4,975 research outputs found
Attitudes to telecare among older people, professional care workers and informal carers: a preventative strategy or crisis management?
This paper reports findings from an attitudinal survey towards telecare that emerged from twenty-two focus groups comprising ninety-two older people, fifty-five professional stakeholders and thirty-nine carers. These were convened in three different regions of England as a precursor to telecare service development. The results from this study suggest that informantsâ views were shaped by prior knowledge of conventional health and social care delivery in their locality and the implication is that expectations and requirements in respect of telecare services in general are likely to be informed by wider perceptions about the extent to which community care should operate as a preventative strategy or as a mechanism for crisis management
MHD simulations of the magnetorotational instability in a shearing box with zero net flux. I. The issue of convergence
We study the properties of MHD turbulence driven by the magnetorotational
instability (MRI) in accretion disks. We adopt the local shearing box model and
focus on the special case for which the initial magnetic flux threading the
disk vanishes. We employ the finite difference code ZEUS to evolve the ideal
MHD equations. Performing a set of numerical simulations in a fixed
computational domain with increasing resolution, we demonstrate that turbulent
activity decreases as resolution increases. We quantify the turbulent activity
by measuring the rate of angular momentum transport through evaluating the
standard alpha parameter. We find alpha=0.004 when (N_x,N_y,N_z)=(64,100,64),
alpha=0.002 when (N_x,N_y,N_z)=(128,200,128) and alpha=0.001 when
(N_x,N_y,N_z)=(256,400,256). This steady decline is an indication that
numerical dissipation, occurring at the grid scale is an important determinant
of the saturated form of the MHD turbulence. Analysing the results in Fourier
space, we demonstrate that this is due to the MRI forcing significant flow
energy all the way down to the grid dissipation scale. We also use our results
to study the properties of the numerical dissipation in ZEUS. Its amplitude is
characterised by the magnitude of an effective magnetic Reynolds number Re_M
which increases from 10^4 to 10^5 as the number of grid points is increased
from 64 to 256 per scale height. The simulations we have carried out do not
produce results that are independent of the numerical dissipation scale, even
at the highest resolution studied. Thus it is important to use physical
dissipation, both viscous and resistive, and to quantify contributions from
numerical effects, when performing numerical simulations of MHD turbulence with
zero net flux in accretion disks at the resolutions normally considered.Comment: 10 pages, 15 figures, accepted in A&A. Numerical results improved,
various numerical issues addressed (boundary conditions, box size, run
durations
MHD simulations of the magnetorotational instability in a shearing box with zero net flux. II. The effect of transport coefficients
We study the influence of the choice of transport coefficients (viscosity and
resistivity) on MHD turbulence driven by the magnetorotational instability
(MRI) in accretion disks. We follow the methodology described in paper I: we
adopt an unstratified shearing box model and focus on the case where the net
vertical magnetic flux threading the box vanishes. For the most part we use the
finite difference code ZEUS, including explicit transport coefficients in the
calculations. However, we also compare our results with those obtained using
other algorithms (NIRVANA, the PENCIL code and a spectral code) to demonstrate
both the convergence of our results and their independence of the numerical
scheme. We find that small scale dissipation affects the saturated state of MHD
turbulence. In agreement with recent similar numerical simulations done in the
presence of a net vertical magnetic flux, we find that turbulent activity
(measured by the rate of angular momentum transport) is an increasing function
of the magnetic Prandtl number Pm for all values of the Reynolds number Re that
we investigated. We also found that turbulence disappears when the Prandtl
number falls below a critical value Pm_c that is apparently a decreasing
function of Re. For the limited region of parameter space that can be probed
with current computational resources, we always obtained Pm_c>1. We conclude
that the magnitudes of the transport coefficients are important in determining
the properties of MHD turbulence in numerical simulations in the shearing box
with zero net flux, at least for Reynolds numbers and magnetic Prandtl numbers
that are such that transport is not dominated by numerical effects and thus can
be probed using current computational resources.Comment: 10 pages, 13 figures, accepted in A&A. Numerical results improved,
minor changes in the tex
First Detection of a Strong Magnetic Field on a Bursty Brown Dwarf: Puzzle Solved
We report the first direct detection of a strong, 5 kG magnetic field on the
surface of an active brown dwarf. LSR J1835+3259 is an M8.5 dwarf exhibiting
transient radio and optical emission bursts modulated by fast rotation. We have
detected the surface magnetic field as circularly polarized signatures in the
819 nm sodium lines when an active emission region faced the Earth. Modeling
Stokes profiles of these lines reveals the effective temperature of 2800 K and
log gravity acceleration of 4.5. These parameters place LSR J1835+3259 on
evolutionary tracks as a young brown dwarf with the mass of 554 M and age of 224 Myr. Its magnetic field is at least 5.1 kG and covers
at least 11% of the visible hemisphere. The active region topology recovered
using line profile inversions comprises hot plasma loops with a vertical
stratification of optical and radio emission sources. These loops rotate with
the dwarf in and out of view causing periodic emission bursts. The magnetic
field is detected at the base of the loops. This is the first time that we can
quantitatively associate brown dwarf non-thermal bursts with a strong, 5 kG
surface magnetic field and solve the puzzle of their driving mechanism. This is
also the coolest known dwarf with such a strong surface magnetic field. The
young age of LSR J1835+3259 implies that it may still maintain a disk, which
may facilitate bursts via magnetospheric accretion, like in higher-mass T
Tau-type stars. Our results pave a path toward magnetic studies of brown dwarfs
and hot Jupiters.Comment: ApJ, in pres
What's the point of knowing how?
