16,541 research outputs found
The spectroscopic Hertzsprung-Russell diagram
The Hertzsprung-Russell diagram is an essential diagnostic diagram for
stellar structure and evolution, which has now been in use for more than 100
years. Our spectroscopic Hertzsprung-Russell (sHR) diagram shows the inverse of
the flux-mean gravity versus the effective temperature. Observed stars whose
spectra have been quantitatively analyzed can be entered in this diagram
without the knowledge of the stellar distance or absolute brightness. Observed
stars can be as conveniently compared to stellar evolution calculations in the
sHR diagram as in the Hertzsprung-Russell diagram. However, at the same time,
our ordinate is proportional to the stellar mass-to-luminosity ratio, which can
thus be directly determined. For intermediate- and low-mass star evolution at
constant mass, we show that the shape of an evolutionary track in the sHR
diagram is identical to that in the Hertzsprung-Russell diagram. We also
demonstrate that for hot stars, their stellar Eddington factor can be directly
read off the sHR diagram. For stars near their Eddington limit, we argue that a
version of the sHR diagram may be useful where the gravity is exchanged by the
effective gravity. We discuss the advantages and limitations of the sHR
diagram, and show that it can be fruitfully applied to Galactic stars, but also
to stars with known distance, e.g., in the LMC or in galaxies beyond the Local
Group.Comment: 9 pages, 8 figures, Astronomy and Astrophysics, in pres
Shear-transformation-zone theory of plastic deformation near the glass transition
The shear-transformation-zone (STZ) theory of plastic deformation in
glass-forming materials is reformulated in light of recent progress in
understanding the roles played the effective disorder temperature and entropy
flow in nonequilibrium situations. A distinction between fast and slow internal
state variables reduces the theory to just two coupled equations of motion, one
describing the plastic response to applied stresses, and the other the dynamics
of the effective temperature. The analysis leading to these equations contains,
as a byproduct, a fundamental reinterpretation of the dynamic yield stress in
amorphous materials. In order to put all these concepts together in a realistic
context, the paper concludes with a reexamination of the experimentally
observed rheological behavior of a bulk metallic glass. That reexamination
serves as a test of the STZ dynamics, confirming that system parameters
obtained from steady-state properties such as the viscosity can be used to
predict transient behaviors.Comment: 15 pages, four figure
ALLOY BROADENING OF THE NEAR-GAP LUMINESCENCE AND THE NATURAL BAND OFFSET IN SEMICONDUCTOR ALLOYS
The inhomogeneous broadening of the near-gap emission (bound excitons (BE) and conduction-band to acceptor (CA)) in semiconductor alloys is reanalysed using the Markoff statistical theory for fluctuations of alloy composition. We give the exact relationship between the linewidth and the Bohr radius of the bound particle. The results of our theory indicate that even in the best GaAlAs samples there is still a significant contribution from other broadening mechanisms. We also show that the linewidth ratio of the CA to BE emission lines may provide a good estimate of the natural band offset in the alloy
Separator development for a heat sterilizable battery Quarterly report, 1 Oct. - 31 Dec. 1966
Composite separator production for heat sterilizable silver-zinc batterie
Separator development for a heat sterilizable battery Quarterly report, 1 Jun. - 30 Sep. 1966
Filler and matrix composite materials for use in silver-zinc battery separator
Binaries are the best single stars
Stellar models of massive single stars are still plagued by major
uncertainties. Testing and calibrating against observations is essential for
their reliability. For this purpose one preferably uses observed stars that
have never experienced strong binary interaction, i.e. "true single stars".
However, the binary fraction among massive stars is high and identifying "true
single stars" is not straight forward. Binary interaction affects systems in
such a way that the initially less massive star becomes, or appears to be,
single. For example, mass transfer results in a widening of the orbit and a
decrease of the luminosity of the donor star, which makes it very hard to
detect. After a merger or disruption of the system by the supernova explosion,
no companion will be present.
The only unambiguous identification of "true single stars" is possible in
detached binaries, which contain two main-sequence stars. For these systems we
can exclude the occurrence of mass transfer since their birth. A further
advantage is that binaries can often provide us with direct measurements of the
fundamental stellar parameters. Therefore, we argue these binaries are worth
the effort needed to observe and analyze them. They may provide the most
stringent test cases for single stellar models.Comment: 5 pages, 1 figure, contribution to the proceedings of "The
multi-wavelength view of hot, massive stars", 39th Li`ege Int. Astroph.
Coll., 12-16 July 201
Partition-dependent framing effects in lab and field prediction markets
Many psychology experiments show that individually judged probabilities of the same event can vary depending on the partition of the state space (a framing effect called "partition-dependence"). We show that these biases transfer to competitive prediction markets in which multiple informed traders are provided economic incentives to bet on their beliefs about events. We report results of a short controlled lab study, a longer field experiment (betting on the NBA playoffs and the FIFA World Cup), and naturally-occurring trading in macro-economic derivatives. The combined evidence suggests that partition-dependence can exist and persist in lab and field prediction markets
Hamiltonians for curves
We examine the equilibrium conditions of a curve in space when a local energy
penalty is associated with its extrinsic geometrical state characterized by its
curvature and torsion. To do this we tailor the theory of deformations to the
Frenet-Serret frame of the curve. The Euler-Lagrange equations describing
equilibrium are obtained; Noether's theorem is exploited to identify the
constants of integration of these equations as the Casimirs of the euclidean
group in three dimensions. While this system appears not to be integrable in
general, it {\it is} in various limits of interest. Let the energy density be
given as some function of the curvature and torsion, . If
is a linear function of either of its arguments but otherwise arbitrary, we
claim that the first integral associated with rotational invariance permits the
torsion to be expressed as the solution of an algebraic equation in
terms of the bending curvature, . The first integral associated with
translational invariance can then be cast as a quadrature for or for
.Comment: 17 page
Evolution of Mass Functions of Coeval Stars through Wind Mass Loss and Binary Interactions
Accurate determinations of stellar mass functions and ages of stellar
populations are crucial to much of astrophysics. We analyse the evolution of
stellar mass functions of coeval main sequence stars including all relevant
aspects of single- and binary-star evolution. We show that the slope of the
upper part of the mass function in a stellar cluster can be quite different to
the slope of the initial mass function. Wind mass loss from massive stars leads
to an accumulation of stars which is visible as a peak at the high mass end of
mass functions, thereby flattening the mass function slope. Mass accretion and
mergers in close binary systems create a tail of rejuvenated binary products.
These blue straggler stars extend the single star mass function by up to a
factor of two in mass and can appear up to ten times younger than their parent
stellar cluster. Cluster ages derived from their most massive stars that are
close to the turn-off may thus be significantly biased. To overcome such
difficulties, we propose the use of the binary tail of stellar mass functions
as an unambiguous clock to derive the cluster age because the location of the
onset of the binary tail identifies the cluster turn-off mass. It is indicated
by a pronounced jump in the mass function of old stellar populations and by the
wind mass loss peak in young stellar populations. We further characterise the
binary induced blue straggler population in star clusters in terms of their
frequency, binary fraction and apparent age.Comment: 21 pages, 22 figures, accepted for publication in Ap
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