10,562 research outputs found
Determination of the Physical Conditions of the Knots in the Helix Nebula from Optical and Infrared Observations
[Abridged] We use new HST and archived images to clarify the nature of the
knots in the Helix Nebula. We employ published far infrared spectrophotometry
and existing 2.12 micron images to establish that the population distribution
of the lowest ro-vibrational states of H2 is close to the distribution of a gas
in LTE at 988 +- 119 K. We derive a total flux from the nebula in H2 lines and
compare this with the power available from the central star for producing this
radiation. We establish that neither soft X-rays nor FUV radiation has enough
energy to power the H2 radiation, only the stellar EUV radiation shortward of
912 Angstrom does. Advection of material from the cold regions of the knots
produces an extensive zone where both atomic and molecular hydrogen are found,
allowing the H2 to directly be heated by Lyman continuum radiation, thus
providing a mechanism that can explain the excitation temperature and surface
brightness of the cusps and tails. New images of the knot 378-801 reveal that
the 2.12 micron cusp and tail lie immediately inside the ionized atomic gas
zone. This firmly establishes that the "tail" structure is an ionization
bounded radiation shadow behind the optically thick core of the knot. A unique
new image in the HeII 4686 Angstrom line fails to show any emission from knots
that might have been found in the He++ core of the nebula. We also re-examined
high signal-to-noise ratio ground-based telescope images of this same inner
region and found no evidence of structures that could be related to knots.Comment: Astronomical Journal, in press. Some figures are shown at reduced
resolution. A full resolution version is available at
http://www.ifront.org/wiki/Helix_Nebula_2007_Pape
Quantum Oscillations in the Underdoped Cuprate YBa2Cu4O8
We report the observation of quantum oscillations in the underdoped cuprate
superconductor YBa2Cu4O8 using a tunnel-diode oscillator technique in pulsed
magnetic fields up to 85T. There is a clear signal, periodic in inverse field,
with frequency 660+/-15T and possible evidence for the presence of two
components of slightly different frequency. The quasiparticle mass is
m*=3.0+/-0.3m_e. In conjunction with the results of Doiron-Leyraud et al. for
YBa2Cu3O6.5, the present measurements suggest that Fermi surface pockets are a
general feature of underdoped copper oxide planes and provide information about
the doping dependence of the Fermi surface.Comment: Contains revisions addressing referees' comments including a
different Fig 1b. 4 pages, 4 figure
Surface activity of titanium alloys contacted with the simulated body fluid
The purpose of this study was to analyze surface chemical species deposited on various implant materials immersed in simulated body fluid (SBF). The investigated materials including pure titanium, two beta titanium alloys and stainless steel (316L) were formed into discs using cold pressing and sintering. The samples were examined using SEM and mid-IR microscopes. All the sample discs exhibited nucleation and growth of calcium phosphates, precipitated from the SBF solution containing calcium, phosphate and other ions. The results have shown that the nucleation and growth of phosphates depend on the alloy composition and become more intensive in the case of the titanium alloy with the niobium additio
Towards a physical interpretation for the Stephani Universes
A physicaly reasonable interpretation is provided for the perfect fluid,
sphericaly symmetric, conformally flat ``Stephani Universes''. The free
parameters of this class of exact solutions are determined so that the ideal
gas relation is identicaly fulfiled, while the full equation of state
of a classical monatomic ideal gas and a matter-radiation mixture holds up to a
good approximation in a near dust, matter dominated regime. Only the models
having spacelike slices with positive curvature admit a regular evolution
domain that avoids an unphysical singularity. In the matter dominated regime
these models are dynamicaly and observationaly indistinguishable from
``standard'' FLRW cosmology with a dust source.Comment: 17 pages, 2 figures, LaTeX with revtex style, submitted to General
Relativity and Gravitatio
Fatigue stress resistance of some composite materials for dental fillings
The paper investigates fatigue stress resistance of some originally made composite materials based on Bis-GMA resin with fluoridated glass and YbF3. The material was used to fill in the cavities in teeth extracted for orthodontic reasons. For laboratory tests a mastication simulator was used. The tooth samples were placed in special holders with resin and exposed to cyclic mechanical loads (100 000 cycles) in order to assess their resistance to fatigue stress. The influence of repeatable mechanical loads on the teeth and the structure of the material were investigated. Additionally, the impact of cyclic loads on fluoride release from the composite materials was estimate
