2,837 research outputs found
Quasi-stationary states and the range of pair interactions
"Quasi-stationary" states are approximately time-independent out of
equilibrium states which have been observed in a variety of systems of
particles interacting by long-range interactions. We investigate here the
conditions of their occurrence for a generic pair interaction V(r \rightarrow
\infty) \sim 1/r^a with a > 0, in d>1 dimensions. We generalize analytic
calculations known for gravity in d=3 to determine the scaling parametric
dependences of their relaxation rates due to two body collisions, and report
extensive numerical simulations testing their validity. Our results lead to the
conclusion that, for a < d-1, the existence of quasi-stationary states is
ensured by the large distance behavior of the interaction alone, while for a >
d-1 it is conditioned on the short distance properties of the interaction,
requiring the presence of a sufficiently large soft-core in the interaction
potential.Comment: 5 pages, 3 figures; final version to appear in Phys. Rev. Let
Is the dark matter halo of the Milky Way flattened?
We performed an extended analysis of the parameter space for the interaction
of the Magellanic System with the Milky Way (MW). The varied parameters cover
the phase space parameters, the masses, the structure, and the orientation of
both Magellanic Clouds, as well as the flattening of the dark matter halo of
the MW. The analysis was done by a specially adopted optimization code
searching for the best match between numerical models and the detailed HI map
of the Magellanic System by Bruens et al. (2005). The applied search algorithm
is a genetic algorithm combined with a code based on the fast, but
approximative restricted N-body method. By this, we were able to analyze more
than 10^6 models, which makes this study one of the most extended ones for the
Magellanic System. Here we focus on the flattening q of the axially symmetric
MW dark matter halo potential, that is studied within the range 0.74<=q<=1.20.
We show that creation of a trailing tail (Magellanic Stream) and a leading
stream (Leading Arm) is quite a common feature of the Magellanic System-MW
interaction, and such structures were modeled across the entire range of halo
flattening values. However, important differences exist between the models,
concerning density distribution and kinematics of HI, and also the dynamical
evolution of the Magellanic System. Detailed analysis of the overall agreement
between modeled and observed distribution of neutral hydrogen shows that the
models assuming an oblate (q<1.0) dark matter halo of the Galaxy allow for
better satisfaction of HI observations than models with other halo
configurations.Comment: 19 pages, 20 figures, 2 appendices, accepted for publication in A&
Measuring dark matter by modeling interacting galaxies
The dark matter content of galaxies is usually determined from galaxies in
dynamical equilibrium, mainly from rotationally supported galactic components.
Such determinations restrict measurements to special regions in galaxies, e.g.
the galactic plane(s), whereas other regions are not probed at all. Interacting
galaxies offer an alternative, because extended tidal tails often probe outer
or off-plane regions of galaxies. However, these systems are neither in
dynamical equilibrium nor simple, because they are composed of two or more
galaxies, by this increasing the associated parameter space.We present our
genetic algorithm based modeling tool which allows to investigate the extended
parameter space of interacting galaxies. From these studies, we derive the
dynamical history of (well observed) galaxies. Among other parameters we
constrain the dark matter content of the involved galaxies. We demonstrate the
applicability of this strategy with examples ranging from stellar streams
around theMilkyWay to extended tidal tails, from proto-typical binary galaxies
(like M51 or the Antennae system) to small group of galaxies.Comment: 4 pages, 3 figures, Conf.: Hunting for the dark, Malta 200
Arthritis Impact on Employment Participation among U.S. Adults: A Population-based Perspective
Background: Arthritis affects 53 million U.S. adults, more than two-thirds of whom are younger than age 65. Approximately 1/3 of working-age (18-64 years) U.S adults with arthritis report arthritis-attributable work limitation.
Objectives: First, to take a population-based perspective to evaluate the association of arthritis with employment participation among U.S. adults. Next, to examine whether this association differs by sex, age, or other characteristics. Finally, to investigate effects of the Great Recession (December 2007 to June 2009) on employment and to determine if arthritis status moderated its effects.
Methods: All three studies were conducted using the National Health Interview Survey (NHIS). The third study also used longitudinal data from the Medical Expenditures Panel Survey (MEPS) linked to NHIS.
