580 research outputs found

### Cosmic Shock Waves on Large Scales of the Universe

In the standard theory of the large scale structure formation, matter
accretes onto high density perturbations via gravitational instability.
Collisionless dark matter forms caustics around such structures, while
collisional baryonic matter forms accretion shocks which then halt and heat the
infalling gas. Here, we discuss the characteristics, roles, and observational
consequences of these accretion shocks.Comment: 3 pages with 1 figure, uses sprocl.sty, to appear in the Proceedings
of the 18th Texas Symposium on Relativistic Astrophysics, ed. A. Olinto, J.
Frieman and D. Schramm, also available upon request to
[email protected]

### Formation of Proto-Globular Cluster Clouds by Thermal Instability

Many models of globular cluster formation assume the presence of cold dense
clouds in early universe. Here we re-examine the Fall & Rees (1985) model for
formation of proto-globular cluster clouds (PGCCs) via thermal instabilities in
a protogalactic halo. We first argue, based on the previous study by others,
that under the protogalactic environments only nonlinear density
inhomogeneities can condense into PGCCs. We then carry out numerical
simulations of the collapse of overdense clouds in one-dimensional spherical
geometry, including self-gravity and radiative cooling down to T=10^4 K. Since
imprinting of Jeans mass at 10^4 K is essential to this model, here we focus on
the cases where external UV background radiation prevents the formation of H2
molecules and so prevent the cloud from cooling below 10^4 K. The quantitative
results from these simulations can be summarized as follows: 1) Perturbations
smaller than M_min ~ (10^{5.6} M_sun) (n_h/0.05 cm3)^{-2} cool isobarically,
while perturbations larger than M_max ~ (10^8 M_sun) (n_h/0.05 cm3)^{-2} cool
isochorically. On the other hand, intermediate size perturbations (M_min<
M_pgcc < M_max) are compressed supersonically. 2) For supersonically collapsing
clouds, the density compression factor after they cool to T_c=10^4 K range
10^{2.5}-10^6. 3) For supersonically collapsing clouds the Jeans mass can be
reduced to as small as 10^{5.5} M_sun (n_h/0.05 cm3)^{-1/2} at the maximum
compression. 4) The density profile of simulated PGCCs can be approximated by a
constant core with a halo of rho ~ r^{-2} rather than a singular isothermal
sphere.Comment: 11 pages, to appear in Journal of Korean Astronomical Society, uses
jkas2.st

### Cluster Accretion Shocks as Possible Acceleration Sites for Ultra High Energy Protons below the Greisen Cutoff

Three-dimensional hydrodynamic simulations of large scale structure in the
Universe have shown that accretion shocks form during the gravitational
collapse of one-dimensional caustics, and that clusters of galaxies formed at
intersections of the caustics are surrounded by these accretion shocks.
Estimated speed and curvature radius of the shocks are 1000-3000 \kms and about
5 Mpc, respectively, in the $\Omega=1$ CDM universe. Assuming that energetic
protons are accelerated by these accretion shocks via the first-order Fermi
process and modeling particle transport around the shocks through Bohm
diffusion, we suggest that protons can be accelerated up to the {\it Greisen
cutoff energy} near $6\times 10^{19}$ eV, provided the mean magnetic field
strength in the region around the shocks is at least of order a microgauss. We
have also estimated the proton flux at earth from the Virgo cluster. Assuming a
few (1-10) \% of the ram pressure of the infalling matter would be transferred
to the cosmic-rays, the estimated flux for $E \sim 10^{19}$eV is consistent
with observations, so that such clusters could be plausible sources of the UHE
CRs.Comment: 14 pages, uuencoded compressed postscript file. Accepted for Jan. 1,
1996 issue of Ap

### Three-Dimensional Simulations of the Parker Instability in a Uniformly-rotating Disk

We investigate the nonlinear effects of uniform rotation on the Parker
instability in an exponentially-stratified disk through high-resolution
simulations. During the linear stage, the speed of gas motion is subsonic and
the evolution with the rotation is not much different from that without the
rotation. This is because the Coriolis force is small. During the nonlinear
stage, oppositely-directed supersonic flows near a magnetic valley are under
the influence of the Coriolis force with different directions, resulting in
twisted magnetic field lines near the valley. Sheet-like structures, which are
tilted with respect to the initial field direction, are formed with an 1.5
enhancement of column density with respect to its initial value. Even though
uniform rotation doesn't give much impact on density enhancement, it generates
helically twisted field lines, which may become an additional support mechanism
of clouds.Comment: 3 pages, uses rmaa.cls, to appear in Proc. of the Conference on
"Astrophysical Plasmas: Codes, Models and Observations", Eds. J. Franco, J.
Arthur, N. Brickhouse, Rev.Mex.AA Conf. Serie

