1,774 research outputs found
The Stability of Double White Dwarf Binaries Undergoing Direct Impact Accretion
We present numerical simulations of dynamically unstable mass transfer in a
double white dwarf binary with initial mass ratio, q = 0.4. The binary
components are approximated as polytropes of index n = 3/2 and the initially
synchronously rotating, semi-detached equilibrium binary is evolved
hydrodynamically with the gravitational potential being computed through the
solution of Poisson's equation. Upon initiating deep contact in our baseline
simulation, the mass transfer rate grows by more than an order of magnitude
over approximately ten orbits, as would be expected for dynamically unstable
mass transfer. However, the mass transfer rate then reaches a peak value, the
binary expands and the mass transfer event subsides. The binary must therefore
have crossed the critical mass ratio for stability against dynamical mass
transfer. Despite the initial loss of orbital angular momentum into the spin of
the accreting star, we find that the accretor's spin saturates and angular
momentum is returned to the orbit more efficiently than has been previously
suspected for binaries in the direct impact accretion mode. To explore this
surprising result, we directly measure the critical mass ratio for stability by
imposing artificial angular momentum loss at various rates to drive the binary
to an equilibrium mass transfer rate. For one of these driven evolutions, we
attain equilibrium mass transfer and deduce that effectively q_crit has evolved
to approximately 2/3. Despite the absence of a fully developed disk, tidal
interactions appear effective in returning excess spin angular momentum to the
orbit.Comment: 27 pages, 6 figures. Please see
http://www.phys.lsu.edu/faculty/tohline/astroph/mftd07/ for animations and
full resolution figures. Accepted for publication in the Astrophysical
Journa
Teachers developing assessment for learning: impact on student achievement
While it is generally acknowledged that increased use of formative assessment (or assessment for learning) leads to higher quality learning, it is often claimed that the pressure in schools to improve the results achieved by students in externally-set tests and examinations precludes its use. This paper reports on the achievement of secondary school students who worked in classrooms where teachers made time to develop formative assessment strategies. A total of 24 teachers (2 science and 2 mathematics teachers, in each of six schools in two LEAs) were supported over a six-month period in exploring and planning their approach to formative assessment, and then, beginning in September 1999, the teachers put these plans into action with selected classes. In order to compute effect sizes, a measure of prior attainment and at least one comparison group was established for each class (typically either an equivalent class taught in the previous year by the same teacher, or a parallel class taught by another teacher). The mean effect size was 0.32
Changes in the source and transport mechanism of terrigenous input to the Indian sector of southern ocean during the late quaternary and its palaeoceanographic implications
Changes in the terrigenous sediment source and transport mechanisms during the late Quaternary
have been investigated using four sediment cores within the Indian sector of Southern Ocean,
using the magnetic susceptibility (MS) and sedimentological records. Sediments deposited during
the Holocene and other interglacial periods were characterised by low MS, low sand content,
reduced ice-rafted detritus (IRD) input and increased illite possibly transported via hydrographic
advection from the south. The glacial intervals are characterised by high MS, high sand content,
increased IRD input and reduced illite clays, derived from both local as well as Antarctic sources.
