13,032 research outputs found
Plectoneme creation reduces the rotational friction of a polymer
The torsional dynamics of a semiflexible polymer with a contour length
larger than its persistence length L_p that is rotated at fixed frequency
omega_0 at one end is studied by scaling arguments and hydrodynamic
simulations. We find a non-equilibrium transition at a critical frequency
omega_*: In the linear regime, omega_0 < omega_*, axial spinning is the
dominant dissipation mode. In the non-linear regime, omega_0 > omega_*, the
twist-dissipation mode involves the continuous creation of plectonemes close to
the driven end and the rotational friction is substantially reduced
Nuclear Bar, Star Formation and Gas Fueling in the Active Galaxy NGC 4303
A combination of Hubble Space Telescope (HST) WFPC2 and NICMOS images are
used to investigate the gas/dust and stellar structure inside the central 300
pc of the nearby active galaxy NGC 4303.
The NICMOS H-band (F160W) image reveals a bright core and a nuclear elongated
bar-like structure of 250 pc in diameter. The bar is centered on the bright
core, and its major axis is oriented in proyection along the spin axis of the
nuclear gaseous rotating disk recently detected (Colina & Arribas 1999).
The V-H (F606W - F160W) image reveals a complex gas/dust distribution with a
two-arm spiral structure of about 225 pc in radius. The southwestern arm is
traced by young star-forming knots while the northeastern arm is detected by
the presence of dust lanes. These spirals do not have a smooth structure but
rather they are made of smaller flocculent spirals or filament-like structures.
The magnitudes and colors of the star-forming knots are typical of clusters of
young stars with masses of 0.5 to 1 x yr^{-1} for about 80 Myr.Comment: ApJ, in press (February 1, 2000
Discrete elastic model for stretching-induced flagellar polymorphs
Force-induced reversible transformations between coiled and normal polymorphs
of bacterial flagella have been observed in recent optical-tweezer experiment.
We introduce a discrete elastic rod model with two competing helical states
governed by a fluctuating spin-like variable that represents the underlying
conformational states of flagellin monomers. Using hybrid Brownian dynamics
Monte-Carlo simulations, we show that a helix undergoes shape transitions
dominated by domain wall nucleation and motion in response to externally
applied uniaxial tension. A scaling argument for the critical force is
presented in good agreement with experimental and simulation results.
Stretching rate-dependent elasticity including a buckling instability are
found, also consistent with the experiment
Non-equilibrium hydrodynamics of a rotating filament
The nonlinear dynamics of an elastic filament that is forced to rotate at its
base is studied by hydrodynamic simulation techniques; coupling between
stretch, bend, twist elasticity and thermal fluctuations is included. The
twirling-overwhirling transition is located and found to be strongly
discontinuous. For finite bend and twist persistence length, thermal
fluctuations lower the threshold rotational frequency, for infinite persistence
length the threshold agrees with previous analytical predictions
On the Interpretation of the l-v Features in the Milky Way Galaxy
We model the gas dynamics of barred galaxies using a three-dimensional,
high-resolution, -body+hydrodynamical simulation and apply it to the Milky
Way in an attempt to reproduce both the large-scale structure and the clumpy
morphology observed in Galactic H\emissiontype{I} and CO diagrams. Owing
to including the multi-phase interstellar medium, self-gravity, star-formation
and supernovae feedback, the clumpy morphology, as well as the large-scale
features, in observed diagrams are naturally reproduced. We identify in
our diagrams with a number of not only large-scale peculiar features such
as the '3-kpc arm', '135-km s arm' and 'Connecting arm' but also clumpy
features such as `Bania clumps', and then link these features in a face-on view
of our model. We give suggestions on the real structure of the Milky Way and on
the fate of gas clumps in the central region.Comment: accepted to PAS
Dynamical Susceptibility in KH2PO4-type Crystals above and below Tc
The time dependent cluster approximation called the path probability method
(PPM) is applied to a pseudo-spin Ising Hamiltonian of the Slater-Takagi model
for KH2PO4-type hydrogen-bonded ferroelectrics in order to calculate the
homogeneous dynamical susceptibility above and below the ferroelectric
transition temperature. Above the transition temperature all the calculations
are carried out analytically in the cactus approximation of the PPM. Below the
transition temperature the dynamical susceptibility is also calculated
accurately since the analytical solution of spontaneous polarization in the
ferroelectric phase can be utilized. When the temperature is approached from
both sides of the transition temperature, only one of relaxation times shows a
critical slowing down and makes a main contribution to the dynamical
susceptibility. The discrepancy from Slater model (ice-rule limit) is discussed
in comparison with some experimental data.Comment: 8 pages, 10 figure
Effects of a Supermassive Black Hole Binary on a Nuclear Gas Disk
We study influence of a galactic central supermassive black hole (SMBH)
binary on gas dynamics and star formation activity in a nuclear gas disk by
making three-dimensional Tree+SPH simulations. Due to orbital motions of SMBHs,
there are various resonances between gas motion and the SMBH binary motion. We
have shown that these resonances create some characteristic structures of gas
in the nuclear gas disk, for examples, gas elongated or filament structures,
formation of gaseous spiral arms, and small gas disks around SMBHs. In these
gaseous dense regions, active star formations are induced. As the result, many
star burst regions are formed in the nuclear region.Comment: 19 pages, 11 figures, accepted for publication in Ap
Obscuring Material around Seyfert Nuclei with Starbursts
The structure of obscuring matter in the environment of active galactic
nuclei with associated nuclear starbursts is investigated using 3-D
hydrodynamical simulations. Simple analytical estimates suggest that the
obscuring matter with energy feedback from supernovae has a torus-like
structure with a radius of several tens of parsecs and a scale height of about
10 pc. These estimates are confirmed by the fully non-linear numerical
simulations, in which the multi-phase inhomogeneous interstellar matter and its
interaction with the supernovae are consistently followed. The globally stable,
torus-like structure is highly inhomogeneous and turbulent. To achieve the high
column densities (> 10^{24} cm^{-2}) as suggested by observations of some
Seyfert 2 galaxies with nuclear starbursts, the viewing angle should be larger
than about 70 degree from the pole-on for a 10^8 solar mass massive black hole.
Due to the inhomogeneous internal structure of the torus, the observed column
density is sensitive to the line-of-sight, and it fluctuates by a factor of
order 100. The covering fraction for N > 10^{23} cm^{-2} is about 0.4. The
average accretion rate toward R < 1 pc is 0.4 solar mass/yr, which is boosted
to twice that in the model without the energy feedback.Comment: ApJL in press (4 pages, 3 figures) A gziped ps file with high
resolution figures is available at http://th.nao.ac.jp/~wada/AGN
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