11,065 research outputs found
Radial Velocity Curves of Ellipsoidal Red Giant Binaries in the Large Magellanic Cloud
Ellipsoidal red giant binaries are close binary systems where an unseen,
relatively close companion distorts the red giant, leading to light variations
as the red giant moves around its orbit. These binaries are likely to be the
immediate evolutionary precursors of close binary planetary nebula and
post-asymptotic giant branch and post-red giant branch stars. Due to the MACHO
and OGLE photometric monitoring projects, the light variability nature of these
ellipsoidal variables has been well studied. However, due to the lack of radial
velocity curves, the nature of their masses, separations, and other orbital
details has so far remained largely unknown. In order to improve this
situation, we have carried out spectral monitoring observations of a large
sample of 80 ellipsoidal variables in the Large Magellanic Cloud and we have
derived radial velocity curves. At least 12 radial velocity points with good
quality were obtained for most of the ellipsoidal variables. The radial
velocity data are provided with this paper. Combining the photometric and
radial velocity data, we present some statistical results related to the binary
properties of these ellipsoidal variables.Comment: 10 pages, 8 figures, 3 table
Radii and Binding Energies of Nuclei in the Alpha-Cluster Model
The alpha-cluster model is based on two assumptions that the proton-neutron
pair interactions are responsible for adherence between alpha-clusters and that
the NN-interaction in the alpha-clusters is isospin independent. It allows one
to estimate the Coulomb energy and the short range inter-cluster bond energy in
dependence on the number of clusters. The charge radii are calculated on the
number of alpha-clusters too. Unlike the Weizsacker formula in this model the
binding energies of alpha-clusters and excess neutrons are estimated
separately. The calculated values are in a good agreement with the experimental
data.Comment: Latex2e 2.09, 13 pages, 4 figure
Charge Radii of beta-Stable Nuclei
In previous work it was shown that the radius of nucleus R is determined by
the alpha-cluster structure and can be estimated on the number of
alpha-clusters disregarding to the number of excess neutrons. A hypothesis also
was made that the radius R_m of a beta-stable isotope, which is actually
measured at electron scattering experiments, is determined by the volume
occupied by the matter of the core plus the volume occupied by the peripheral
alpha-clusters. In this paper it is shown that the condition R_m = R restricts
the number of excess neutrons filling the core to provide the beta-stability.
The number of peripheral clusters can vary from 1 to 5 and the value of R for
heavy nuclei almost do not change, whereas the number of excess neutrons should
change with the number of peripheral clusters to get the value of R_m close to
R. It can explain the path of the beta-stability and its width. The radii R_m
of the stable isotopes with 12 =< Z =< 83 and the alpha-decay isotopes with 84
=< Z =< 116 that are stable to beta-decay have been calculated.Comment: Latex2e 2.09, 10 pages, 3 figure
A newly discovered stellar type: dusty post-red giant branch stars in the Magellanic Clouds
Context: We present a newly discovered class of low-luminosity, dusty,
evolved objects in the Magellanic Clouds. These objects have dust excesses,
stellar parameters, and spectral energy distributions similar to those of dusty
post-asymptotic giant branch (post-AGB) stars. However, they have lower
luminosities and hence lower masses. We suggest that they have evolved off the
red giant branch (RGB) instead of the AGB as a result of binary interaction.
Aims: In this study we aim to place these objects in an evolutionary context
and establish an evolutionary connection between RGB binaries (such as the
sequence E variables) and our new sample of objects. Methods: We compared the
theoretically predicted birthrates of the progeny of RGB binaries to the
observational birthrates of the new sample of objects. Results: We find that
there is order-of-magnitude agreement between the observed and predicted
birthrates of post-RGB stars. The sources of uncertainty in the birthrates are
discussed; the most important sources are probably the observational
incompleteness factor and the post-RGB evolution rates. We also note that
mergers are relatively common low on the RGB and that stars low on the RGB with
mid-IR excesses may recently have undergone a merger. Conclusions: Our sample
of dusty post-RGB stars most likely provides the first observational evidence
for a newly discovered phase in binary evolution: post-RGB binaries with
circumstellar dust.Comment: Accepted for publication in Astronomy and Astrophysics Letter
Optical spectroscopy study of Nd(O,F)BiS2 single crystals
We present an optical spectroscopy study on F-substituted NdOBiS
superconducting single crystals grown using KCl/LiCl flux method. The
measurement reveals a simple metallic response with a relatively low screened
plasma edge near 5000 \cm. The plasma frequency is estimated to be 2.1 eV,
which is much smaller than the value expected from the first-principles
calculations for an electron doping level of x=0.5, but very close to the value
based on a doping level of 7 of itinerant electrons per Bi site as
determined by ARPES experiment. The energy scales of the interband transitions
are also well reproduced by the first-principles calculations. The results
suggest an absence of correlation effect in the compound, which essentially
rules out the exotic pairing mechanism for superconductivity or scenario based
on the strong electronic correlation effect. The study also reveals that the
system is far from a CDW instability as being widely discussed for a doping
level of x=0.5.Comment: 5 pages, 5 figure
Impacts of Strategic Grazing on Herbage Accumulation and Nutritive Value of Naturalised Hill Pasture
Impacts of Strategic Grazing on Density and Ground Cover of Naturalised Hill Pasture
Low ground cover by perennial species is a major problem in naturalised pasture on steep hill country in southern Australia. This leads to water and nutrient runoff, recharge to groundwater, and soil erosion, all of which impact on the environmental sustainability and profitability of grazing enterprises. Restoration of perennial components, particularly the native grasses for these marginal land classes, is of great importance for improving water balance, halting land degradation (Ridley et al. 1997), extending growing season, and increasing pasture production. The objective of this study was to use strategic grazing management to increase the ground cover and plant population density of perennial species in steep hill country
Forcing and Velocity Correlations in a Vibrated Granular Monolayer
The role of forcing on the dynamics of a vertically shaken granular monolayer
is investigated. Using a flat plate, surprising negative velocity correlations
are measured. A mechanism for this anti-correlation is proposed with support
from both experimental results and molecular dynamics simulations. Using a
rough plate, velocity correlations are positive, and the velocity distribution
evolves from a gaussian at very low densities to a broader distribution at high
densities. These results are interpreted as a balance between stochastic
forcing, interparticle collisions, and friction with the plate.Comment: 4 pages, 5 figure
Concurrent bandits and cognitive radio networks
We consider the problem of multiple users targeting the arms of a single
multi-armed stochastic bandit. The motivation for this problem comes from
cognitive radio networks, where selfish users need to coexist without any side
communication between them, implicit cooperation or common control. Even the
number of users may be unknown and can vary as users join or leave the network.
We propose an algorithm that combines an -greedy learning rule with a
collision avoidance mechanism. We analyze its regret with respect to the
system-wide optimum and show that sub-linear regret can be obtained in this
setting. Experiments show dramatic improvement compared to other algorithms for
this setting
- …