### Supersonic Collisions between Two Gas Streams

A star around a massive black hole can be disrupted tidally by the gravity of
the black hole. Then, its debris may form a precessing stream which may even
collide with itself. In order to understand the dynamical effects of the
stream-stream collision on the eventual accretion of the stellar debris onto
the black hole, we have studied how gas flow behaves when the outgoing stream
collides supersonically with the incoming stream. We have investigated the
problem analytically with one-dimensional plane-parallel streams and
numerically with more realistic three-dimensional streams. A shock formed
around the contact surface converts the bulk of the orbital streaming kinetic
energy into thermal energy. In three-dimensional simulations, the accumulated
hot post-shock gas then expands adiabatically and drives another shock into the
low density ambient region. Through this expansion, thermal energy is converted
back to the kinetic energy associated with the expanding motion. Thus, in the
end, only a small fraction of the orbital kinetic energy is actually converted
to the thermal energy, while most of it is transferred to the kinetic energy of
the expanding gas. Nevertheless the collision is effective in circularizing the
debris orbit, because the shock efficiently transforms the ordered motion of
the streams into the expanding motion in directions perpendicular to the
streams. The circularization efficiency decreases, if two colliding streams
have a large ratio of cross sections and a large density contrast. But even in
such cases, the main shock extends beyond the overlapping contact surface and
the high pressure region behind the shock keeps the stream of the larger cross
section from passing freely. Thus the stream-stream collisions are still
expected to circularize the stellar debris rather efficiently, unless the ratioComment: ApJ accepted, 14 pages with 9 figures, uuencoded, gzipped, tarred
postscript files, or available upon request to [email protected]

### A Divergence-Free Upwind Code for Multidimensional Magnetohydrodynamic Flows

A description is given for preserving {\bmsy\nabla}\cdot{\vec B}=0 in a
magnetohydrodynamic (MHD) code that employs the upwind, Total Variation
Diminishing (TVD) scheme and the Strang-type operator splitting for
multi-dimensionality. The method is based on the staggered mesh technique to
constrain the transport of magnetic field: the magnetic field components are
defined at grid interfaces with their advective fluxes on grid edges, while
other quantities are defined at grid centers. The magnetic field at grid
centers for the upwind step is calculated by interpolating the values from grid
interfaces. The advective fluxes on grid edges for the magnetic field evolution
are calculated from the upwind fluxes at grid interfaces. Then, the magnetic
field can be maintained with {\bmsy\nabla}\cdot{\vec B}=0 exactly, if this is
so initially, while the upwind scheme is used for the update of fluid
quantities. The correctness of the code is demonstrated through tests comparing
numerical solutions either with analytic solutions or with numerical solutions
from the code using an explicit divergence-cleaning method. Also the robustness
is shown through tests involving realistic astrophysical problems.Comment: 15 pages of text, 8 figures (in degraded gif format), to appear in
The Astrophysical Journal (Dec. 10, 1998), original quality figures available
via anonymous ftp at ftp://ftp.msi.umn.edu/pub/users/twj/mhddivb5.uu or
ftp://canopus.chungnam.ac.kr/ryu/mhddivb5.u

### Neutron stars and white dwarfs in galactic halos

The possibility that galactic halos are composed of stellar remnants such as neutron stars and white dwarfs is discussed. On the basis of a simple model for the evolution of galactic halos, researchers follow the history of halo matter, luminosity, and metal and helium abundances. They assume conventional yields for helium and the heavier elements. By comparing with the observational constraints, which may be considered as fairly conservative, it is found that, for an exponentially decreasing star formation rate (SFR) with e-folding time tau, only values between 6 x 10(8) less than similar to tau less than similar to 2 x 10(9) years are allowed together with a very limited range of masses for the initial mass function (IMF). Star formation is allowed for 2 solar mass less than similar to m less than similar to 8 solar mass if tau = 2 x 10(9) years, and for 4 solar mass less than similar to m less than similar to 6 solar mass if tau = 10(9) years. For tau = 6 x 10(8) years, the lower and upper mass limits merge to similar to 5 solar mass. Researchers conclude that, even though the possibility of neutron stars as halo matter may be ruled out, that of white dwarfs may still be a viable hypothesis, though with very stringent constraints on allowed parameters, that merits further consideration

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