92,522 research outputs found
Far-infrared study of K giants in the solar neighborhood: Connection between Li enrichment and mass-loss
We searched for a correlation between the two anomalous properties of K
giants: Li enhancement and IR excess from an unbiased survey of a large sample
of RGB stars. A sample of 2000 low-mass K giants with accurate astrometry from
the Hipparcos catalog was chosen for which Li abundances have been determined
from low-resolution spectra. Far-infrared data were collected from the
and catalogs. To probe the correlation between the two anomalies, we
supplemented 15 Li-rich K giants discovered from this sample with 25 known
Li-rich K giants from other studies. Dust shell evolutionary models and
spectral energy distributions were constructed using the code DUSTY to estimate
different dust shell properties, such as dust evolutionary time scales, dust
temperatures, and mass-loss rates. Among 2000 K giants, we found about two
dozen K giants with detectable far-IR excess, and surprisingly, none of them
are Li-rich. Similarly, the 15 new Li-rich K giants that were identified from
the same sample show no evidence of IR excess. Of the total 40 Li-rich K
giants, only 7 show IR excess. Important is that K giants with Li enhancement
and/or IR excess begin to appear only at the bump on the RGB. Results show that
K giants with IR excess are very rare, similar to K giants with Li enhancement.
This may be due to the rapid differential evolution of dust shell and Li
depletion compared to RGB evolutionary time scales. We also infer from the
results that during the bump evolution, giants probably undergo some internal
changes, which are perhaps the cause of mass-loss and Li-enhancement events.
However, the available observational results do not ascertain that these
properties are correlated. That a few Li-rich giants have IR excess seems to be
pure coincidence.Comment: Accepted for Publication in Astronomy & Astrophysics, 6 figures, 5
tables, 19 page
Upflows in the upper transition region of the quiet Sun
We investigate the physical meaning of the prominent blue shifts of Ne VIII,
which is observed to be associated with quiet-Sun network junctions (boundary
intersections), through data analyses combining force-free-field extrapolations
with EUV spectroscopic observations. For a middle-latitude region, we
reconstruct the magnetic funnel structure in a sub-region showing faint
emission in EIT-Fe 195. This funnel appears to consist of several smaller
funnels that originate from network lanes, expand with height and finally merge
into a single wide open-field region. However, the large blue shifts of Ne VIII
are generally not associated with open fields, but seem to be associated with
the legs of closed magnetic loops. Moreover, in most cases significant upflows
are found in both of the funnel-shaped loop legs. These quasi-steady upflows
are regarded as signatures of mass supply to the coronal loops rather than the
solar wind. Our observational result also reveals that in many cases the
upflows in the upper transition region (TR) and the downflows in the middle TR
are not fully cospatial. Based on these new observational results, we suggest
different TR structures in coronal holes and in the quiet Sun.Comment: 4 pages, 4 figures, will appear in the Proceedings of the Solar wind
12 conferenc
A New Kind of Uniformly Accelerated Reference Frames
A new kind of uniformly accelerated reference frames with a line-element
different from the M{\o}ller and Rindler ones is presented, in which every
observer at consts. has the same constant acceleration. The laws of
mechanics are checked in the new kind of frames. Its thermal property is
studied. The comparison with the M{\o}ller and Rindler uniform accelerated
reference frames is also made.Comment: 10 pages, 2 figures. to appear in Int. J. Mod. Phys.
Re-examining the electronic structure of germanium: A first-principle study
We report results from an efficient, robust, ab-initio method for
self-consistent calculations of electronic and structural properties of Ge. Our
non-relativistic calculations employed a generalized gradient approximation
(GGA) potential and the linear combination of atomic orbitals (LCAO) formalism.
The distinctive feature of our computations stem from the use of
Bagayoko-Zhao-Williams-Ekuma-Franklin (BZW-EF) method. Our results are in
agreement with experimental ones where the latter are available. In particular,
our theoretical, indirect band gap of 0.65 eV, at the experimental lattice
constant of 5.66 \AA{}, is in excellent agreement with experiment. Our
predicted, equilibrium lattice constant is 5.63 \AA{}, with a corresponding
indirect band gap of 0.65 eV and a bulk modulus of 80 GPa. We also calculated
the effective masses in various directions with respect to the point.Comment: 10 Pages, 3 Figures, and 1 tabl
WxN1–x alloys as diffusion barriers between Al and Si
Reactively sputtered tungsten nitride (WxN1–x) layers are investigated as diffusion barriers between Al overlayers and Si shallow n + -p junctions. Both amorphous W80 N20 and polycrystalline W60 N40 films were found to be very effective in preserving the integrity of the n + -p diodes for 30-min vacuum annealing up to 575 °C. Diode failure at higher temperatures is caused by localized penetration of Al into through the WxN1–x barriers. The effectiveness of the barrier decreases for polycrystalline W90 N10 and is worse for pure W
Controlling spin in an electronic interferometer with spin-active interfaces
We consider electronic current transport through a ballistic one-dimensional
quantum wire connected to two ferromagnetic leads. We study the effects of the
spin-dependence of interfacial phase shifts (SDIPS) acquired by electrons upon
scattering at the boundaries of the wire. The SDIPS produces a spin splitting
of the wire resonant energies which is tunable with the gate voltage and the
angle between the ferromagnetic polarizations. This property could be used for
manipulating spins. In particular, it leads to a giant magnetoresistance effect
with a sign tunable with the gate voltage and the magnetic field applied to the
wire.Comment: 5 pages, 3 figures. to be published in Europhysics Letter
Local Density Approximation Description of Electronic Properties of Wurtzite Cadmium Sulfide (w-CdS)
We present calculated, electronic and related properties of wurtzite cadmium
sulfide (w-CdS). Our ab-initio, non-relativistic calculations employed a local
density functional approximation (LDA) potential and the linear combination of
atomic orbitals (LCAO). Following the Bagayoko, Zhao, and Williams (BZW)
method, we solved self-consistently both the Kohn-Sham equation and the
equation giving the ground state density in terms of the wave functions of the
occupied states. Our calculated, direct band gap of 2.47 eV, at the point, is
in excellent agreement with experiment. So are the calculated density of states
and the electron effective mass. In particular, our results reproduce the peaks
in the conduction band density of states, within the experimental
uncertainties.Comment: 22 Pages 4 Figure
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