Structural analysis, magnetic and transport properties of the (Ru1-xCox)Sr2GdCu2O8 system
The effects of Co substitution on structural and superconducting properties
of RuSr2GdCu2O8 compound have been studied. Rietveld refinements of the X-ray
diffraction patterns indicate that the cobalt ion progressively replaces
ruthenium sites. This replacement induces significant changes on the crystal
structure and on the magnetic and superconducting properties. The effects Co
substitution on the superconducting behaviour, and more particulary on the
changes induce by the hole doping mechanism, were investigated in
(Ru1-xCox)Sr2GdCu2O8 by a "bond valence sum" analysis with Co content from x=
0.0 to x = 0.2. The weak ferromagnetic transition at Tm= 138.2 K is shifted to
lower temperature, and suppressed at higher Co content. From the
crystallographic point of view the Ru-O(1)-Cu bond angle, associated to the
rotation of the RuO6 octahedra, around the c-axis remain essetially constant
when Ru is substituted by Co. Furthermore, increasing Co content has the effect
to increase the weak ferromagnetic moment, which may be interpreted as the main
responsible for breaking the delicate balance between magnetic and
superconducting ordering.Comment: 21 pages, 8 figure
Spin polarized neutron matter within the Dirac-Brueckner-Hartree-Fock approach
The relation between energy and density (known as the nuclear equation of
state) plays a major role in a variety of nuclear and astrophysical systems.
Spin and isospin asymmetries can have a dramatic impact on the equation of
state and possibly alter its stability conditions. An example is the possible
manifestation of ferromagnetic instabilities, which would indicate the
existence, at a certain density, of a spin-polarized state with lower energy
than the unpolarized one. This issue is being discussed extensively in the
literature and the conclusions are presently very model dependent. We will
report and discuss our recent progress in the study of spin-polarized neutron
matter. The approach we take is microscopic and relativistic. The calculated
neutron matter properties are derived from realistic nucleon-nucleon
interactions. This makes it possible to understand the nature of the EOS
properties in terms of specific features of the nuclear force model.Comment: 6 pages, 11 figures, revised/extended calculation
Magnetic Stress at the Marginally Stable Orbit: Altered Disk Structure, Radiation, and Black Hole Spin Evolution
Magnetic connections to the plunging region can exert stresses on the inner
edge of an accretion disk around a black hole. We recompute the relativistic
corrections to the thin-disk dynamics equations when these stresses take the
form of a time-steady torque on the inner edge of the disk. The additional
dissipation associated with these stresses is concentrated relatively close
outside the marginally stable orbit, scaling as r to the -7/2 at large radius.
As a result of these additional stresses: spin-up of the central black hole is
retarded; the maximum spin-equilibrium accretion efficiency is 36%, and occurs
at a/M=0.94; the disk spectrum is extended toward higher frequencies; line
profiles (such as Fe K-alpha) are broadened if the line emissivity scales with
local flux; limb-brightening, especially at the higher frequencies, is
enhanced; and the returning radiation fraction is substantially increased, up
to 58%. This last effect creates possible explanations for both synchronized
continuum fluctuations in AGN, and polarization rises shortward of the Lyman
edge in quasars. We show that no matter what additional stresses occur, when
a/M < 0.36, the second law of black hole dynamics sets an absolute upper bound
on the accretion efficiency.Comment: 11 pages, 15 figures, accepted for publication in the Astrophysical
Journa
New Isotropic and Anisotropic Sudden Singularities
We show the existence of an infinite family of finite-time singularities in
isotropically expanding universes which obey the weak, strong, and dominant
energy conditions. We show what new type of energy condition is needed to
exclude them ab initio. We also determine the conditions under which
finite-time future singularities can arise in a wide class of anisotropic
cosmological models. New types of finite-time singularity are possible which
are characterised by divergences in the time-rate of change of the
anisotropic-pressure tensor. We investigate the conditions for the formation of
finite-time singularities in a Bianchi type universe with anisotropic
pressures and construct specific examples of anisotropic sudden singularities
in these universes.Comment: Typos corrected. Published versio
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