231 research outputs found
Vertical distribution of stars and gas in a galactic disk
We study the vertical density distribution of stars and gas (HI and H_2) in a
galactic disk which is embedded in a dark matter halo. The new feature of this
work is the inclusion of gas, and the gravitational coupling between stars and
gas, which has led to a more realistic treatment of a multi-component galactic
disk. The gas gravity is shown to be crucially important despite the low gas
mass fraction. This approach physically explains the observed scaleheight
distribution of all the three disk components, including the long-standing
puzzle (Oort 1962) of a constant HI scaleheight observed in the inner Galaxy.
The above model is applied to two external galaxies: NGC 891 and NGC 4565, and
the stellar disk is shown to be not strictly flat as was long believed but
rather it shows a moderate flaring of a factor of about 2 within the optical
radius.Comment: 4 pages, 2 figures; to appear in the Proceedings of "Island
Universes: Structure and evolution of disk galaxies" (Terschelling, The
Netherlands, July 2005), ed. R. de Jon
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Status of the ORNL liquid low-level waste management upgrades
The strategy for management of the Oak Ridge National Laboratory`s (ORNL`s) radioactively contaminated liquid waste was reviewed. The latest information on waste characterization, regulations, US Department of Energy (DOE) budget guidance, and research and development programs was evaluated to determine how the strategy should be revised. Few changes are needed to update the strategy to reflect new waste characterization, research, and regulatory information. However, recent budget guidance from DOE indicates that minimum funding will not be sufficient to accomplish original objectives to upgrade the liquid low-level waste (LLLW) system to be in compliance with the Federal Facilities Agreement compliance, provide long-term LLLW treatment capability, and minimize Environmental Safety & Health risks. Options are presented that might allow the ORNL LLLW system to continue operations temporarily but significantly reduce its capabilities to handle emergency situations, provide treatment for new waste streams, and accommodate waste from the Environmental Restoration Program and from decontamination and decommissioning of surplus facilities. These options are also likely to increase worker radiation exposure, risk of environmental insult, and generation of solid waste for on-site and off-site disposal/storage beyond existing facility capacities. The strategy will be fully developed after receiving additional guidance. The proposed budget limitations are too severe to allow ORNL to meet regulatory requirements or continue operations long term
HI in the Outskirts of Nearby Galaxies
The HI in disk galaxies frequently extends beyond the optical image, and can
trace the dark matter there. I briefly highlight the history of high spatial
resolution HI imaging, the contribution it made to the dark matter problem, and
the current tension between several dynamical methods to break the disk-halo
degeneracy. I then turn to the flaring problem, which could in principle probe
the shape of the dark halo. Instead, however, a lot of attention is now devoted
to understanding the role of gas accretion via galactic fountains. The current
cold dark matter theory has problems on galactic scales, such as
the core-cusp problem, which can be addressed with HI observations of dwarf
galaxies. For a similar range in rotation velocities, galaxies of type Sd have
thin disks, while those of type Im are much thicker. After a few comments on
modified Newtonian dynamics and on irregular galaxies, I close with statistics
on the HI extent of galaxies.Comment: 38 pages, 17 figures, invited review, book chapter in "Outskirts of
Galaxies", Eds. J. H. Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and
Space Science Library, Springer, in pres
Morphologies of AGN host galaxies using HST/ACS in the CDFS-GOODS field
Using HST/ACS images in four bands F435W, F606W, F775W and F850LP, we
identify optical counterparts to the X-ray sources in the Chandra Deep Field
South in the GOODS South field. A detailed study has been made of these sources
to study their morphological types. We use methods like decomposition of galaxy
luminosity profiles, color maps and visual inspection of 192 galaxies which are
identified as possible optical counterparts of Chandra X-ray sources in the
CDFS-GOODS field. We find that most moderate luminosity AGN hosts are bulge
dominated in the redshift range (z \approx 0.4-1.3), but not
merging/interacting galaxies. This implies probable fueling of the moderate
luminosity AGN by mechanisms other than those merger driven.Comment: pdflatex, accepted in ApSS. revisions in tex
Micro-Hall Magnetometry Studies of Thermally Assisted and Pure Quantum Tunneling in Single Molecule Magnet Mn12-Acetate
We have studied the crossover between thermally assisted and pure quantum
tunneling in single crystals of high spin (S=10) uniaxial single molecule
magnet Mn12-acetate using micro-Hall effect magnetometry. Magnetic hysteresis
experiments have been used toinvestigate the energy levels that determine the
magnetization reversal as a function of magnetic field and temperature. These
experiments demonstrate that the crossover occurs in a narrow (~0.1 K) or broad
(~1 K) temperature interval depending on the magnitude and direction of the
applied field. For low external fields applied parallel to the easy axis, the
energy levels that dominate the tunneling shift abruptly with temperature. In
the presence of a transverse field and/or large longitudinal field these energy
levels change with temperature more gradually. A comparison of our experimental
results with model calculations of this crossover suggest that there are
