193,823 research outputs found
Fluorine Abundances of Galactic Low-Metallicity Giants
With abundances and 2{\sigma} upper limits of fluorine (F) in seven
metal-poor field giants, nucleosynthesis of stellar F at low metallicity is
discussed. The measurements are derived from the HF(1-0) R9 line at 23358{\AA}
using nearinfrared K-band high-resolution spectra obtained with CRIRES at the
Very Large Telescope. The sample reaches lower metallicities than previous
studies on F of field giants, ranging from [Fe/H] = -1.56 down to -2.13.
Effects of three-dimensional model atmospheres on the derived F and O
abundances are quantitatively estimated and shown to be insignificant for the
program stars. The observed F yield in the form of [F/O] is compared with two
sets of Galactic chemical evolution models, which quantitatively demonstrate
the contribution of Type II supernova (SN II) {\nu}-process and asymptotic
giant branch/Wolf-Rayet stars. It is found that at this low-metallicity region,
models cannot well predict the observed distribution of [F/O], while the
observations are better fit by models considering an SN II {\nu}-process with a
neutrino energy of E_{\nu} = 3 x 10^53 erg. Our sample contains HD 110281, a
retrograde orbiting low-{\alpha} halo star, showing a similar F evolution as
globular clusters. This supports the theory that such halo stars are possibly
accreted from dwarf galaxy progenitors of globular clusters in the halo.Comment: 8 pages, 8 figures, 2 tables, published in The Astrophysical Journa
Towards First-principles Electrochemistry
Chemisorbed molecules at a fuel cell electrode are a very sensitive probe of
the surrounding electrochemical environment, and one that can be accurately
monitored with different spectroscopic techniques. We develop a comprehensive
electrochemical model to study molecular chemisorption at either constant
charge or fixed applied voltage, and calculate from first principles the
voltage dependence of vibrational frequencies -- the vibrational Stark effect
-- for CO adsorbed on close-packed platinum electrodes. The predicted
vibrational Stark slopes are found to be in very good agreement with
experimental electrochemical spectroscopy data, thereby resolving previous
controversies in the quantitative interpretation of in-situ experiments and
elucidating the relation between canonical and grand-canonicaldescriptions of
vibrational surface phenomena.Comment: 10 pages, 2 figure
The design of a linear L-band high power amplifier for mobile communication satellites
A linear L-band solid state high power amplifier designed for the space segment of the Mobile Satellite (MSAT) mobile communication system is described. The amplifier is capable of producing 35 watts of RF power with multitone signal at an efficiency of 25 percent and with intermodulation products better than 16 dB below carrier
Threshold resummation for exclusive B meson decays
We argue that double logarithmic corrections need to be
resumed in perturbative QCD factorization theorem for exclusive meson
decays, when the end-point region with a momentum fraction is
important. These double logarithms, being of the collinear origin, are absorbed
into a quark jet function, which is defined by a matrix element of a quark
field attached by a Wilson line. The factorization of the jet function from the
decay is proved to all orders. Threshold resummation for
the jet function leads to a universal, {\it i.e.}, process-independent, Sudakov
factor, whose qualitative behavior is analyzed and found to smear the end-point
singularities in heavy-to-light transition form factors.Comment: 10 pages, more details are include
Eulerian and modified Lagrangian approaches to multi-dimensional condensation and collection
Turbulence is argued to play a crucial role in cloud droplet growth. The
combined problem of turbulence and cloud droplet growth is numerically
challenging. Here, an Eulerian scheme based on the Smoluchowski equation is
compared with two Lagrangian superparticle (or su- perdroplet) schemes in the
presence of condensation and collection. The growth processes are studied
either separately or in combination using either two-dimensional turbulence, a
steady flow, or just gravitational acceleration without gas flow. Good
agreement between the differ- ent schemes for the time evolution of the size
spectra is observed in the presence of gravity or turbulence. Higher moments of
the size spectra are found to be a useful tool to characterize the growth of
the largest drops through collection. Remarkably, the tails of the size spectra
are reasonably well described by a gamma distribution in cases with gravity or
turbulence. The Lagrangian schemes are generally found to be superior over the
Eulerian one in terms of computational performance. However, it is shown that
the use of interpolation schemes such as the cloud-in-cell algorithm is
detrimental in connection with superparticle or superdroplet approaches.
Furthermore, the use of symmetric over asymmetric collection schemes is shown
to reduce the amount of scatter in the results.Comment: 36 pages, 17 figure
Temperature dependence of thermal conductivity in 1D nonlinear lattices
We examine the temperature dependence of thermal conductivity of one
dimensional nonlinear (anharmonic) lattices with and without on-site potential.
It is found from computer simulation that the heat conductivity depends on
temperature via the strength of nonlinearity. Based on this correlation, we
make a conjecture in the effective phonon theory that the mean-free-path of the
effective phonon is inversely proportional to the strength of nonlinearity. We
demonstrate analytically and numerically that the temperature behavior of the
heat conductivity is not universal for 1D harmonic lattices
with a small nonlinear perturbation. The computer simulations of temperature
dependence of heat conductivity in general 1D nonlinear lattices are in good
agreements with our theoretic predictions. Possible experimental test is
discussed.Comment: 6 pages and 2 figures. Accepted for publication in Europhys. Let
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