680 research outputs found
Nonlinear Hydromagnetic Wave Support of a Stratified Molecular Cloud
We perform numerical simulations of nonlinear MHD waves in a gravitationally
stratified molecular cloud that is bounded by a hot and tenuous external
medium. We study the relation between the strength of the turbulence and
various global properties of a molecular cloud, within a 1.5-dimensional
approximation. Under the influence of a driving source of Alfvenic
disturbances, the cloud is lifted up by the pressure of MHD waves and reaches a
steady-state characterized by oscillations about a new time-averaged
equilibrium state. The nonlinear effect results in the generation of
longitudinal motions and many shock waves; however, the wave kinetic energy
remains predominantly in transverse, rather than longitudinal, motions. There
is an approximate equipartition of energy between the transverse velocity and
fluctuating magnetic field (aspredicted by small-amplitude theory) in the
region of the stratified cloud which contains most of the mass; however, this
relation breaks down in the outer regions, particularly near the cloud surface,
where the motions have a standing-wave character. This means that the
Chandrasekhar-Fermi formula applied to molecular clouds must be significantly
modified in such regions. Models of an ensemble of clouds show that, for
various strengths of the input energy, the velocity dispersion in the cloud
, where is a characteristic size of the
cloud.Furthermore, is always comparable to the mean Alfven velocity of
the cloud, consistent with observational results.Comment: 16 pages, 15 figures, emulateapj, to appear in ApJ, 2003 Oct 1,
higher resolution figures at http://www.astro.uwo.ca/~basu/pub.html or
http://www.astro.uwo.ca/~kudoh/pub.htm
Thermodynamics of four-dimensional black objects in the warped compactification
We reinvestigate the thermodynamics of black objects (holes and strings) in
four-dimensional braneworld models that are originally constructed by Emparan,
Horowitz and Myers based on the anti-de Sitter (AdS) C-metric. After proving
the uniqueness of slicing the AdS C-metric, we derive thermodynamic quantities
of the black objects by means of the Euclidean formulation and find that we
have no necessity of requiring any regularization to calculate their classical
action. We show that there exist the Bekenstein-Hawking law and the
thermodynamic first law. The thermodynamic mass of the localized black hole on
a flat brane is negative, and it differs from the one previously derived. We
discuss the thermodynamic stabilities and show that the BTZ black string is
more stable than the localized black holes in a canonical ensemble, except for
an extreme case. We also find a braneworld analogue of the Hawking-Page
transition between the BTZ black string and thermal AdS branes. The localized
black holes on a de Sitter brane is discussed by considering Nariai instanton,
comparing the study of "black cigar" in the five-dimensional braneworld model.Comment: 15 pages, 4 figures, RevTex4, typos fixed, minor correction
Three-dimensional MHD Simulations of Jets from Accretion Disks
We report the results of 3-dimensional magnetohydrodynamic (MHD) simulations
of a jet formation by the interaction between an accretion disk and a large
scale magnetic field. The disk is not treated as a boundary condition but is
solved self-consistently. To investigate the stability of MHD jet, the
accretion disk is perturbed with a non-axisymmetric sinusoidal or random
fluctuation of the rotational velocity. The dependences of the jet velocity
, mass outflow rate , and mass accretion rate
on the initial magnetic field strength in both non-axisymmetric cases are
similar to those in the axisymmetric case. That is, ,
and where is the
initial magnetic field strength. The former two relations are consistent with
the Michel's steady solution, , although
the jet and accretion do not reach the steady state. In both perturbation
cases, a non-axisymmetric structure with appears in the jet, where
means the azimuthal wave number. This structure can not be explained by
Kelvin-Helmholtz instability and seems to originate in the accretion disk.