Why is it useful to talk and think about knowledge-how? Using Edward Craigâs discussion of the function of the concepts of knowledge and knowledge-how as a jumping off point, this paper argues that considering this question can offer us new angles on the debate about knowledge-how. We consider two candidate functions for the concept of knowledge-how: pooling capacities, and mutual reliance. Craig makes the case for pooling capacities, which connects knowledge-how to our need to pool practical capacities. I argue that the evidence is much more equivocal. My suggested diagnosis is that the concept of knowledge-how plays both functions, meaning that the concept of knowledge-how is inconsistent, and that the debate about knowledge-how is at least partly a metalinguistic negotiation. In closing, I suggest a way to revise the philosophical concept of knowledge how
Accretion of low angular momentum material onto black holes: 2D magnetohydrodynamical case
We report on the second phase of our study of slightly rotating accretion
flows onto black holes. We consider magnetohydrodynamical (MHD) accretion flows
with a spherically symmetric density distribution at the outer boundary, but
with spherical symmetry broken by the introduction of a small,
latitude-dependent angular momentum and a weak radial magnetic field. We study
accretion flows by means of numerical 2D, axisymmetric, MHD simulations with
and without resistive heating. Our main result is that the properties of the
accretion flow depend mostly on an equatorial accretion torus which is made of
the material that has too much angular momentum to be accreted directly. The
torus accretes, however, because of the transport of angular momentum due to
the magnetorotational instability (MRI). Initially, accretion is dominated by
the polar funnel, as in the hydrodynamic inviscid case, where material has zero
or very low angular momentum. At the later phase of the evolution, the torus
thickens towards the poles and develops a corona or an outflow or both.
Consequently, the mass accretion through the funnel is stopped. The accretion
of rotating gas through the torus is significantly reduced compared to the
accretion of non-rotating gas (i.e., the Bondi rate). It is also much smaller
than the accretion rate in the inviscid, weakly rotating case.Our results do
not change if we switch on or off resistive heating. Overall our simulations
are very similar to those presented by Stone, Pringle, Hawley and Balbus
despite different initial and outer boundary conditions. Thus, we confirm that
MRI is very robust and controls the nature of radiatively inefficient accretion
flows.Comment: submitted in Ap
Observation of Buried Phosphorus Dopants near Clean Si(100)-(2x1) with Scanning Tunneling Microscopy
We have used scanning tunneling microscopy to identify individual phosphorus
dopant atoms near the clean silicon (100)-(2x1) reconstructed surface. The
charge-induced band bending signature associated with the dopants shows up as
an enhancement in both filled and empty states and is consistent with the
appearance of n-type dopants on compound semiconductor surfaces and passivated
Si(100)-(2x1). We observe dopants at different depths and see a strong
dependence of the signature on the magnitude of the sample voltage. Our results
suggest that, on this clean surface, the antibonding surface state band acts as
an extension of the bulk conduction band into the gap. The positively charged
dimer vacancies that have been observed previously appear as depressions in the
filled states, as opposed to enhancements, because they disrupt these surface
bands.Comment: 4 pages, 3 figures. TeX for OSX from Wierde
All Paired Up with No Place to Go: Pairing, Synapsis, and DSB Formation in a Balancer Heterozygote
The multiply inverted X chromosome balancer FM7 strongly suppresses, or eliminates, the occurrence of crossing over when heterozygous with a normal sequence homolog. We have utilized the LacI-GFP: lacO system to visualize the effects of FM7 on meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes. Surprisingly, the analysis of meiotic pairing and synapsis for three lacO reporter couplets in FM7/X heterozygotes revealed they are paired and synapsed during zygotene/pachytene in 70%â80% of oocytes. Moreover, the regions defined by these lacO couplets undergo double-strand break formation at normal frequency. Thus, even complex aberration heterozygotes usually allow high frequencies of meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes. However, the frequencies of failed pairing and synapsis were still 1.5- to 2-fold higher than were observed for corresponding regions in oocytes with two normal sequence X chromosomes, and this effect was greatest near a breakpoint. We propose that heterozygosity for breakpoints creates a local alteration in synaptonemal complex structure that is propagated across long regions of the bivalent in a fashion analogous to chiasma interference, which also acts to suppress crossing over
Unstable magnetohydrodynamical continuous spectrum of accretion disks. A new route to magnetohydrodynamical turbulence in accretion disks
We present a detailed study of localised magnetohydrodynamical (MHD)
instabilities occuring in two--dimensional magnetized accretion disks. We model
axisymmetric MHD disk tori, and solve the equations governing a
two--dimensional magnetized accretion disk equilibrium and linear wave modes
about this equilibrium. We show the existence of novel MHD instabilities in
these two--dimensional equilibria which do not occur in an accretion disk in
the cylindrical limit. The disk equilibria are numerically computed by the
FINESSE code. The stability of accretion disks is investigated analytically as
well as numerically. We use the PHOENIX code to compute all the waves and
instabilities accessible to the computed disk equilibrium. We concentrate on
strongly magnetized disks and sub--Keplerian rotation in a large part of the
disk. These disk equilibria show that the thermal pressure of the disk can only
decrease outwards if there is a strong gravitational potential. Our theoretical
stability analysis shows that convective continuum instabilities can only
appear if the density contours coincide with the poloidal magnetic flux
contours. Our numerical results confirm and complement this theoretical
analysis. Furthermore, these results show that the influence of gravity can
either be stabilizing or destabilizing on this new kind of MHD instability. In
the likely case of a non--constant density, the height of the disk should
exceed a threshold before this type of instability can play a role. This
localised MHD instability provides an ideal, linear route to MHD turbulence in
strongly magnetized accretion disk tori.Comment: 20 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
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