited at a sublethal level by differentiation to the non-replicative "stumpy" form and by the host immune response. Here, we derive continuous time equations to model the time-course, cell types and level of trypanosome parasitaemia, and compare the best fits with experimental data. The best fits that were obtained favour a model in which both density-dependent trypanosome differentiation and host immune response have a role in limiting the increase of parasites, much poorer fits being obtained when differentiation and immune response are considered independently of one another. Best fits also favour a model in which the slender-to-stumpy differentiation progresses in a manner that is essentially independent of the cell cycle. Finally, these models also make the prediction that the density-dependent trypanosome differentiation mechanism can give rise to oscillations in parasitaemia level. These oscillations are independent of the immune system and are not due to antigenic variation
- …