14,915 research outputs found
Asteroseismic Theory of Rapidly Oscillating Ap Stars
This paper reviews some of the important advances made over the last decade
concerning theory of roAp stars.Comment: 9 pages, 5 figure
Testing excitation models of rapidly oscillating Ap stars with interferometry
Rapidly oscillating Ap stars are unique objects in the potential they offer
to study the interplay between a number of important physical phenomena, in
particular, pulsations, magnetic fields, diffusion, and convection.
Nevertheless, the simple understanding of how the observed pulsations are
excited in these stars is still in progress. In this work we perform a test to
what is possibly the most widely accepted excitation theory for this class of
stellar pulsators. The test is based on the study of a subset of members of
this class for which stringent data on the fundamental parameters are available
thanks to interferometry. For three out of the four stars considered in this
study, we find that linear, non-adiabatic models with envelope convection
suppressed around the magnetic poles can reproduce well the frequency region
where oscillations are observed. For the fourth star in our sample no agreement
is found, indicating that a new excitation mechanism must be considered. For
the three stars whose observed frequencies can be explained by the excitation
models under discussion, we derive the minimum angular extent of the region
where convection must be suppressed. Finally, we find that the frequency
regions where modes are expected to be excited in these models is very
sensitive to the stellar radius. This opens the interesting possibility of
determining this quantity and related ones, such as the effective temperature
or luminosity, from comparison between model predictions and observations, in
other targets for which these parameters are not well determined.Comment: Accepted for publication in the MNRA
Tomographic Characterization of Three-Qubit Pure States with Only Two-Qubit Detectors
A tomographic process for three-qubit pure states using only pairwise
detections is presented.Comment: 3 pages; revtex4; v2: the focus on tomography was emphasized and the
experimental procedure detailed; v3: the text was improved in clarity, some
mistakes were correcte
Bifurcation points in the theory of axially symmetric arc cathodes
Steady-state current transfer from arc plasmas to axially symmetric cathodes is treated in the framework of
the model of nonlinear surface heating. An approach is developed to calculate the bifurcation points at which
three-dimensional spot-mode solutions branch off from solutions describing the diffuse mode and axially
symmetric spot modes. In particular, the first bifurcation point positioned on the diffuse-mode solution has
been calculated, and thus its stability limit, i.e., the current below which the diffuse mode becomes unstable.
Calculation results are given for the case of a tungsten cathode in the form of a circular cylinder in high pressure plasmas. The effect produced on the stability limit by variations of control parameters ~cathode
dimensions, work function of the cathode material, plasma-producing gas, and its pressure! is studied and
found to conform to trends observed experimentally. The stability limit is found to be much more sensitive to
variations of control parameters than characteristics of the diffuse mode are, the strongest effect being pro duced by variations of cathode dimensions and of the work function of the cathode material. This finding
conforms to the fact that the diffuse-spot transition is difficult to reproduce in the experiment.info:eu-repo/semantics/publishedVersio
Joule heat generation in thermionic cathodes of high-pressure arc discharges
The nonlinear surface heating model of plasma-cathode interaction in high-pressure arcs is
extended to take into account the Joule effect inside the cathode body. Calculation results are given
for different modes of current transfer to tungsten cathodes of different configurations in argon
plasmas of atmospheric or higher pressures. Special attention is paid to analysis of energy balances
of the cathode and the near-cathode plasma layer. In all the cases, the variation of potential inside
the cathode is much smaller than the near-cathode voltage drop. However, this variation can be
comparable to the volt equivalent of the energy flux from the plasma to the cathode and then the
Joule effect is essential. Such is the case of the diffuse and mixed modes on rod cathodes at high
currents, where the Joule heating causes a dramatic change of thermal and electrical regimes of the
cathode. The Joule heating has virtually no effect over characteristics of spots on rod and infinite
planar cathodes.info:eu-repo/semantics/publishedVersio
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