Non-axisymmetric modes in the jet reach almost constant levels after about 1.5
orbital periods of the accretion disk, while all modes in the accretion disk
grow with oscillation. As for the angular momentum transport by Maxwell stress,
the vertical component, , in the wide range of initial magnetic field
strength.Comment: Accepted for publication in ApJ. The pdf file with high resolution
figures can be downloaded at
http://www.kusastro.kyoto-u.ac.jp/~hiromitu/3j050806.pd
Six-dimensional localized black holes: numerical solutions
To test the strong-gravity regime in Randall-Sundrum braneworlds, we consider
black holes bound to a brane. In a previous paper, we studied numerical
solutions of localized black holes whose horizon radii are smaller than the AdS
curvature radius. In this paper, we improve the numerical method and discuss
properties of the six dimensional (6D) localized black holes whose horizon
radii are larger than the AdS curvature radius. At a horizon temperature
, the thermodynamics of the localized black
hole undergo a transition with its character changing from a 6D Schwarzschild
black hole type to a 6D black string type. The specific heat of the localized
black holes is negative, and the entropy is greater than or nearly equal to
that of the 6D black strings with the same thermodynamic mass. The large
localized black holes show flattened horizon geometries, and the intrinsic
curvature of the horizon four-geometry becomes negative near the brane. Our
results indicate that the recovery mechanism of lower-dimensional Einstein
gravity on the brane works even in the presence of the black holes.Comment: 17 pages, 9 figures, RevTeX4, typos correcte
Quadrupole formula for Kaluza-Klein modes in the braneworld
The quadrupole formula in four-dimensional Einstein gravity is a useful tool
to describe gravitational wave radiation. We derive the quadrupole formula for
the Kaluza-Klein (KK) modes in the Randall-Sundrum braneworld model. The
quadrupole formula provides transparent representation of the exterior weak
gravitational field induced by localized sources. We find that a general
isolated dynamical source gives rise to the 1/r^2 correction to the leading 1/r
gravitational field. We apply the formula to an evaluation of the effective
energy carried by the KK modes from the viewpoint of an observer on the brane.
Contrary to the ordinary gravitational waves (zero mode), the flux of the
induced KK modes by the non-spherical part of the quadrupole moment vanishes at
infinity and only the spherical part contributes to the flux. Since the effect
of the KK modes appears in the linear order of the metric perturbations, the
effective energy flux observed on the brane is not always positive, but can
become negative depending on the motion of the localized sources.Comment: 9 pages, no figures, REVTeX 4; version accepted for publication in
CQ
Remarkable morphological characteristics of Milnesium sp. from Inhovde, East Antarctica
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Formation of protostellar jets - effects of magnetic diffusion
We investigate the evolution of a disk wind into a collimated jet under the
influence of magnetic diffusivity, assuming that the turbulent pattern in the
disk will also enter the disk corona and the jet. Using the ZEUS-3D code in the
axisymmetry option we solve the time-dependent resistive MHD equations for a
model setup of a central star surrounded by an accretion disk. We find that the
diffusive jets propagate slower into the ambient medium. Close to the star we
find that a quasi stationary state evolves after several hundred (weak
diffusion) or thousand (strong diffusion) disk rotations. Magnetic diffusivity
affects the protostellar jet structure as follows. The jet poloidal magnetic
field becomes de-collimated. The jet velocity increases with increasing
diffusivity, while the degree of collimation for the hydrodynamic flow remains
more or less the same. We suggest that the mass flux is a proper tracer for the
degree of jet collimation and find indications of a critical value for the
magnetic diffusivity above which the jet collimation is only weak.Comment: 16 pages, 12 figs, accepted by Astron. and Astrop
Clathrate hydrates as a sink of noble gases in Titan's atmosphere
We use a statistical thermodynamic approach to determine the composition of
clathrate hydrates which may form from a multiple compound gas whose
composition is similar to that of Titan's atmosphere. Assuming that noble gases
are initially present in this gas phase, we calculate the ratios of xenon,
krypton and argon to species trapped in clathrate hydrates. We find that these
ratios calculated for xenon and krypton are several orders of magnitude higher
than in the coexisting gas at temperature and pressure conditions close to
those of Titan's present atmosphere at ground level. Furthermore we show that,
by contrast, argon is poorly trapped in these ices. This trapping mechanism
implies that the gas-phase is progressively depleted in xenon and krypton when
the coexisting clathrate hydrates form whereas the initial abundance of argon
remains almost constant. Our results are thus compatible with the deficiency of
Titan's atmosphere in xenon and krypton measured by the {\it Huygens} probe
during its descent on January 14, 2005. However, in order to interpret the
subsolar abundance of primordial Ar also revealed by {\it Huygens}, other
processes that occurred either during the formation of Titan or during its
evolution must be also invoked.Comment: Astronomy & Astrophysics Letters, in pres
Analytical solutions for black-hole critical behaviour
Dynamical Einstein cluster is a spherical self-gravitating system of
counterrotating particles, which may expand, oscillate and collapse. This
system exhibits critical behaviour in its collapse at the threshold of black
hole formation. It appears when the specific angular momentum of particles is
tuned finely to the critical value. We find the unique exact self-similar
solution at the threshold. This solution begins with a regular surface,
involves timelike naked singularity formation and asymptotically approaches a
static self-similar cluster.Comment: 4 pages, 3 figures, accepted for publication in General Relativity
and Gravitation, typos correcte
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