60,042 research outputs found
Thermomechanical Behavior of the HL-LHC 11 Tesla Nb3Sn Magnet Coil Constituents during Reaction Heat Treatment
The knowledge of the temperature induced changes of the superconductor
volume, and of the thermo-mechanical behaviour of the different coil and
tooling materials is required for predicting the coil geometry and the stress
distribution in the coil after the Nb3Sn reaction heat treatment. In the
present study we have measured the Young's and shear moduli of the HL-LHC 11 T
Nb3Sn dipole magnet coil and reaction tool constituents during in situ heat
cycles with the dynamic resonance method. The thermal expansion behaviours of
the coil components and of a free standing Nb3Sn wire were compared based on
dilation experiments.Comment: 6 pages, 12 figures, presented at MT25 conferenc
Spectral Models of Convection-Dominated Accretion Flows
For small values of the dimensionless viscosity parameter, namely
, the dynamics of non-radiating accretion flows is
dominated by convection; convection strongly suppresses the accretion of matter
onto the central object and transports a luminosity from small to large radii in the flow. A fraction of this convective
luminosity is likely to be radiated at large radii via thermal bremsstrahlung
emission. We show that this leads to a correlation between the frequency of
maximal bremsstrahlung emission and the luminosity of the source, . Accreting black holes with X-ray luminosities are expected to
have hard X-ray spectra, with photon indices , and sources with
are expected to have soft spectra, with
. This is testable with {\it Chandra} and {\it XMM}.Comment: final version accepted by ApJ; significant modifications from
previous versio
A Note on Bimodal Accretion Disks
The existence of bimodal disks is investigated. Following a simple argument
based on energetic considerations we show that stationary, bimodal accretion
disk models in which a Shakura--Sunyaev disk (SSD) at large radii matches an
advection dominated accretion flow (ADAF) at smaller radii are never possible
using the standard slim disk approach, unless some extra energy flux is
present. The same argument, however, predicts the possibility of a transition
from an outer Shapiro--Lightman--Eardley (SLE) disk to an ADAF, and from a SLE
disk to a SSD. Both types of solutions have been found.Comment: 9 pages including 9 figures, accepted for publication in The
Astrophysical Journa
Threshold concepts and metalearning capacity
This study operationalises the empowering concept of metalearning in the specific context of engagement with a threshold concept. An experience of metalearning was constituted in two parts. First students' awareness of themselves as learners is prompted by, and focuses on, a learning profile that is generated online through the completion of the Reflections on Learning Inventory (RoLI). Second, students are given an opportunity to interpret their respective profiles and write a short and undirected reflective account of their interpretation. The second part of the experience focuses not only on students' awareness but also on their capacity to control their future learning on the basis of their heightened awareness.
The Interstellar N/O Abundance Ratio: Evidence for Local Infall?
Sensitive measurements of the interstellar gas-phase oxygen abundance have
revealed a slight oxygen deficiency ( 15%) toward stars within 500 pc of
the Sun as compared to more distant sightlines. Recent observations of
the interstellar gas-phase nitrogen abundance indicate larger variations, but
no trends with distance were reported due to the significant measurement
uncertainties for many sightlines. By considering only the highest quality
( 5 ) N/O abundance measurements, we find an intriguing trend in
the interstellar N/O ratio with distance. Toward the seven stars within
500 pc of the Sun, the weighted mean N/O ratio is 0.217 0.011, while for
the six stars further away the weighted mean value (N/O = 0.142 0.008) is
curiously consistent with the current Solar value (N/O =
0.138). It is difficult to imagine a scenario invoking
environmental (e.g., dust depletion, ionization, etc.) variations alone that
explains this abundance anomaly. Is the enhanced nitrogen abundance localized
to the Solar neighborhood or evidence of a more widespread phenomenon? If it is
localized, then recent infall of low metallicity gas in the Solar neighborhood
may be the best explanation. Otherwise, the N/O variations may be best
explained by large-scale differences in the interstellar mixing processes for
AGB stars and Type II supernovae.Comment: accepted for publication in the Astrophysical Journal Letter
Heavy Meson Production in NN Collisions with Polarized Beam and Target -- A new facility for COSY
The study of near--threshold meson production in pp and pd collisions
involving polarized beams and polarized targets offers the rare opportunity to
gain insight into short--range features of the nucleon--nucleon interaction.
The Cooler Synchrotron COSY at FZ--J\"ulich is a unique environment to perform
such studies. Measurements of polarization observables require a cylindrically
symmetrical detector, capable to measure the momenta and the directions of
outgoing charged hadrons. The wide energy range of COSY leads to momenta of
outgoing protons to be detected in a single meson production reaction between
300 and 2500 MeV/c. Scattering angles of protons to be covered extend to about
in the laboratory system. An azimuthal angular coverage of the
device around 98% seems technically achievable. The required magnetic
spectrometer could consist of a superconducting toroid, providing fields around
3 T.Comment: 6 pages, 1 figure, submitted to Czechoslovak Journal of Physic
Morphological Three-Dimensional Analysis
This paper presents the main results of our research on mathematical morphology for three-dimensional images. The first issue is to decide which grid must be used. The face-centred cubic grid and the centred cubic grid seem to be more suitable than the cubic grid in terms of possible rotations. For these two grids we derive the formulae for the basic Minkovski measures. Then we show through several examples that extension from 2D to 3D is straightforward for most transformations. The efficiency of direct 3D processing is illustrated by applications to filtering, overlapping particle separation and grey-scale image segmentation
Ionization, Magneto-rotational, and Gravitational Instabilities in Thin Accretion Disks Around Supermassive Black Holes
We consider the combined role of the thermal ionization, magneto-rotational
and gravitational instabilities in thin accretion disks around supermassive
black holes. We find that in the portions of the disk unstable to the
ionization instability, the gas remains well coupled to the magnetic field even
on the cold, neutral branch of the thermal limit cycle. This suggests that the
ionization instability is not a significant source of large amplitude
time-dependent accretion in AGN. We also argue that, for accretion rates
greater than 10^{-2} solar masses per year, the gravitationally unstable and
magneto-rotationally unstable regions of the accretion disk overlap; for lower
accretion rates they may not. Some low-luminosity AGN, e.g. NGC 4258, may thus
be in a transient phase in which mass is building up in a non-accreting
gravitationally and magneto-rotationally stable ``dead zone.'' We comment on
possible implications of these findings.Comment: Accepted for publication in Ap
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