7,007 research outputs found
N-body Simulations of the Small Magellanic Cloud and the Magellanic Stream
An extensive set of N-body simulations has been carried out on the
gravitational interaction of the Small Magellanic Cloud (SMC) with the Galaxy
and the Large Magellanic Cloud (LMC). The SMC is assumed to have been a barred
galaxy with a disc-to-halo mass ratio of unity before interaction and modelled
by a large number of self-gravitating particles, whereas the Galaxy and LMC
have been represented by rigid spherical potentials. The best model we have
found succeeded in reproducing the Magellanic Stream (MS), as well as a leading
counterpart to the Magellanic Stream (the leading arm), on the opposite side of
the Magellanic Clouds to the Stream, which mimicks the overall distribution of
several neutral hydrogen clumps observed in the corresponding region of the
sky. The elongation of the SMC bar along the line-of-sight direction suggested
by Cepheid observations has been partially reproduced, alongside its projected
appearance on the sky. The model successfully explains some major trends in the
kinematics of young populations in the SMC bar and older populations in the
`halo' of the SMC, as well as the overall velocity pattern for the gas, young
stars, and carbon stars in the inter-Cloud region.Comment: 26 pages plain LaTeX. 1 hardcopy table and 12 hardcopy figures
available on request from [email protected]
Reversibility of Red blood Cell deformation
The ability of cells to undergo reversible shape changes is often crucial to
their survival. For Red Blood Cells (RBCs), irreversible alteration of the cell
shape and flexibility often causes anemia. Here we show theoretically that RBCs
may react irreversibly to mechanical perturbations because of tensile stress in
their cytoskeleton. The transient polymerization of protein fibers inside the
cell seen in sickle cell anemia or a transient external force can trigger the
formation of a cytoskeleton-free membrane protrusion of micrometer dimensions.
The complex relaxation kinetics of the cell shape is shown to be responsible
for selecting the final state once the perturbation is removed, thereby
controlling the reversibility of the deformation. In some case, tubular
protrusion are expected to relax via a peculiar "pearling instability".Comment: 4 pages, 3 figure
Dynamic Modes of Microcapsules in Steady Shear Flow: Effects of Bending and Shear Elasticities
The dynamics of microcapsules in steady shear flow was studied using a
theoretical approach based on three variables: The Taylor deformation parameter
, the inclination angle , and the phase angle of
the membrane rotation. It is found that the dynamic phase diagram shows a
remarkable change with an increase in the ratio of the membrane shear and
bending elasticities. A fluid vesicle (no shear elasticity) exhibits three
dynamic modes: (i) Tank-treading (TT) at low viscosity of
internal fluid ( and relaxes to constant values), (ii)
Tumbling (TB) at high ( rotates), and (iii) Swinging
(SW) at middle and high shear rate (
oscillates). All of three modes are accompanied by a membrane ()
rotation. For microcapsules with low shear elasticity, the TB phase with no
rotation and the coexistence phase of SW and TB motions are induced by
the energy barrier of rotation. Synchronization of rotation with
TB rotation or SW oscillation occurs with integer ratios of rotational
frequencies. At high shear elasticity, where a saddle point in the energy
potential disappears, intermediate phases vanish, and either or
rotation occurs. This phase behavior agrees with recent simulation results of
microcapsules with low bending elasticity.Comment: 11 pages, 14 figure
Rubidium and lead abundances in giant stars of the globular clusters M 13 and NGC 6752
We present measurements of the neutron-capture elements Rb and Pb in five
giant stars of the globular cluster NGC 6752 and Pb measurements in four giants
of the globular cluster M 13. The abundances were derived by comparing
synthetic spectra with high resolution, high signal-to-noise ratio spectra
obtained using HDS on the Subaru telescope and MIKE on the Magellan telescope.
The program stars span the range of the O-Al abundance variation. In NGC 6752,
the mean abundances are [Rb/Fe] = -0.17 +/- 0.06 (sigma = 0.14), [Rb/Zr] =
-0.12 +/- 0.06 (sigma = 0.13), and [Pb/Fe] = -0.17 +/- 0.04 (sigma = 0.08). In
M 13 the mean abundance is [Pb/Fe] = -0.28 +/- 0.03 (sigma = 0.06). Within the
measurement uncertainties, we find no evidence for a star-to-star variation for
either Rb or Pb within these clusters. None of the abundance ratios [Rb/Fe],
[Rb/Zr], or [Pb/Fe] are correlated with the Al abundance. NGC 6752 may have
slightly lower abundances of [Rb/Fe] and [Rb/Zr] compared to the small sample
of field stars at the same metallicity. For M 13 and NGC 6752 the Pb abundances
are in accord with predictions from a Galactic chemical evolution model. If
metal-poor intermediate-mass asymptotic giant branch stars did produce the
globular cluster abundance anomalies, then such stars do not synthesize
significant quantities of Rb or Pb. Alternatively, if such stars do synthesize
large amounts of Rb or Pb, then they are not responsible for the abundance
anomalies seen in globular clusters.Comment: Accepted for publication in Ap
Dynamical regimes and hydrodynamic lift of viscous vesicles under shear
The dynamics of two-dimensional viscous vesicles in shear flow, with
different fluid viscosities and inside and
outside, respectively, is studied using mesoscale simulation techniques.
