3,390 research outputs found
Non-LTE treatment of molecules in the photospheres of cool stars
We present a technique to treat systems with very many levels, like
molecules, in non-LTE. This method is based on a superlevel formalism coupled
with rate operator splitting. Superlevels consist of many individual levels
that are assumed to be in LTE relative to each other. The usage of superlevels
reduces the dimensionality of the rate equations dramatically and, thereby,
makes the problem computationally more easily treatable. Our superlevel
formalism retains maximum accuracy by using direct opacity sampling (dOS) when
calculating the radiative transitions and the opacities. We developed this
method in order to treat molecules in cool dwarf model calculations in non-LTE.
Cool dwarfs have low electron densities and a radiation field that is far from
a black body radiation field, both properties may invalidate the conditions for
the common LTE approximation. Therefore, the most important opacity sources,
the molecules, need to be treated in non-LTE. As a case study we applied our
method to carbon monoxide. We find that our method gives accurate results since
the conditions for the superlevel method are very well met for molecules. Due
to very high collisional cross sections with hydrogen, and the high densities
of H_2 the population of CO itself shows no significant deviation from LTE.Comment: AASTeX v50, 35 pages including 12 figures, accepted by Ap
Conductivity in a symmetry broken phase: Spinless fermions with corrections
The dynamic conductivity of strongly correlated electrons in
a symmetry broken phase is investigated in the present work. The model
considered consists of spinless fermions with repulsive interaction on a simple
cubic lattice. The investigated symmetry broken phase is the charge density
wave (CDW) with wave vector which occurs at
half-filling. The calculations are based on the high dimensional approach, i.e.
an expansion in the inverse dimension is used. The finite dimensionality
is accounted for by the inclusion of linear terms in and the true finite
dimensional DOS. Special care is paid to the setup of a conserving
approximation in the sense of Baym/Kadanoff without inconsistencies. The
resulting Bethe-Salpeter equation is solved for the dynamic conductivity in the
non symmetry broken and in the symmetry broken phase (AB-CDW). The
dc-conductivity is reduced drastically in the CDW. Yet it does not vanish in
the limit due to a subtle cancellation of diverging mobility and
vanishing DOS. In the dynamic conductivity the energy gap
induced by the symmetry breaking is clearly discernible. In addition, the
vertex corrections of order lead to an excitonic resonance lying within
the gap.Comment: 23 pages, 19 figures included with psfig, Revtex; Physical Review
B15, in press (October/November 1996) depending on the printer/screen driver,
it might be necessary to comment out figures 3,4,5,10,11,12,19 and have them
printed separatel
Revised metallicity classes for low-mass stars: dwarfs (dM), subdwarfs (sdM), extreme subdwarfs (esdM), and ultra subdwarfs (usdM)
The current classification system of M stars on the main sequence
distinguishes three metallicity classes (dwarfs - dM, subdwarfs - sdM, and
extreme subdwarfs - esdM). The spectroscopic definition of these classes is
based on the relative strength of prominent CaH and TiO molecular absorption
bands near 7000A, as quantified by three spectroscopic indices (CaH2, CaH3, and
TiO5). We re-examine this classification system in light of our ongoing
spectroscopic survey of stars with proper motion \mu > 0.45 "/yr, which has
increased the census of spectroscopically identified metal-poor M stars to over
400 objects. Kinematic separation of disk dwarfs and halo subdwarfs suggest
deficiencies in the current classification system. Observations of common
proper motion doubles indicates that the current dM/sdM and sdM/esdM boundaries
in the [TiO5,CaH2+CaH3] index plane do not follow iso-metallicity contours,
leaving some binaries inappropriately classified as dM+sdM or sdM+esdM. We
propose a revision of the classification system based on an empirical
calibration of the TiO/CaH ratio for stars of near solar metallicity. We
introduce the parameter \zeta_{TiO/CaH} which quantifies the weakening of the
TiO bandstrength due to metallicity effect, with values ranging from
\zeta_{TiO/CaH}=1 for stars of near-solar metallicity to \zeta_{TiO/CaH}~0 for
the most metal-poor (and TiO depleted) subdwarfs. We redefine the metallicity
classes based on the value of the parameter \zeta_{TiO/CaH}; and refine the
scheme by introducing an additional class of ultra subdwarfs (usdM). We
introduce sequences of sdM, esdM, and usdM stars to be used as formal
classification standards.Comment: 15 pages, accepted for publication in the Astrophysical Journa
Critical level spacing distribution of two-dimensional disordered systems with spin-orbit coupling
The energy level statistics of 2D electrons with spin-orbit scattering are
considered near the disorder induced metal-insulator transition. Using the Ando
model, the nearest-level-spacing distribution is calculated numerically at the
critical point. It is shown that the critical spacing distribution is size
independent and has a Poisson-like decay at large spacings as distinct from the
Gaussian asymptotic form obtained by the random-matrix theory.Comment: 7 pages REVTeX, 2 uuencoded, gzipped figures; J. Phys. Condensed
