773 research outputs found
Viscosities of the Gay-Berne nematic liquid crystal
We present molecular dynamics simulation measurements of the viscosities of
the Gay-Berne phenomenological model of liquid crystals in the nematic and
isotropic phases. The temperature dependence of the rotational and shear
viscosities, including the nonmonotonic behavior of one shear viscosity are in
good agreement with experimental data. The bulk viscosities are significantly
larger than the shear viscosities, again in agreement with experiment.Comment: 11 pages, 4 Postscript figures, Revte
A multiwavelength approach to the SFR estimation in galaxies at intermediate redshifts
We use a sample of 7 starburst galaxies at intermediate redshifts (z ~ 0.4
and z ~ 0.8) with observations ranging from the observed ultraviolet to 1.4
GHz, to compare the star formation rate (SFR) estimators which are used in the
different wavelength regimes. We find that extinction corrected Halpha
underestimates the SFR, and the degree of this underestimation increases with
the infrared luminosity of the galaxies. Galaxies with very different levels of
dust extinction as measured with SFR(IR)/SFR(Halpha, uncorrected for
extinction) present a similar attenuation A[Halpha], as if the Balmer lines
probed a different region of the galaxy than the one responsible for the bulk
of the IR luminosity for large SFRs. In addition, SFR estimates derived from
[OII]3727 match very well those inferred from Halpha after applying the
metallicity correction derived from local galaxies. SFRs estimated from the UV
luminosities show a dichotomic behavior, similar to that previously reported by
other authors in galaxies at z <~ 0.4. Here we extend this result up to z ~
0.8. Finally, one of the studied objects is a luminous compact galaxy (LCG)
that may be suffering similar dust-enshrouded star formation episodes. These
results highlight the relevance of quantifying the actual L(IR) of LCGs, as
well as that of a much larger and generic sample of luminous infrared galaxies,
which will be possible after the launch of SIRTF.Comment: Accepted for publication in The Astrophysical Journa
Non-LTE Model Atmospheres for Late-Type Stars II. Restricted NLTE Calculations for a Solar-Like Atmosphere
We test our knowledge of the atomic opacity in the solar UV spectrum. Using
the atomic data compiled in Paper I from modern, publicly available, databases,
we perform calculations that are confronted with space-based observations of
the Sun. At wavelengths longer than about 260 nm, LTE modeling can reproduce
quite closely the observed fluxes; uncertainties in the atomic line data
account fully for the differences between calculated and observed fluxes. At
shorter wavelengths, departures from LTE appear to be important, as our LTE and
restricted NLTE calculations differ. Analysis of visible-near infrared Na I and
O I lines, two species that produce a negligible absorption in the UV, shows
that observed departures from LTE for theses species can be reproduced very
accurately with restricted (fixed atmospheric structure) NLTE calculations.Comment: 13 pages, 11 figures, to appear in Ap
Investigation of the Exclusive 3He(e,e'pp)n Reaction
Cross sections for the 3He(e,e'pp)n reaction were measured over a wide range
of energy and three- momentum transfer. At a momentum transfer q=375 MeV/c,
data were taken at transferred energies omega ranging from 170 to 290 MeV. At
omega=220 MeV, measurements were performed at three q values (305, 375, and 445
MeV/c). The results are presented as a function of the neutron momentum in the
final-state, as a function of the energy and momentum transfer, and as a
function of the relative momentum of the two-proton system. The data at neutron
momenta below 100 MeV/c, obtained for two values of the momentum transfer at
omega=220 MeV, are well described by the results of continuum-Faddeev
calculations. These calculations indicate that the cross section in this domain
is dominated by direct two-proton emission induced by a one-body hadronic
current. Cross section distributions determined as a function of the relative
momentum of the two protons are fairly well reproduced by continuum-Faddeev
calculations based on various realistic nucleon-nucleon potential models. At
higher neutron momentum and at higher energy transfer, deviations between data
and calculations are observed that may be due to contributions of isobar
currents.Comment: 14 pages, 1 table, 17 figure
High-Resolution Spectroscopy of the Transiting Planet Host Star TrES-1
We report on a spectroscopic determination of the stellar parameters and
chemical abundances for the parent star of the transiting planet TrES-1. Based
on a detailed analysis of iron lines in our Keck and HET spectra we derive
K, , and [Fe/H] . By measuring the \ion{Ca}{2} activity indicator and by putting useful
upper limits on the Li abundance we constrain the age of TrES-1 to be Gyr. By comparing theoretical stellar evolution models with the
observational parameters we obtain , and
. Our improved estimates of the stellar
parameters are utilized in a new analysis of the transit photometry of TrES-1
to derive a mass , a radius , and an inclination . The improved planetary mass and radius estimates
provide the grounds for new crucial tests of theoretical models of evolution
and evaporation of irradiated extrasolar giant planets.Comment: 14 pages, 3 figures, Astrophysical Journal Letters, accepte
The solar chromosphere at high resolution with IBIS. I. New insights from the Ca II 854.2 nm line
(Abridged)
Aims: In this paper, we seek to establish the suitability of imaging
spectroscopy performed in the Ca II 854.2 nm line as a means to investigate the
solar chromosphere at high resolution.
Methods: We utilize monochromatic images obtained with the Interferometric
BIdimensional Spectrometer (IBIS) at multiple wavelengths within the Ca II
854.2 nm line and over several quiet areas. We analyze both the morphological
properties derived from narrow-band monochromatic images and the average
spectral properties of distinct solar features such as network points,
internetwork areas and fibrils.
Results: The spectral properties derived over quiet-Sun targets are in full
agreement with earlier results obtained with fixed-slit spectrographic
observations, highlighting the reliability of the spectral information obtained
with IBIS. Furthermore, the very narrowband IBIS imaging reveals with much
clarity the dual nature of the Ca II 854.2 nm line: its outer wings gradually
sample the solar photosphere, while the core is a purely chromospheric
indicator. The latter displays a wealth of fine structures including bright
points, akin to the Ca II H2V and K2V grains, as well as fibrils originating
from even the smallest magnetic elements. The fibrils occupy a large fraction
of the observed field of view even in the quiet regions, and clearly outline
atmospheric volumes with different dynamical properties, strongly dependent on
the local magnetic topology. This highlights the fact that 1-D models
stratified along the vertical direction can provide only a very limited
representation of the actual chromospheric physics.Comment: 13 pages, 8 figures. Accepted in A&A. Revised version after referee's
comments. New Fig. 1 and 7. Higher quality figures in
http://www.arcetri.astro.it/~gcauzzi/papers/ibis.caii.pd
Beryllium abundances in stars hosting giant planets
We have derived beryllium abundances in a wide sample of stars hosting
planets, with spectral types in the range F7V-K0V, aimed at studying in detail
the effects of the presence of planets on the structure and evolution of the
associated stars. Predictions from current models are compared with the derived
abundances and suggestions are provided to explain the observed
inconsistencies. We show that while still not clear, the results suggest that
theoretical models may have to be revised for stars with Teff<5500K. On the
other hand, a comparison between planet host and non-planet host stars shows no
clear difference between both populations. Although preliminary, this result
favors a ``primordial'' origin for the metallicity ``excess'' observed for the
planetary host stars. Under this assumption, i.e. that there would be no
differences between stars with and without giant planets, the light element
depletion pattern of our sample of stars may also be used to further
investigate and constraint Li and Be depletion mechanisms.Comment: A&A in press -- accepted on the 22/02/2002 (11 pages, 6 figures
included
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