5,909 research outputs found
The phase diagram of ice: a quasi-harmonic study based on a flexible water model
The phase diagram of ice is studied by a quasi-harmonic approximation. The
free energy of all experimentally known ice phases has been calculated with the
flexible q-TIP4P/F model of water. The only exception is the high pressure ice
X, in which the presence of symmetric O-H-O bonds prevents its modeling with
this empirical interatomic potential. The simplicity of our approach allows us
to study ice phases at state points of the T-P plane that have been omitted in
previous simulations using free energy methods based on thermodynamic
integration. The effect in the phase diagram of averaging the proton disorder
that appears in several ice phases has been studied. It is found particularly
relevant for ice III, at least for cell sizes typically used in phase
coexistence simulations. New insight into the capability of the employed water
model to describe the coexistence of ice phases is presented. We find that the
H-ordered ices IX and XIV, as well as the H-disordered ice XII, are
particularly stable for this water model. This fact disagrees with experimental
data. The unexpected large stability of ice IX is a property related to the
TIP4P-character of the water model. Only after omission of these three stable
ice phases, the calculated phase diagram becomes in reasonable qualitative
agreement to the experimental one in the T-P region corresponding to ice Ih,
II, III, V, and VI. The calculation of the phase diagram in the quantum and
classical limits shows that the most important quantum effect is the
stabilization of ice II due to its lower zero-point energy when compared to
that one of ices Ih, III, and V.Comment: 13 pages, 8 figures, 5 table
Can new generations explain neutrino masses?
In this talk we explore the possibility that the smallness of the observed
neutrino masses is naturally understood in a modified version of the standard
model with N extra generations of fermions and N right-handed neutrinos, in
which light neutrino masses are generated at two loops. We find that with N = 1
it is not possible to fit the observed spectrum of masses and mixings while
with N = 2 it is. Within this extension, we analyse the parameters which are
allowed and the possible phenomenological signals of the model in future
experiments. Contribution to the proceedings of Les Rencontres de Moriond EW
2011, Young Scientist Forum
Spectroscopic and physical parameters of Galactic O-type stars. II. Observational constraints on projected rotational and extra broadening velocities as a function of fundamental parameters and stellar evolution
Rotation is of key importance for the evolution of hot massive stars,
however, the rotational velocities of these stars are difficult to determine.
Based on our own data for 31 Galactic O stars and incorporating similar data
for 86 OB supergiants from the literature, we aim at investigating the
properties of rotational and extra line-broadening as a function of stellar
parameters and at testing model predictions about the evolution of stellar
rotation. Fundamental stellar parameters were determined by means of the code
FASTWIND. Projected rotational and extra broadening velocities originate from a
combined Ft + GOF method. Model calculations published previously were used to
estimate the initial evolutionary masses. The sample O stars with Minit > 50
Msun rotate with less that 26% of their break-up velocity, and they also lack
objects with v sin i 35 Msun on the
hotter side of the bi-stability jump, the observed and predicted rotational
rates agree quite well; for those on the cooler side of the jump, the measured
velocities are systematically higher than the predicted ones. In general, the
derived extra broadening velocities decrease toward cooler Teff, whilst for
later evolutionary phases they appear, at the same v sin i, higher for
high-mass stars than for low-mass ones. None of the sample stars shows extra
broadening velocities higher than 110 km/s. For the majority of the more
massive stars, extra broadening either dominates or is in strong competition
with rotation. Conclusions: For OB stars of solar metallicity, extra broadening
is important and has to be accounted for in the analysis. When appearing at or
close to the zero-age main sequence, most of the single and more massive stars
rotate slower than previously thought. Model predictions for the evolution of
rotation in hot massive stars may need to be updated.Comment: 15 pages, 10 figures, accepted for publication in A &
Rare-gas solids under pressure: A path-integral Monte Carlo simulation
Rare-gas solids (Ne, Ar, Kr, and Xe) under hydrostatic pressure up to 30 kbar
have been studied by path-integral Monte Carlo simulations in the
isothermal-isobaric ensemble. Results of these simulations have been compared
with available experimental data and with those obtained from a quasiharmonic
approximation (QHA). This comparison allows us to quantify the overall
anharmonicity of the lattice vibrations and its influence on several structural
and thermodynamic properties of rare-gas solids. The vibrational energy
increases with pressure, but this increase is slower than that of the elastic
energy, which dominates at high pressures. In the PIMC simulations, the
vibrational kinetic energy is found to be larger than the corresponding
potential energy, and the relative difference between both energies decreases
as the applied pressure is raised. The accuracy of the QHA increases for rising
pressure.Comment: 9 pages, 6 figure
Infrared 3D Observations of Nearby Active Galaxies
We present multi-wavelength imaging observations of three nearby and famous
active galaxies obtained with NICMOS, ISOCAM and the MPE near-IR integral field
spectrometer. The data reveal a variety of features and properties that are
missed in optical studies and in traditional IR monodimensional spectroscopy.Comment: 6 pages, to appear in "Imaging the Universe in Three Dimensions:
Astrophysics with Advanced Multi-Wavelength Imaging Devices", eds. W. van
Breugel and J. Bland-Hawthorn, needs pasp3D.st
Profit efficiency among Kenyan smallholders milk producers: A case study of Meru-South district, Kenya
Production inefficiency is usually analyzed by economical efficiency, which is composed of two components-technical and allocative efficiencies. This study provided a direct measure of production efficiency of the smallholder milk producers in Kenya using a stochastic profit frontier and inefficiency model. The primary data were collected, using IMPACT (intergrated modeling platform for mixed animal crops systems) structured questionnaire and includes four conventional inputs and socio-economic factors affecting production. The result showed that profit efficiencies of the sampled farmers varied widely between 26% and 73% with a mean of 60% suggesting that an estimated 40% of the profit is lost due to a combination of both technical and allocative inefficiencies in the smallholder dairy milk production. This study further observed that level of education, experience, and the size of the farm influenced profit efficiency positively while profit efficiency decreased with age. This implies that profit inefficiency among smallholder dairy milk producers can be reduced significantly with improvement in the level of education of sampled farmer
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