352,622 research outputs found
On Rotations as Spin Matrix Polynomials
Recent results for rotations expressed as polynomials of spin matrices are
derived here by elementary differential equation methods. Structural features
of the results are then examined in the framework of biorthogonal systems, to
obtain an alternate derivation. The central factorial numbers play key roles in
both derivations.Comment: 6 Figures. References updated in v2, along with some editing of tex
Shock-triggered formation of magnetically-dominated clouds
To understand the formation of a magnetically dominated molecular cloud out
of an atomic cloud, we follow the dynamical evolution of the cloud with a
time-dependent axisymmetric magnetohydrodynamic code. A thermally stable warm
atomic cloud is initially in static equilibrium with the surrounding hot
ionised gas. A shock propagating through the hot medium interacts with the
cloud. As a fast-mode shock propagates through the cloud, the gas behind it
becomes thermally unstable. The value of the gas also becomes much
smaller than the initial value of order unity. These conditions are ideal for
magnetohydrodynamic waves to produce high-density clumps embedded in a rarefied
warm medium. A slow-mode shock follows the fast-mode shock. Behind this shock a
dense shell forms, which subsequently fragments. This is a primary region for
the formation of massive stars. Our simulations show that only weak and
moderate-strength shocks can form cold clouds which have properties typical of
giant molecular clouds.Comment: 7 pages, 6 figures, accepted by Astronomy and Astrophysic
IP Eri: A surprising long-period binary system hosting a He white dwarf
We determine the orbital elements for the K0 IV + white dwarf (WD) system IP
Eri, which appears to have a surprisingly long period of 1071 d and a
significant eccentricity of 0.25. Previous spectroscopic analyses of the WD,
based on a distance of 101 pc inferred from its Hipparcos parallax, yielded a
mass of only 0.43 M, implying it to be a helium-core WD. The orbital
properties of IP Eri are similar to those of the newly discovered long-period
subdwarf B star (sdB) binaries, which involve stars with He-burning cores
surrounded by extremely thin H envelopes, and are therefore close relatives to
He WDs. We performed a spectroscopic analysis of high-resolution spectra from
the HERMES/Mercator spectrograph and concluded that the atmospheric parameters
of the K0 component are K, , [Fe/H] = 0.09
and km/s. The detailed abundance analysis focuses on C, N, O
abundances, carbon isotopic ratio, light (Na, Mg, Al, Si, Ca, Ti) and s-process
(Sr, Y, Zr, Ba, La, Ce, Nd) elements. We conclude that IP Eri abundances agree
with those of normal field stars of the same metallicity. The long period and
non-null eccentricity indicate that this system cannot be the end product of a
common-envelope phase; it calls instead for another less catastrophic
binary-evolution channel presented in detail in a companion paper (Siess et al.
2014).Comment: 14 pages, 10 figures, 4 tables, accepted for publication in A&A
(Update of Table 3, Fig. 8 and text in Sect. 5.1, 5.3 and 6 due to minor
corrections on N and Y II
Parametrization of coronal heating: spatial distribution and observable consequences
We investigate the difference in the spatial distribution of the energy input
for parametrizations of different mechanisms to heat the corona of the Sun and
possible impacts on the coronal emission. We use a 3D MHD model of a solar
active region as a reference and compare the Ohmic-type heating in this model
to parametrizations for alternating current (AC) and direct current (DC)
heating models, in particular, we use Alfven wave and MHD turbulence heating.
We extract the quantities needed for these two parametrizations from the
reference model and investigate the spatial distribution of the heat input in
all three cases, globally and along individual field lines. To study
differences in the resulting coronal emission we employ 1D loop models with a
prescribed heat input based on the heating rate we extracted along a bundle of
field lines. On average, all heating implementations show a roughly drop of the
heating rate with height. This also holds for individual field lines. While all
mechanism show a concentration of the energy input towards the low parts of the
atmosphere, for individual field lines the concentration towards the footpoints
is much stronger for the DC mechanisms than for the Alfven wave AC case. In
contrast, the AC model gives a stronger concentration of the emission towards
the footpoints. This is because the more homogeneous distribution of the energy
input leads to higher coronal temperatures and a more extended transition
region. The significant difference in the concentration of the heat input
towards the foot points for the AC and DC mechanisms, and the pointed
difference in the spatial distribution of the coronal emission for these cases
shows that the two mechanisms should be discriminable by observations. Before
drawing final conclusions, these parametrizations should be implemented in new
3D models in a more self-consistent way.Comment: accepted for publication in A&A, 10 pages, 9 figure
Entropic Stabilization of Tunable Planar Modulated Superstructures
Self-assembling novel ordered structures with nanoparticles has recently
received much attention. Here we use computer simulations to study a
two-dimensional model system characterized by a simple isotropic interaction
that could be realized with building blocks on the nanoscale. We find that the
particles arrange themselves into hexagonal superstructures of twin boundaries
whose superlattice vector can be tuned reversibly by changing the temperature.
Thermodynamic stability is confirmed by calculating the free energy with a
combination of thermodynamic integration and the Frenkel-Ladd method. Different
contributions to the free energy difference are discussed.Comment: 4 pages, 5 figures plus 7 pages of supplementary figure
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