97 research outputs found
Formation of chondrules in radiative shock waves I. First results, spherical dust particles, stationary shocks
The formation of chondrules in the protoplanetary nebulae causes many
questions concerning the formation process, the source of energy for melting
the rims, and the composition of the origin material. The aim of this work is
to explore the heating of the chondrule in a single precursor as is typical for
radiation hydrodynamical shock waves. We take into account the gas-particle
friction for the duration of the shock transition and calculate the heat
conduction into the chondrules. These processes are located in the
protoplanetary nebulae at a region around 2.5 AU, which is considered to be the
most likely place of chondrule formation. The present models are a first step
towards computing radiative shock waves occurring in a particle-rich
environment. We calculated the shock waves using one-dimensional,
time-independent equations of radiation hydrodynamics involving realistic gas
and dust opacities and gas-particle friction. The evolution of spherical
chondrules was followed by solving the heat conduction equation on an adaptive
grid. The results for the shock-heating event are consistent with the
cosmochemical constraints of chondrule properties. The calculations yield a
relative narrow range for density or temperature to meet the requested heating
rates of as extracted from cosmochemical constraints.
Molecular gas, opacities with dust, and a protoplanetary nebula with accretion
are necessary requirements for a fast heating process. The thermal structure in
the far post-shock region is not fully consistent with experimental constraints
on chondrule formation since the models do not include additional molecular
cooling processes.Comment: 8 pages,5 figure
Acceleration of cosmic rays in supernova-remnants
It is commonly accepted that supernova-explosions are the dominant source of cosmic rays up to an energy of 10 to the 14th power eV/nucleon. Moreover, these high energy particles provide a major contribution to the energy density of the interstellar medium (ISM) and should therefore be included in calculations of interstellar dynamic phenomena. For the following the first order Fermi mechanism in shock waves are considered to be the main acceleration mechanism. The influence of this process is twofold; first, if the process is efficient (and in fact this is the cas) it will modify the dynamics and evolution of a supernova-remnant (SNR), and secondly, the existence of a significant high energy component changes the overall picture of the ISM. The complexity of the underlying physics prevented detailed investigations of the full non-linear selfconsistent problem. For example, in the context of the energy balance of the ISM it has not been investigated how much energy of a SN-explosion can be transfered to cosmic rays in a time-dependent selfconsistent model. Nevertheless, a lot of progress was made on many aspects of the acceleration mechanism
A cosmic ray driven instability
The interaction between energetic charged particles and thermal plasma which forms the basis of diffusive shock acceleration leads also to interesting dynamical phenomena. For a compressional mode propagating in a system with homogeneous energetic particle pressure it is well known that friction with the energetic particles leads to damping. The linear theory of this effect has been analyzed in detail by Ptuskin. Not so obvious is that a non-uniform energetic particle pressure can addition amplify compressional disturbances. If the pressure gradient is sufficiently steep this growth can dominate the frictional damping and lead to an instability. It is important to not that this effect results from the collective nature of the interaction between the energetic particles and the gas and is not connected with the Parker instability, nor with the resonant amplification of Alfven waves
Time-dependent galactic winds I. Structure and evolution of galactic outflows accompanied by cosmic ray acceleration
Cosmic rays are transported out of the galaxy by diffusion and advection due
to streaming along magnetic field lines and resonant scattering off
self-excited MHD waves. Thus momentum is transferred to the plasma via the
frozen-in waves as a mediator assisting the thermal pressure in driving a
galactic wind. The bulk of the Galactic CRs are accelerated by shock waves
generated in SNRs, a significant fraction of which occur in OB associations on
a timescale of several years. We examine the effect of changing boundary
conditions at the base of the galactic wind due to sequential SN explosions on
the outflow. Thus pressure waves will steepen into shock waves leading to in
situ post-acceleration of GCRs. We performed simulations of galactic winds in
flux tube geometry appropriate for disk galaxies, describing the CR
diffusive-advective transport in a hydrodynamical fashion along with the energy
exchange with self-generated MHD waves. Our time-dependent CR hydrodynamic
simulations confirm the existence of time asymptotic outflow solutions (for
constant boundary conditions). It is also found that high-energy particles
escaping from the Galaxy and having a power-law distribution in energy
() similar to the Milky Way with an upper energy cut-off at
eV are subjected to efficient and rapid post-SNR acceleration in
the lower galactic halo up to energies of eV by multiple
shock waves propagating through the halo. The particles can gain energy within
less than kpc from the galactic plane corresponding to flow times less