additional mechanisms that enhance the tunneling rate of low lying energy
levels and broaden the crossover for small transverse fields.Comment: 5 pages, 5 figure
Thermal treatment for radioactive waste minimisation
Safe management of radioactive waste is challenging to waste producers and waste management organisations. Deployment of thermal treatment technologies can provide significant improvements: volume reduction, waste passivation, organics destruction, safety demonstration facilitation, etc. The EC-funded THERAMIN project enables an EU-wide strategic review and assessment of the value of thermal treatment technologies applicable to Low and Intermediate Level waste streams (ion exchange media, soft operational waste, sludges, organic waste, and liquids). THERAMIN compiles an EU-wide database of wastes, which could be treated by thermal technologies and documents available thermal technologies. Applicability and benefits of technologies to the identified waste streams will be evaluated through full-scale demonstration tests by project partners. Safety case implications will also be assessed through the study of the disposability of thermally treated waste products. This paper will communicate the strategic aims of the ongoing project and highlight some key findings and results achieved to date
Anisotropic distribution functions for spherical galaxies
A method is presented for finding anisotropic distribution functions for
stellar systems with known, spherically symmetric, densities, which depends
only on the two classical integrals of the energy and the magnitude of the
angular momentum. It requires the density to be expressed as a sum of products
of functions of the potential and of the radial coordinate. The solution
corresponding to this type of density is in turn a sum of products of functions
of the energy and of the magnitude of the angular momentum. The products of the
density and its radial and transverse velocity dispersions can be also
expressed as a sum of products of functions of the potential and of the radial
coordinate. Several examples are given, including some of new anisotropic
distribution functions. This device can be extended further to the related
problem of finding two-integral distribution functions for axisymmetric
galaxies.Comment: 5 figure
Quantum Communication in Rindler Spacetime
A state that an inertial observer in Minkowski space perceives to be the
vacuum will appear to an accelerating observer to be a thermal bath of
radiation. We study the impact of this Davies-Fulling-Unruh noise on
communication, particularly quantum communication from an inertial sender to an
accelerating observer and private communication between two inertial observers
in the presence of an accelerating eavesdropper. In both cases, we establish
compact, tractable formulas for the associated communication capacities
assuming encodings that allow a single excitation in one of a fixed number of
modes per use of the communications channel. Our contributions include a
rigorous presentation of the general theory of the private quantum capacity as
well as a detailed analysis of the structure of these channels, including their
group-theoretic properties and a proof that they are conjugate degradable.
Connections between the Unruh channel and optical amplifiers are also
discussed.Comment: v3: 44 pages, accepted in Communications in Mathematical Physic
Generalized Einstein Theory on Solar and Galactic Scales
We study a generalized Einstein theory with the following two criteria:{\it
i}) on the solar scale, it must be consistent with the classical tests of
general relativity, {\it ii}) on the galactic scale, the gravitational
potential is a sum of Newtonian and Yukawa potentials so that it may explain
the flat rotation curves of spiral galaxies. Under these criteria, we find that
such a generalized Einstein action must include at least one scalar field and
one vector field as well as the quadratic term of the scalar curvature.Comment: 13 pages, Latex, SLAC-PUB-596
The 3-Band Hubbard-Model versus the 1-Band Model for the high-Tc Cuprates: Pairing Dynamics, Superconductivity and the Ground-State Phase Diagram
One central challenge in high- superconductivity (SC) is to derive a
detailed understanding for the specific role of the - and
- orbital degrees of freedom. In most theoretical studies an
effective one-band Hubbard (1BH) or t-J model has been used. Here, the physics
is that of doping into a Mott-insulator, whereas the actual high- cuprates
are doped charge-transfer insulators. To shed light on the related question,
where the material-dependent physics enters, we compare the competing magnetic
and superconducting phases in the ground state, the single- and two-particle
excitations and, in particular, the pairing interaction and its dynamics in the
three-band Hubbard (3BH) and 1BH-models. Using a cluster embedding scheme, i.e.
the variational cluster approach (VCA), we find which frequencies are relevant
for pairing in the two models as a function of interaction strength and doping:
in the 3BH-models the interaction in the low- to optimal-doping regime is
dominated by retarded pairing due to low-energy spin fluctuations with
surprisingly little influence of inter-band (p-d charge) fluctuations. On the
other hand, in the 1BH-model, in addition a part comes from "high-energy"
excited states (Hubbard band), which may be identified with a non-retarded
contribution. We find these differences between a charge-transfer and a Mott
insulator to be renormalized away for the ground-state phase diagram of the
3BH- and 1BH-models, which are in close overall agreement, i.e. are
"universal". On the other hand, we expect the differences - and thus, the
material dependence to show up in the "non-universal" finite-T phase diagram
(-values).Comment: 17 pages, 9 figure
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