20,557 research outputs found
Mechanism of Magnetic Flux Loss in Molecular Clouds
We investigate the detailed processes working in the drift of magnetic fields
in molecular clouds. To the frictional force, whereby the magnetic force is
transmitted to neutral molecules, ions contribute more than half only at cloud
densities , and charged grains contribute more
than 90% at . Thus grains play a decisive role
in the process of magnetic flux loss. Approximating the flux loss time by
a power law , where is the mean field strength in
the cloud, we find , characteristic to ambipolar diffusion,
only at . At higher densities,
decreases steeply with , and finally at , where magnetic fields
effectively decouple from the gas, is attained, reminiscent of
Ohmic dissipation, though flux loss occurs about 10 times faster than by Ohmic
dissipation. Ohmic dissipation is dominant only at . While ions and electrons drift in the direction of
magnetic force at all densities, grains of opposite charges drift in opposite
directions at high densities, where grains are major contributors to the
frictional force. Although magnetic flux loss occurs significantly faster than
by Ohmic dissipation even at very high densities as , the process going on at high densities is quite different from ambipolar
diffusion in which particles of opposite charges are supposed to drift as one
unit.Comment: 34 pages including 9 postscript figures, LaTex, accepted by
Astrophysical Journal (vol.573, No.1, July 1, 2002
Giant nonlinear conduction and thyristor-like negative derivative resistance in BaIrO3 single crystals
We synthesized single-crystalline samples of monoclinic BaIrO3 using a molten
flux method, and measured their magnetization, resistivity, Seebeck coefficient
and nonlinear voltage-current characteristics. The magnetization rapidly
increases below a ferromagnetic transition temperature TC of 180 K, where the
resistivity concomitantly shows a hump-type anomaly, followed by a sharp
increase below 30 K. The Seebeck coefficient suddenly increases below TC, and
shows linear temperature dependence below 50 K. A most striking feature of this
compound is that the anomalously giant nonlinear conduction is observed below
30 K, where a small current density of 20 A/cm2 dramatically suppresses the
sharp increase in resistivity to induce a metallic conduction down to 4 K.Comment: 10 pages, 4 figures Submitted to Physical Review Letter
Comments on differential cross section of phi-meson photoproduction at threshold
We show that the differential cross section d_sigma/d_t of gamma p --> \phi p
reaction at the threshold is finite and its value is crucial to the mechanism
of the phi meson photoproduction and for the models of phi-N interaction.Comment: 8 pages, 2 figure
Finite-temperature phase structures of hard-core bosons in an optical lattice with an effective magnetic field
We study finite-temperature phase structures of hard-core bosons in a
two-dimensional optical lattice subject to an effective magnetic field by
employing the gauged CP model. Based on the extensive Monte Carlo
simulations, we study their phase structures at finite temperatures for several
values of the magnetic flux per plaquette of the lattice and mean particle
density. Despite the presence of the particle number fluctuation, the
thermodynamic properties are qualitatively similar to those of the frustrated
XY model with only the phase as a dynamical variable. This suggests that cold
atom simulators of the frustrated XY model are available irrespective of the
particle filling at each site.Comment: 13 pages, 9 figure
The 1953 Cosmic Ray Conference at Bagneres de Bigorre
The cosmic ray conference at Bagn`eres de Bigorre in July, 1953 organized by
Patrick Blackett and Louis Leprince-Ringuet was a seminal one. It marked the
beginning of sub atomic physics and its shift from cosmic ray research to
research at the new high energy accelerators. The knowledge of the heavy
unstable particles found in the cosmic rays was essentially correct in fact and
interpretation and defined the experiments that needed to be carried out with
the new accelerators. A large fraction of the physicists who had been using
cosmic rays for their research moved to the accelerators. This conference can
be placed in importance in the same category as two other famous conferences,
the Solvay congress of 1927 and the Shelter Island Conference of 1948
A Spherical Model for "Starless" Cores of Magnetic Molecular Clouds and Dynamical Effects of Dust Grains
In the standard picture of isolated star formation, dense ``starless'' cores
are formed out of magnetic molecular clouds due to ambipolar diffusion. Under
the simplest spherical geometry, I demonstrate that ``starless'' cores formed
this way naturally exhibit a large scale inward motion, whose size and speed
are comparable to those detected recently by Taffala et al. and Williams et al.
in ``starless'' core L1544. My model clouds have a relatively low mass (of
order 10 ) and low field strength (of order 10 G) to begin with.
They evolve into a density profile with a central plateau surrounded by a
power-law envelope, as found previously. The density in the envelope decreases
with radius more steeply than those found by Mouschovias and collaborators for
the more strongly magnetized, disk-like clouds.
At high enough densities, dust grains become dynamically important by greatly
enhancing the coupling between magnetic field and the neutral cloud matter. The
trapping of magnetic flux associated with the enhanced coupling leads, in the
spherical geometry, to a rapid assemblage of mass by the central protostar,
which exacerbates the so-called ``luminosity problem'' in star formation.Comment: 27 pages, 4 figures, accepted by Ap
Compressional properties of nuclear matter in the relativistic mean field theory with the excluded volume effects
Compressional properties of nuclear matter are studied by using the mean
field theory with the excluded volume effects of the nucleons. It is found that
the excluded volume effects make it possible to fit the empirical data of the
Coulomb coefficient of nucleus incompressibility, even if the volume
coefficient is small(MeV). However, the symmetry properties favor
MeV as in the cases of the mean field theory of point-like
nucleons.Comment: PACS numbers, 21.65.+f, 21.30.+
Antiproton Production in p+d Reaction at Subthreshold Energies
An enhancement of antiprotons produced in p+d reaction in comparison with
ones in p+p elementary reaction is investigated.
In the neighborhood of subthreshold energy the enhancement is caused by the
difference of available energies for antiproton production. The cross section
in p+d reaction, on the other hand, becomes just twice of the one in elementary
p+p reaction at the incident energy far from the threshold energy when
non-nucleonic components in deuteron target are not considered.Comment: LaTeX,7 pages with 5 eps figure
Volume, Coulomb, and volume-symmetry coefficients of nucleus incompressibility in the relativistic mean field theory with the excluded volume effects
The relation among the volume coefficient (=incompressibility of the
nuclear matter), the Coulomb coefficient , and the volume-symmetry
coefficient of the nucleus incompressibility are studied in the
framework of the relativistic mean field theory with the excluded volume
effects of the nucleons, under the assumption of the scaling model. It is found
that MeV is necessary to account for the empirical values of ,
, and , simultaneously, as is in the case of the point-like
nucleons. The result is independent on the detail descriptions of the potential
of the -meson self-interaction and is almost independent on the
excluded volume of the nucleons.Comment: PACS numbers, 21.65.+f, 21.30.+
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