10,098 research outputs found
Neutrinos and the synthesis of heavy elements: the role of gravity
The synthesis of heavy elements in the Universe presents several challenges.
From one side the astrophysical site is still undetermined and on other hand
the input from nuclear physics requires the knowledge of properties of exotic
nuclei, some of them perhaps accessible in ion beam facilities. Black hole
accretion disks have been proposed as possible r-process sites. Analogously to
Supernovae these objects emit huge amounts of neutrinos. We discuss the
neutrino emission from black hole accretion disks. In particular we show the
influence that the black hole strong gravitational field has on changing the
electron fraction relevant to the synthesis of elements.Comment: 5 pages, 5 figures, Invited talk at the 15th International Symposium
on Capture Gamma-Ray Spectroscopy and Related Topics (CGS15), to appear in
EPJ Web of Conference
Non-universality of dark-matter halos: cusps, cores, and the central potential
Dark-matter halos grown in cosmological simulations appear to have central
NFW-like density cusps with mean values of , and
some dispersion, which is generally parametrized by the varying index
in the Einasto density profile fitting function. Non-universality in profile
shapes is also seen in observed galaxy clusters and possibly dwarf galaxies.
Here we show that non-universality, at any given mass scale, is an intrinsic
property of DARKexp, a theoretically derived model for collisionless
self-gravitating systems. We demonstrate that DARKexp - which has only one
shape parameter, - fits the dispersion in profile shapes of massive
simulated halos as well as observed clusters very well. DARKexp also allows for
cored dark-matter profiles, such as those found for dwarf spheroidal galaxies.
We provide approximate analytical relations between DARKexp , Einasto
, or the central logarithmic slope in the Dehnen-Tremaine analytical
-models. The range in halo parameters reflects a substantial variation
in the binding energies per unit mass of dark-matter halos.Comment: ApJ, in press, 10 pages, 7 figure
Neutron capture rates and r-process nucleosynthesis
Simulations of r-process nucleosynthesis require nuclear physics information
for thousands of neutron-rich nuclear species from the line of stability to the
neutron drip line. While arguably the most important pieces of nuclear data for
the r-process are the masses and beta decay rates, individual neutron capture
rates can also be of key importance in setting the final r-process abundance
pattern. Here we consider the influence of neutron capture rates in forming the
A~80 and rare earth peaks.Comment: 10 pages, 5 figures, appears in the Proceedings of the 14th
International Symposium on Capture Gamma-Ray Spectroscopy and Related Topic
The Epidemiology of Multiple Sclerosis in Scotland: Inferences from Hospital Admissions
PMCID: PMC3029296This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
IrSr_2Sm_{1.15}Ce_{0.85}Cu_{2.175}O_{10}: A Novel Reentrant Spin-Glass Material
A new iridium containing layered cuprate material,
IrSr_2Sm_{1.15}Ce_{0.85}Cu_{2.175}O_{10, has been synthesized by conventional
ambient-pressure solid-state techniques. The material's structure has been
fully characterized by Rietveld refinement of high resolution synchrotron X-ray
diffraction data; tilts and rotations of the IrO_6 octahedra are observed as a
result of a bond mismatch between in-plane Ir-O and Cu-O bond lengths.
DC-susceptibility measurements evidence a complex set of magnetic transitions
upon cooling that are characteristic of a reentrant spin-glass ground-state.
The glassy character of the lowest temperature, Tg=10 K, transition is further
confirmed by AC-susceptibility measurements, showing a characteristic frequency
dependence that can be well fitted by the Vogel-Fulcher law and yields a value
of \Delta_(T_f)/[T_f \Delta log({\omega})] =0.015(1), typical of dilute
magnetic systems. Electronic transport measurements show the material to be
semiconducting at all temperatures with no transition to a superconducting
state. Negative magnetoresistance is observed when the material is cooled below
25 K, and the magnitude of this magnetoresistance is seen to increase upon
cooling to a value of MR = -9 % at 8 K
Phase Mixing of Alfvén Waves Near a 2D Magnetic Null Point
The propagation of linear Alfvén wave pulses in an inhomogeneous plasma near a 2D coronal null point is investigated. When a uniform plasma density is considered, it is seen that an initially planar Alfvén wavefront remains planar, despite the varying equilibrium Alfvén speed, and that all the wave collects at the separatrices. Thus, in the non-ideal case, these Alfvénic disturbances preferentially dissipate their energy at these locations. For a non-uniform equilibrium density, it is found that the Alfvén wavefront is significantly distorted away from the initially planar geometry, inviting the possibility of dissipation due to phase mixing. Despite this however, we conclude that for the Alfvén wave, current density accumulation and preferential heating still primarily occur at the separatrices, even when an extremely non-uniform density profile is considered
Gamma-Ray Bursts Black hole accretion disks as a site for the vp-process
We study proton rich nucleosynthesis in windlike outflows from gamma-ray
bursts accretion disks with the aim to determine if such outflows are a site of
the vp-process. The efficacy of this vp-process depends on thermodynamic and
hydrodynamic factors. We discuss the importance of the entropy of the material,
the outflow rate, the initial ejection point and accretion rate of the disk. In
some cases the vp-process pushes the nucleosynthesis out to A~100 and produces
light p-nuclei. However, even when these nuclei are not produced, neutrino
induced interactions can significantly alter the abundance pattern and cannot
be neglected.Comment: 9 pages, 16 figures, accepted for publication in Phys. Rev.
A flight investigation with a STOL airplane flying curved, descending instrument approach paths
A flight investigation using a De Havilland Twin Otter airplane was conducted to determine the configurations of curved, 6 deg descending approach paths which would provide minimum airspace usage within the requirements for acceptable commercial STOL airplane operations. Path configurations with turns of 90 deg, 135 deg, and 180 deg were studied; the approach airspeed was 75 knots. The length of the segment prior to turn, the turn radius, and the length of the final approach segment were varied. The relationship of the acceptable path configurations to the proposed microwave landing system azimuth coverage requirements was examined
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