Matter, in prin
Sustainable growth in complex networks
Based on the empirical analysis of the dependency network in 18 Java
projects, we develop a novel model of network growth which considers both: an
attachment mechanism and the addition of new nodes with a heterogeneous
distribution of their initial degree, . Empirically we find that the
cumulative degree distributions of initial degrees and of the final network,
follow power-law behaviors: , and
, respectively. For the total number of links as a
function of the network size, we find empirically ,
where is (at the beginning of the network evolution) between 1.25 and
2, while converging to for large . This indicates a transition from
a growth regime with increasing network density towards a sustainable regime,
which revents a collapse because of ever increasing dependencies. Our
theoretical framework is able to predict relations between the exponents
, , , which also link issues of software engineering and
developer activity. These relations are verified by means of computer
simulations and empirical investigations. They indicate that the growth of real
Open Source Software networks occurs on the edge between two regimes, which are
either dominated by the initial degree distribution of added nodes, or by the
preferential attachment mechanism. Hence, the heterogeneous degree distribution
of newly added nodes, found empirically, is essential to describe the laws of
sustainable growth in networks.Comment: 5 pages, 2 figures, 1 tabl
A k-shell decomposition method for weighted networks
We present a generalized method for calculating the k-shell structure of
weighted networks. The method takes into account both the weight and the degree
of a network, in such a way that in the absence of weights we resume the shell
structure obtained by the classic k-shell decomposition. In the presence of
weights, we show that the method is able to partition the network in a more
refined way, without the need of any arbitrary threshold on the weight values.
Furthermore, by simulating spreading processes using the
susceptible-infectious-recovered model in four different weighted real-world
networks, we show that the weighted k-shell decomposition method ranks the
nodes more accurately, by placing nodes with higher spreading potential into
shells closer to the core. In addition, we demonstrate our new method on a real
economic network and show that the core calculated using the weighted k-shell
method is more meaningful from an economic perspective when compared with the
unweighted one.Comment: 17 pages, 6 figure
Risk-Seeking versus Risk-Avoiding Investments in Noisy Periodic Environments
We study the performance of various agent strategies in an artificial
investment scenario. Agents are equipped with a budget, , and at each
time step invest a particular fraction, , of their budget. The return on
investment (RoI), , is characterized by a periodic function with
different types and levels of noise. Risk-avoiding agents choose their fraction
proportional to the expected positive RoI, while risk-seeking agents
always choose a maximum value if they predict the RoI to be positive
("everything on red"). In addition to these different strategies, agents have
different capabilities to predict the future , dependent on their
internal complexity. Here, we compare 'zero-intelligent' agents using technical
analysis (such as moving least squares) with agents using reinforcement
learning or genetic algorithms to predict . The performance of agents is
measured by their average budget growth after a certain number of time steps.
We present results of extensive computer simulations, which show that, for our
given artificial environment, (i) the risk-seeking strategy outperforms the
risk-avoiding one, and (ii) the genetic algorithm was able to find this optimal
strategy itself, and thus outperforms other prediction approaches considered.Comment: 27 pp. v2 with minor corrections. See http://www.sg.ethz.ch for more
inf
Determining ethylene group disorder levels in -(BEDT-TTF)Cu[N(CN)]Br
We present a detailed structural investigation of the organic superconductor
-(BEDT-TTF)Cu[N(CN)]Br at temperatures from 9 to 300 K.
Anomalies in the dependence of the lattice parameters are associated with a
glass-like transition previously reported at = 77 K. From structure
refinements at 9, 100 and 300 K, the orthorhombic crystalline symmetry, space
group {\it Pnma}, is established at all temperatures. Further, we extract the
dependence of the occupation factor of the eclipsed conformation of the
terminal ethylene groups of the BEDT-TTF molecule. At 300 K, we find 67(2) %,
with an increase to 97(3) % at 9 K. We conclude that the glass-like transition
is not primarily caused by configurational freezing-out of the ethylene groups
The Anderson Transition in Two-Dimensional Systems with Spin-Orbit Coupling
We report a numerical investigation of the Anderson transition in
two-dimensional systems with spin-orbit coupling. An accurate estimate of the
critical exponent for the divergence of the localization length in this
universality class has to our knowledge not been reported in the literature.
Here we analyse the SU(2) model. We find that for this model corrections to
scaling due to irrelevant scaling variables may be neglected permitting an
accurate estimate of the exponent
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