than years. The mechanism described here offers a natural
solution to explain the power-law distribution of CRs between the "knee" and
the "ankle". The mechanism described here offers a natural and elegant solution
to explain the power-law distribution of CRs between the "knee" and the
"ankle".Comment: 15 pages, 7 figure
Detection of the evolutionary stages of variables in M3
The large number of variables in M3 provides a unique opportunity to study an
extensive sample of variables with the same apparent distance modulus. Recent,
high accuracy CCD time series of the variables show that according to their
mean magnitudes and light curve shapes, the variables belong to four separate
groups. Comparing the properties of these groups (magnitudes and periods) with
horizontal branch evolutionary models, we conclude that these samples can be
unambiguously identified with different stages of the horizontal branch stellar
evolution. Stars close to the zero age horizontal branch (ZAHB) show Oosterhoff
I type properties, while the brightest stars have Oosterhoff II type statistics
regarding their mean periods and RRab/RRc number ratios. This finding
strengthens the earlier suggestion of Lee et al. (1990) connecting the
Oosterhoff dichotomy to evolutionary effects, however, it is unexpected to find
large samples of both of the Oosterhoff type within a single cluster, which is,
moreover, the prototype of the Oosterhoff I class globular clusters. The very
slight difference between the Fourier parameters of the stars (at a given
period) in the three fainter samples spanning over about 0.15 mag range in M_V
points to the limitations of any empirical methods which aim to determine
accurate absolute magnitudes of RR Lyrae stars solely from the Fourier
parameters of the light curves.Comment: 4 pages, 4 figures. Submitted to Astrophys. J. Letter
Collapse of Rotating Magnetized Molecular Cloud Cores and Mass Outflows
Collapse of the rotating magnetized molecular cloud core is studied with the
axisymmetric magnetohydrodynamical (MHD) simulations. Due to the change of the
equation of state of the interstellar gas, the molecular cloud cores experience
several different phases as collapse proce eds. In the isothermal run-away
collapse (), a pseudo-disk is formed and
it continues to contract till the opaque core is fo rmed at the center. In this
disk, a number of MHD fast and slow shock pairs appear running parallelly to
the disk. After the equation of state becomes hard, an adiabatic core is
formed, which is separated from the isothermal contracting pseudo-disk by the
accretion shock front facing radially outwards. By the effect of the magnetic
tension, the angular momentum is transferred from the disk mid-plane to the
surface. The gas with excess angular momentum near the surface is finally
ejected, which explains the molecular bipolar outflow. Two types of outflows
are observed. When the poloidal magnetic field is strong (magnetic energy is
comparable to the thermal one), a U-shaped outflow is formed in which fast
moving gas is confined to the wall whose shape looks like a capit al letter U.
The other is the turbulent outflow in which magnetic field lines and velocity
fi elds are randomly oriented. In this case, turbulent gas moves out almost
perpendicularly from the disk. The continuous mass accretion leads to the
quasistatic contraction of the first core. A second collapse due to
dissociation of H in the first core follows. Finally another quasistatic
core is again formed by atomic hydrogen (the second core). It is found that
another outflow is ejected around the second atomic core, which seems to
correspond to the optical jets or the fast neutral winds.Comment: submitted to Ap
Evolution of Rotating Molecular Cloud Core with Oblique Magnetic Field
We studied the collapse of rotating molecular cloud cores with inclined
magnetic fields, based on three-dimensional numerical simulations.The numerical
simulations start from a rotating Bonnor-Ebert isothermal cloud in a uniform
magnetic field. The magnetic field is initially taken to be inclined from the
rotation axis. As the cloud collapses, the magnetic field and rotation axis
change their directions. When the rotation is slow and the magnetic field is
relatively strong, the direction of the rotation axis changes to align with the
magnetic field, as shown earlier by Matsumoto & Tomisaka. When the magnetic
field is weak and the rotation is relatively fast, the magnetic field inclines
to become perpendicular to the rotation axis. In other words, the evolution of
the magnetic field and rotation axis depends on the relative strength of the
rotation and magnetic field. Magnetic braking acts to align the rotation axis
and magnetic field, while the rotation causes the magnetic field to incline
through dynamo action. The latter effect dominates the former when the ratio of
the angular velocity to the magnetic field is larger than a critical value
\Omega_0/ B_0 > 0.39 G^1/2 c_s^-1, where B_0, \Omega_0, G, and c_s^-1 denote
the initial magnetic field, initial angular velocity, gravitational constant,
and sound speed, respectively. When the rotation is relatively strong, the
collapsing cloud forms a disk perpendicular to the rotation axis and the
magnetic field becomes nearly parallel to the disk surface in the high density
region. A spiral structure appears due to the rotation and the wound-up
magnetic field in the disk.Comment: 45 pages, 17 figures, Submitted to ApJ, For high resolution figures
see http://www2.scphys.kyoto-u.ac.jp/~machidam/ms060201.pd
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