2,961 research outputs found
Active primate simulator Final report
Systems engineering data and design specifications for Biosatellite active primate simulato
An Empirical Relation Between The Large-Scale Magnetic Field And The Dynamical Mass In Galaxies
The origin and evolution of cosmic magnetic fields as well as the influence
of the magnetic fields on the evolution of galaxies are unknown. Though not
without challenges, the dynamo theory can explain the large-scale coherent
magnetic fields which govern galaxies, but observational evidence for the
theory is so far very scarce. Putting together the available data of
non-interacting, non-cluster galaxies with known large-scale magnetic fields,
we find a tight correlation between the integrated polarized flux density,
S(PI), and the rotation speed, v(rot), of galaxies. This leads to an almost
linear correlation between the large-scale magnetic field B and v(rot),
assuming that the number of cosmic ray electrons is proportional to the star
formation rate, and a super-linear correlation assuming equipartition between
magnetic fields and cosmic rays. This correlation cannot be attributed to an
active linear alpha-Omega dynamo, as no correlation holds with global shear or
angular speed. It indicates instead a coupling between the large-scale magnetic
field and the dynamical mass of the galaxies, B ~ M^(0.25-0.4). Hence, faster
rotating and/or more massive galaxies have stronger large-scale magnetic
fields. The observed B-v(rot) correlation shows that the anisotropic turbulent
magnetic field dominates B in fast rotating galaxies as the turbulent magnetic
field, coupled with gas, is enhanced and ordered due to the strong gas
compression and/or local shear in these systems. This study supports an
stationary condition for the large-scale magnetic field as long as the
dynamical mass of galaxies is constant.Comment: 23 pages, 4 figures, accepted for publication in the Astrophysical
Journal Letter
Ideal barriers to polarization reversal and domain-wall motion in strained ferroelectric thin films
The ideal intrinsic barriers to domain switching in c-phase PbTiO_3 (PTO),
PbZrO_3 (PZO), and PbZr_{1-x}Ti_xO_3 (PZT) are investigated via
first-principles computational methods. The effects of epitaxial strain on the
atomic structure, ferroelectric response, barrier to coherent domain reversal,
domain-wall energy, and barrier to domain-wall translation are studied. It is
found that PTO has a larger polarization, but smaller energy barrier to domain
reversal, than PZO. Consequentially the idealized coercive field is over two
times smaller in PTO than PZO. The Ti--O bond length is more sensitive to
strain than the other bonds in the crystals. This results in the polarization
and domain-wall energy in PTO having greater sensitivity to strain than in PZO.
Two ordered phases of PZT are considered, the rock-salt structure and a (100)
PTO/PZO superlattice. In these simple structures we find that the ferroelectric
properties do not obey Vergard's law, but instead can be approximated as an
average over individual 5-atom unit cells.Comment: 9 pages, 13 figure
Mixed Early and Late-Type Properties in the Bar of NGC 6221: Evidence for Evolution along the Hubble Sequence?
Rotation curves and velocity dispersion profiles are presented for both the
stellar and gaseous components along five different position angles (P.A.=5,
50, 95, 125 and 155 degrees) of the nearby barred spiral NGC 6221. The observed
kinematics extends out to about 80" from the nucleus. Narrow and broad-band
imaging is also presented. The radial profiles of the fluxes ratio [NII]/Halpha
reveal the presence of a ring-like structure of ionized gas, with a radius of
about 9" and a deprojected circular velocity of about 280 km/s. The analysis of
the dynamics of the bar indicates this ring is related to the presence of an
inner Lindblad resonance (ILR) at 1.3 kpc. NGC6221 is found to exhibit
intermediate properties between those of the early-type barred galaxies: the
presence of a gaseous ring at an ILR, the bar edge located between the ILR's
and the corotation radius beyond the steep rising portion of the rotation
curve, the dust-lane pattern, and those of the late-type galaxies: an almost
exponential surface brightness profile, the presence of Halpha regions along
all the bar, the spiral-arm pattern. It is consistent with scenarios of
bar-induced evolution from later to earlier-type galaxies.Comment: 1 File ds7406.tar.gz which contains: one latex file (ds7406.tex), and
10 encsulated postscript figures (ds7406f**.eps). To be compiled with aa-l
latex2e macro style. To be published in A&A Sup. Serie
Semicausal operations are semilocalizable
We prove a conjecture by DiVincenzo, which in the terminology of Preskill et
al. [quant-ph/0102043] states that ``semicausal operations are
semilocalizable''. That is, we show that any operation on the combined system
of Alice and Bob, which does not allow Bob to send messages to Alice, can be
represented as an operation by Alice, transmitting a quantum particle to Bob,
and a local operation by Bob. The proof is based on the uniqueness of the
Stinespring representation for a completely positive map. We sketch some of the
problems in transferring these concepts to the context of relativistic quantum
field theory.Comment: 4 pages, 1 figure, revte
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