91 research outputs found
Promoter prediction using physico-chemical properties of DNA
The ability to locate promoters within a section of DNA is known to be a very difficult and very important task in DNA analysis. We document an approach that incorporates the concept of DNA as a complex molecule using several models of its physico-chemical properties. A support vector machine is trained to recognise promoters by their distinctive physical and chemical properties. We demonstrate that by combining models, we can improve upon the classification accuracy obtained with a single model. We also show that by examining how the predictive accuracy of these properties varies over the promoter, we can reduce the number of attributes needed. Finally, we apply this method to a real-world problem. The results demonstrate that such an approach has significant merit in its own right. Furthermore, they suggest better results from a planned combined approach to promoter prediction using both physicochemical and sequence based techniques
Magnetohydrodynamical Accretion Flows: Formation of Magnetic Tower Jet and Subsequent Quasi-Steady State
We present three-dimensional (3-D) magnetohydrodynamical (MHD) simulations of
radiatively inefficient accretion flow around black holes. General relativistic
effects are simulated by using the pseudo-Newtonian potential. We start
calculations with a rotating torus threaded by localized poloidal magnetic
fields with plasma beta, a ratio of the gas pressure to the magnetic pressure,
and 100. When the bulk of torus material reaches the innermost
region close to a central black hole, a magnetically driven jet emerges. This
magnetic jet is derived by vertically inflating toroidal fields (`magnetic
tower') and has a two-component structure: low- (\lsim 1) plasmas
threaded with poloidal (vertical) fields are surrounded by that with toroidal
fields. The collimation width of the jet depends on external pressure, pressure
of ambient medium; the weaker the external pressure is, the wider and the
longer-lasting becomes the jet. Unless the external pressure is negligible, the
bipolar jet phase ceases after several dynamical timescales at the original
torus position and a subsequent quasi-steady state starts. The black hole is
surrounded by quasi-spherical zone with highly inhomogeneous structure in which
toroidal fields are dominant except near the rotation axis. Mass accretion
takes place mainly along the equatorial plane. Comparisons with other MHD
simulation results and observational implications are discussed.Comment: Accepted for publication in ApJ. 15 pages, 3 tables, 11 figures. A
paper with high-resolution figures and movies available at
http://www2.yukawa.kyoto-u.ac.jp/~ykato/research
New Panoramic View of CO and 1.1 mm Continuum Emission in the Orion A Molecular Cloud. I. Survey Overview and Possible External Triggers of Star Formation
We present new, wide and deep images in the 1.1 mm continuum and the
CO (=1-0) emission toward the northern part of the Orion A Giant
Molecular Cloud (Orion-A GMC). The 1.1 mm data were taken with the AzTEC camera
mounted on the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope
in Chile, and the CO (=1-0) data were with the 25 beam receiver
(BEARS) on the NRO 45 m telescope in the On-The-Fly (OTF) mode. The present
AzTEC observations are the widest (\timeform{1.D7}
\timeform{2.D3}, corresponding to 12 pc 17 pc) and the
highest-sensitivity (9 mJy beam) 1.1 mm dust-continuum imaging of
the Orion-A GMC with an effective spatial resolution of 40\arcsec. The
CO (=1-0) image was taken over the northern \timeform{1D.2}
\times\timeform{1D.2} (corresponding 9 pc 9 pc) area with a
sensitivity of 0.93 K in , a velocity resolution of 1.0 km
s, and an effective spatial resolution of 21\arcsec. With these data,
together with the MSX 8 m, Spitzer 24 m and the 2MASS data, we have
investigated the detailed structure and kinematics of molecular gas associated
with the Orion-A GMC and have found evidence for interactions between molecular
clouds and the external forces that may trigger star formation. Two types of
possible triggers were revealed; 1) Collision of the diffuse gas on the cloud
surface, particularly at the eastern side of the OMC-2/3 region, and 2)
Irradiation of UV on the pre-existing filaments and dense molecular cloud
cores. Our wide-field and high-sensitivity imaging have provided the first
comprehensive view of the potential sites of triggered star formation in the
Orion-A GMC.Comment: 32 pages, 20 figures, accepted for publication in PAS
Mass Outflow Rate From Accretion Discs around Compact Objects
We compute mass outflow rates from accretion disks around compact objects,
such as neutron stars and black holes. These computations are done using
combinations of exact transonic inflow and outflow solutions which may or may
not form standing shock waves. Assuming that the bulk of the outflow is from
the effective boundary layers of these objects, we find that the ratio of the
outflow rate and inflow rate varies anywhere from a few percent to even close
to a hundred percent (i.e., close to disk evacuation case) depending on the
initial parameters of the disk, the degree of compression of matter near the
centrifugal barrier, and the polytropic index of the flow. Our result, in
general, matches with the outflow rates obtained through a fully time-dependent
numerical simulation. In some region of the parameter space when the standing
shock does not form, our results indicate that the disk may be evacuated and
may produce quiescence states.Comment: 30 Latex pages and 13 figures. crckapb.sty; Published in Class.