Besides the well-known tank-treading and tumbling motions, an oscillatory
swinging motion is observed in the simulations for large shear rate. The
existence of this swinging motion requires the excitation of higher-order
undulation modes (beyond elliptical deformations) in two dimensions.
Keller-Skalak theory is extended to deformable two-dimensional vesicles, such
that a dynamical phase diagram can be predicted for the reduced shear rate and
the viscosity contrast . The simulation results
are found to be in good agreement with the theoretical predictions, when
thermal fluctuations are incorporated in the theory. Moreover, the hydrodynamic
lift force, acting on vesicles under shear close to a wall, is determined from
simulations for various viscosity contrasts. For comparison, the lift force is
calculated numerically in the absence of thermal fluctuations using the
boundary-integral method for equal inside and outside viscosities. Both methods
show that the dependence of the lift force on the distance of
the vesicle center of mass from the wall is well described by an effective
power law for intermediate distances with vesicle radius .
The boundary-integral calculation indicates that the lift force decays
asymptotically as far from the wall.Comment: 13 pages, 13 figure
Exploring Millions of 6-State FSSP Solutions: the Formal Notion of Local CA Simulation
In this paper, we come back on the notion of local simulation allowing to
transform a cellular automaton into a closely related one with different local
encoding of information. This notion is used to explore solutions of the Firing
Squad Synchronization Problem that are minimal both in time (2n -- 2 for n
cells) and, up to current knowledge, also in states (6 states). While only one
such solution was proposed by Mazoyer since 1987, 718 new solutions have been
generated by Clergue, Verel and Formenti in 2018 with a cluster of machines. We
show here that, starting from existing solutions, it is possible to generate
millions of such solutions using local simulations using a single common
personal computer
K band Spectroscopy of Ultraluminous Infrared Galaxies: The 2 Jy Sample
We present near-infrared spectroscopy for a complete sample of 33
ultraluminous infrared galaxies at a resolution of R\approx 1000. Most of the
wavelength range from 1.80-2.20 microns in the rest frame is covered, including
the Pa-alpha and Br-gamma hydrogen recombination lines, and the molecular
hydrogen vibration-rotation 1-0 S(1) and S(3) lines. Other species, such as He
I, [Fe II], and [Si VI] appear in the spectra as well, in addition to a number
of weaker molecular hydrogen lines. Nuclear extractions for each of the
individual galaxies are presented here, along with spectra of secondary nuclei,
where available. The Pa-alpha emission is seen to be highly concentrated on the
nuclei, typically with very little emision extending beyond a radius of 1 kpc.
Signatures of active nuclei are rare in the present sample, occurring in only
two of the 33 galaxies. It is found that visual extinctions to the nuclei via
the Pa-alpha/Br-gamma line ratio in excess of 10 magnitudes are relatively
common among ULIRGs, and that visual extinctions greater than 25 mag are
necessary to conceal a QSO emitting half the total bolometric luminosity. The
vibration-rotation lines of molecular hydrogen appear to be predominantly
thermal in origin, with effective temperatures generally around 2200 K. The
relative nuclear velocities between double nucleus ULIRGs are investigated,
through which it is inferred that the maximum deprojected velocity difference
is about 200 km/s. This figure is lower than the velocities predicted by
physical models of strong interactions/mergers of large, gas-rich galaxies.Comment: 52 pages (19 with just figures), 9 figures, accepted for publication
in the Astronomical Journal; Table 3 not formatted properly on astro-ph: get
source and print Murphy.tab3.p
Transport coefficients of off-lattice mesoscale-hydrodynamics simulation techniques
The viscosity and self-diffusion constant of particle-based mesoscale
hydrodynamic methods, multi-particle collision dynamics (MPC) and dissipative
particle dynamics (DPD), are investigated, both with and without
angular-momentum conservation. Analytical results are derived for fluids with
an ideal-gas equation of state and a finite-time-step dynamics, and compared
with simulation data. In particular, the viscosity is derived in a general form
for all variants of the MPC method. In general, very good agreement between
theory and simulations is obtained.Comment: 12 pages, 9 figure
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