Significant reduction in clay fraction and illite content during glacials suggests that the erosive
and transporting capabilities of the deep and bottom waters could have reduced compared to the
interglacial times. The changes in terrigenous influx to this region were significantly influenced by
the rhythmic glacial–interglacial fluctuations in bottom circulation and the position of the Polar
Front
Numerical Simulations of the Onset and Stability of Dynamical Mass Transfer in Binaries
Hydrodynamical simulations of semi-detached, polytropic binary stars are
presented in an effort to study the onset and stability of dynamical mass
transfer events. Initial, synchronously rotating equilibrium models are
constructed using a self-consistent-field technique and then evolved with an
Eulerian hydrodynamics code in a fully self-consistent manner. We describe code
improvements introduced over the past few years that permit us to follow
dynamical mass-transfer events through more than 30 orbits. Mass-transfer
evolutions are presented for two different initial configurations: A
dynamically unstable binary with initial mass ratio (donor/accretor) that leads to a complete merger in orbits; and a
double-degenerate binary with initial mass ratio that, after some
initial unstable growth of mass transfer, tends to separate as the
mass-transfer rate levels off.Comment: 47 pages, 11 figures, submitted to the Astrophysical Journal. See
http://www.phys.lsu.edu/faculty/tohline/astroph/dmtf05 for high resolution
figures and mpeg animation
Long and short paths in uniform random recursive dags
In a uniform random recursive k-dag, there is a root, 0, and each node in
turn, from 1 to n, chooses k uniform random parents from among the nodes of
smaller index. If S_n is the shortest path distance from node n to the root,
then we determine the constant \sigma such that S_n/log(n) tends to \sigma in
probability as n tends to infinity. We also show that max_{1 \le i \le n}
S_i/log(n) tends to \sigma in probability.Comment: 16 page
Algorithm for normal random numbers
We propose a simple algorithm for generating normally distributed pseudo
random numbers. The algorithm simulates N molecules that exchange energy among
themselves following a simple stochastic rule. We prove that the system is
ergodic, and that a Maxwell like distribution that may be used as a source of
normally distributed random deviates follows when N tends to infinity. The
algorithm passes various performance tests, including Monte Carlo simulation of
a finite 2D Ising model using Wolff's algorithm. It only requires four simple
lines of computer code, and is approximately ten times faster than the
Box-Muller algorithm.Comment: 5 pages, 3 encapsulated Postscript Figures. Submitted to
Phys.Rev.Letters. For related work, see http://pipe.unizar.es/~jf
Compensation of B-L charge of matter with relic sneutrinos
We consider massless gauge boson connected to B-L charge with and without
compensation to complete the investigation of the gauging of B and L charges.
Relic sneutrinos predicted by SUSY and composite models may compensate B-L
charge of matter. As a consequence of the possible compensation mechanism we
have shown that the available experimental data admit the range of the B-L
interaction constant, 10^{-29} < {\alpha}_{B-L} < 10^{-12}, in addition to
{\alpha}_{B-L} < 10^{-49} obtained without compensation.Comment: 6 page
Bond percolation on a class of correlated and clustered random graphs
We introduce a formalism for computing bond percolation properties of a class
of correlated and clustered random graphs. This class of graphs is a
generalization of the Configuration Model where nodes of different types are
connected via different types of hyperedges, edges that can link more than 2
nodes. We argue that the multitype approach coupled with the use of clustered
hyperedges can reproduce a wide spectrum of complex patterns, and thus enhances
our capability to model real complex networks. As an illustration of this
claim, we use our formalism to highlight unusual behaviors of the size and
composition of the components (small and giant) in a synthetic, albeit
realistic, social network.Comment: 16 pages and 4 figure
Experimental modulation of capsule size in Cryptococcus neoformans
Experimental modulation of capsule size is an important technique for the study of the virulence of the encapsulated pathogen Cryptococcus neoformans. In this paper, we summarize the techniques available for experimental modulation of capsule size in this yeast and describe improved methods to induce capsule size changes. The response of the yeast to the various stimuli is highly dependent on the cryptococcal strain. A high CO(2) atmosphere and a low iron concentration have been used classically to increase capsule size. Unfortunately, these stimuli are not reliable for inducing capsular enlargement in all strains. Recently we have identified new and simpler conditions for inducing capsule enlargement that consistently elicited this effect. Specifically, we noted that mammalian serum or diluted Sabouraud broth in MOPS buffer pH 7.3 efficiently induced capsule growth. Media that slowed the growth rate of the yeast correlated with an increase in capsule size. Finally, we summarize the most commonly used media that induce capsule growth in C. neoformans
TEGDMA Reduces Mineralization in Dental Pulp Cells
Direct application of dentin bonding agents onto the exposed pulp has been advocated, but in vivo studies indicate a lack of reparative dentin formation. Our objective was to investigate the role of triethylene glycol dimethacrylate (TEGDMA), a commonly used compound in dentin bonding agents, as a potential inhibitor of mineralization. Human pulp cells were exposed to different concentrations of TEGDMA, and expression of the mineralization-related genes collagen I, alkaline phosphatase, bone sialoprotein, osteocalcin, Runx2, and dentin sialophosphoprotein was analyzed. Gene expression studies by real-time polymerase chain-reaction revealed a concentration- and timedependent decrease of mineralization markers. A subtoxic TEGDMA concentration (0.3 mM) reduced expression levels by 5 to 20% after 4 hrs and by 50% after 12 hrs. Furthermore, alkaline phosphatase activity and calcium deposition were significantly lower in dental pulp cells treated with TEGDMA over 14 days. These findings indicate that even low TEGDMA concentrations might inhibit mineralization induced by dental pulp cells, thus impairing reparative dentin formation after pulp capping with dentin bonding agents
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