Quantum Grav. Vol. 16. No. 12. Pg. 387
Compton Scattering in Static and Moving Media. II. System-Frame Solutions for Spherically Symmetric Flows
I study the formation of Comptonization spectra in spherically symmetric,
fast moving media in a flat spacetime. I analyze the mathematical character of
the moments of the transfer equation in the system-frame and describe a
numerical method that provides fast solutions of the time-independent radiative
transfer problem that are accurate in both the diffusion and free-streaming
regimes. I show that even if the flows are mildly relativistic (V~0.1, where V
is the electron bulk velocity in units of the speed of light), terms that are
second-order in V alter the emerging spectrum both quantitatively and
qualitatively. In particular, terms that are second-order in V produce
power-law spectral tails, which are the dominant feature at high energies, and
therefore cannot be neglected. I further show that photons from a static source
are upscattered by the bulk motion of the medium even if the velocity field
does not converge. Finally, I discuss these results in the context of radial
accretion onto and outflows from compact objects.Comment: 28 pages, 9 figures; minor changes, to appear in the Astrophysical
Journa
Compton Scattering by Static and Moving Media I. The Transfer Equation and Its Moments
Compton scattering of photons by nonrelativistic particles is thought to play
an important role in forming the radiation spectrum of many astrophysical
systems. Here we derive the time-dependent photon kinetic equation that
describes spontaneous and induced Compton scattering as well as absorption and
emission by static and moving media, the corresponding radiative transfer
equation, and their zeroth and first moments, in both the system frame and in
the frame comoving with the medium. We show that it is necessary to use the
correct relativistic differential scattering cross section in order to obtain a
photon kinetic equation that is correct to first order in epsilon/m_e, T_e/m_e,
and V, where epsilon is the photon energy, T_e and m_e are the electron
temperature and rest mass, and V is the electron bulk velocity in units of the
speed of light. We also demonstrate that the terms in the radiative transfer
equation that are second-order in V usually should be retained, because if the
radiation energy density is sufficiently large compared to the radiation flux,
the effects of bulk Comptonization described by the terms that are second-order
in V are at least as important as the effects described by the terms that are
first-order in V, even when V is small. Our equations are valid for systems of
arbitrary optical depth and can therefore be used in both the free-streaming
and the diffusion regimes. We demonstrate that Comptonization by the electron
bulk motion occurs whether or not the radiation field is isotropic or the bulk
flow converges and that it is more important than thermal Comptonization if V^2
> 3 T_e/m_e.Comment: 31 pages, accepted for publication in The Astrophysical Journa
Mass-losing accretion discs around supermassive black holes
We study the effects of outflow/wind on the gravitational stability of
accretion discs around supermassive black holes using a set of analytical
steady-state solutions. Mass-loss rate by the outflow from the disc is assumed
to be a power-law of the radial distance and the amount of the energy and the
angular momentum which are carried away by the wind are parameterized
phenomenologically. We show that the mass of the first clumps at the
self-gravitating radius linearly decreases with the total mass-loss rate of the
outflow. Except for the case of small viscosity and high accretion rate,
generally, the self-gravitating radius increases as the amount of mass-loss by
the outflow increases. Our solutions show that as more angular momentum is lost
by the outflow, then reduction to the mass of the first clumps is more
significant.Comment: Accepted for publication in Astrophysics & Space Scienc
Self-Collimation and Magnetic Field Generation of Astrophysical Jets
A novel model for collimation and transport of electron-positron-ion jets is
presented. Analytical results show that the filamentary structures can be
sustained by self-induced toroidal magnetic fields permeating through the
filaments, whose widths significantly expand in the pair-dominant regimes. The
magnetic field strength reflects a characteristic of equipartition of excess
kinetic energy of the jets. It is also shown that growth of the hoselike
instability is strongly suppressed. Essential features derived from this model
are consistent with recent results observed by using very long baseline
telescopes.Comment: 10 pages including 2 eps figures, to be published in 10 April 2002,
ApJ
Kinematics of classical Cepheids in the Nuclear Stellar Disk
Classical Cepheids are useful tracers of the Galactic young stellar
population because their distances and ages can be determined from their
period-luminosity and period-age relations. In addition, the radial velocities
and chemical abundance of the Cepheids can be derived from spectroscopic
observations, providing further insights into the structure and evolution of
the Galaxy. Here, we report the radial velocities of classical Cepheids near
the Galactic Center, three of which were reported in 2011, the other reported
for the first time. The velocities of these Cepheids suggest that the stars
orbit within the Nuclear Stellar Disk, a group of stars and interstellar matter
occupying a region of 200 pc around the Center, although the three-dimensional
velocities cannot be determined until the proper motions are known. According
to our simulation, these four Cepheids formed within the Nuclear Stellar Disk
like younger stars and stellar clusters therein.Comment: Accepted for publication in ApJ; 8 pages, 7 figures, 6 table
- …