Results: These manuscripts are under peer-review for publication; limited results are presented:
Study 1- Employment participation was always statistically significantly and substantially lower (e.g., \u3e10 percentage points) among adults with arthritis compared with those without arthritis.
Study 2- Overall, 20.1 million adults (10.4% [95% CI=10.1-10.8] of the working-age population) reported work disability.
Study 3- During the period of the Great Recession, people with arthritis stopped work at higher rates and started work at lower rates than those without arthritis, suggesting at least some differential effect among those with arthritis.
Conclusion: This work contributes new knowledge by establishing long-term patterns and benchmark information for employment participation, work disability, transitions, and macro economic effects among adults with and without arthritis in the U.S. A population-based, non-condition-specific approach of this type has not been previously reported
Physical Processes in Star-Gas Systems
First we present a recently developed 3D chemodynamical code for galaxy
evolution from the K**2 collaboration. It follows the evolution of all
components of a galaxy such as dark matter, stars, molecular clouds and diffuse
interstellar matter (ISM). Dark matter and stars are treated as collisionless
N-body systems. The ISM is numerically described by a smoothed particle
hydrodynamics (SPH) approach for the diffuse (hot) gas and a sticky particle
scheme for the (cool) molecular clouds. Physical processs such as star
formation, stellar death or condensation and evaporation processes of clouds
interacting with the ISM are described locally. An example application of the
model to a star forming dwarf galaxy will be shown for comparison with other
codes. Secondly we will discuss new kinds of exotic chemodynamical processes,
as they occur in dense gas-star systems in galactic nuclei, such as
non-standard ``drag''-force interactions, destructive and gas producing stellar
collisions. Their implementation in 1D dynamical models of galactic nuclei is
presented. Future prospects to generalize these to 3D are work in progress and
will be discussed.Comment: 4 pages, 4 figures, "The 5th Workshop on Galactic Chemodynamics" -
Swinburne University (9-11 July 2003). To be published in the Publications of
the Astronomical Society of Australia in 2004 (B.K. Gibson and D. Kawata,
eds.). Accepted version, minor changes relative to origina
Impact of dark matter subhalos on extended HI disks of galaxies: Possible formation of HI fine structures and stars
Recent observations have discovered star formation activities in the extreme
outer regions of disk galaxies. However it remains unclear what physical
mechanisms are responsible for triggering star formation in such low-density
gaseous environments of galaxies. In order to understand the origin of these
outer star-forming regions, we numerically investigate how the impact of dark
matter subhalos orbiting a gas-rich disk galaxy embedded in a massive dark
matter halo influences the dynamical evolution of outer HI gas disk of the
galaxy. We find that if the masses of the subhalos () in a galaxy
with an extended HI gas disk are as large as , where
is the total mass of the galaxy's dark halo, local fine structures
can be formed in the extended HI disk. We also find that the gas densities of
some apparently filamentary structures can exceed a threshold gas density for
star formation and thus be likely to be converted into new stars in the outer
part of the HI disk in some models with larger . These results thus
imply that the impact of dark matter subhalos (``dark impact'') can be
important for better understanding the origin of recent star formation
discovered in the extreme outer regions of disk galaxies. We also suggest that
characteristic morphologies of local gaseous structures formed by the dark
impact can indirectly prove the existence of dark matter subhalos in galaxies.
We discuss the origin of giant HI holes observed in some gas-rich galaxies
(e.g., NGC 6822) in the context of the dark impact.Comment: 8 pages, 4 figures, accepted by ApJ
Hypermultiplets and Topological Strings
The c-map relates classical hypermultiplet moduli spaces in compactifications
of type II strings on a Calabi-Yau threefold to vector multiplet moduli spaces
via a further compactification on a circle. We give an off-shell description of
the c-map in N=2 superspace. The superspace Lagrangian for the hypermultiplets
is a single function directly related to the prepotential of special geometry,
and can therefore be computed using topological string theory. Similarly, a
class of higher derivative terms for hypermultiplets can be computed from the
higher genus topological string amplitudes. Our results provide a framework for
studying quantum corrections to the hypermultiplet moduli space, as well as for
understanding the black hole wave-function as a function of the hypermultiplet
moduli.Comment: 21 pages, references adde
- âŠ