34,501 research outputs found
Large-scale Vortices in Protoplanetary Disks: On the observability of possible early stages of planet formation
We investigate the possibility of mapping large-scale anti-cyclonic vortices,
resulting from a global baroclinic instability, as pre-cursors of planet
formation in proto-planetary disks with the planned Atacama Large Millimeter
Array (ALMA). On the basis of three-dimensional radiative transfer simulations,
images of a hydrodynamically calculated disk are derived which provide the
basis for the simulation of ALMA. We find that ALMA will be able to trace the
theoretically predicted large-scale anti-cyclonic vortex and will therefore
allow testing of existing models of this very early stage of planet formation
in circumstellar disks.Comment: Accepted by ApJ (Letters section). A preprint version with
high-quality figures can be downloaded from
http://spider.ipac.caltech.edu/staff/swolf/homepage/public/preprints/
vortex.ps.g
Ferrimagnetism of MnV_2O_4 spinel
The spinel MnV_2O_4 is a two-sublattice ferrimagnet, with site A occupied by
the Mn^{2+} ion and site B by the V^{3+} ion. The magnon of the system, the
transversal fluctuation of the total magnetization, is a complicated mixture of
the sublattice A and B transversal magnetic fluctuations. As a result, the
magnons' fluctuations suppress in a different way the manganese and vanadium
magnetic orders and one obtains two phases. At low temperature (0,T^*) the
magnetic orders of the Mn and V ions contribute to the magnetization of the
system, while at the high temperature (T^*,T_N), the vanadium magnetic order is
suppressed by magnon fluctuations, and only the manganese ions have non-zero
spontaneous magnetization. A modified spin-wave theory is developed to describe
the two phases and to calculate the magnetization as a function of temperature.
The anomalous curve reproduces the experimentally obtained ZFC
magnetization.Comment: 4 pages, one figur
Electronic structure of single-crystalline MgAlB probed by x-ray diffraction multipole refinements and polarization-dependent x-ray absorption spectroscopy
X-ray diffraction multipole refinements of single-crystalline
MgAlB and polarization-dependent near-edge x-ray absorption
fine structure at the B 1 edge reveal a strongly anisotropic electronic
structure. Comparing the data for superconducting compounds (, 1.0)
with those for the non-superconductor () gives direct evidence for a
rearrangement of the hybridizations of the boron bonds and underline the
importance of holes in the -bonded covalent states for the
superconducting properties of the diborides. The data indicate that Mg is
approximately divalent in MgB and suggest predominantly ionic bonds between
the Mg ions and the two-dimensional B rings. For AlB (), on the other
hand, about 1.5 electrons per Al atom are transferred to the B sheets while the
residual 1.5 electrons remain at the Al site which suggests significant
covalent bonding between the Al ions and the B sheets. This finding together
with the static electron deformation density points to almost equivalent
electron counts on B sheets of MgB and AlB\@, yet with a completely
different electron/hole distribution between the and bonds
Analytical Approximations for Calculating the Escape and Absorption of Radiation in Clumpy Dusty Environments
We present analytical approximations for calculating the scattering,
absorption and escape of nonionizing photons from a spherically symmetric
two-phase clumpy medium, with either a central point source of isotropic
radiation, a uniform distribution of isotropic emitters, or uniformly
illuminated by external sources. The analytical approximations are based on the
mega-grains model of two-phase clumpy media, as proposed by Hobson & Padman,
combined with escape and absorption probability formulae for homogeneous media.
The accuracy of the approximations is examined by comparison with 3D Monte
Carlo simulations of radiative transfer, including multiple scattering. Our
studies show that the combined mega-grains and escape/absorption probability
formulae provide a good approximation of the escaping and absorbed radiation
fractions for a wide range of parameters characterizing the medium. A realistic
test is performed by modeling the absorption of a starlike source of radiation
by interstellar dust in a clumpy medium, and by calculating the resulting
equilibrium dust temperatures and infrared emission spectrum of both the clumps
and the interclump medium. In particular, we find that the temperature of dust
in clumps is lower than in the interclump medium if clumps are optically thick.
Comparison with Monte Carlo simulations of radiative transfer in the same
environment shows that the analytic model yields a good approximation of dust
temperatures and the emerging UV to FIR spectrum of radiation for all three
types of source distributions mentioned above. Our analytical model provides a
numerically expedient way to estimate radiative transfer in a variety of
interstellar conditions and can be applied to a wide range of astrophysical
environments, from star forming regions to starburst galaxies.Comment: 55 pages, 27 figures. ApJ 523 (1999), in press. Corrected equations
and text so as to be same as ApJ versio
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