634 research outputs found
Predmet istraživanja: kataliza razvijanja vodika na živinim elektrodama
Cases of electrochemical hydrogen evolution catalysis often represent problems that electrode kineticists find difficult to solve. On the other hand, they offer chances of finding electrochemical methods for specific and sensitive studies of a variety of substances, notably those of biological importance.Procesi katalize elektrokemijskog razvijanja vodika često predstavljaju teško rješive probleme za istraživače kinetike elektrodnih reakcija. Oni se, međutim, mogu iskoristiti za razvoj jedinstvenih i osjetljivih elektrokemijskih metoda za proučavanje različitih tvari, posebice onih od biološke važnosti. U radu se opisuje povijest istraživanja kataliziranog razvijanja vodika na živinim elektrodama i navode se tri tipa katalizatora
Limb darkening in spherical stellar atmospheres
(Abridged) Context. Stellar limb darkening, I({\mu} = cos{\theta}), is an
important constraint for microlensing, eclipsing binary, planetary transit, and
interferometric observations, but is generally treated as a parameterized
curve, such as a linear-plus-square-root law. Many analyses assume
limb-darkening coefficients computed from model stellar atmospheres. However,
previous studies, using I({\mu}) from plane- parallel models, have found that
fits to the flux-normalized curves pass through a fixed point, a common {\mu}
location on the stellar disk, for all values of T eff, log g and wavelength.
Aims. We study this fixed {\mu}-point to determine if it is a property of the
model stellar atmospheres or a property of the limb-darkening laws.
Furthermore, we use this limb-darkening law as a tool to probe properties of
stellar atmospheres for comparison to limb- darkening observations. Methods.
Intensities computed with plane-parallel and spherically-symmetric Atlas models
(characterized by the three fundamental parameters L\star, M\star and R\star)
are used to reexamine the existence of the fixed {\mu}-point for the
parametrized curves. Results. We find that the intensities from our spherical
models do not have a fixed point, although the curves do have a minimum spread
at a {\mu}-value similar to the parametrized curves. We also find that the
parametrized curves have two fixed points, {\mu}1 and {\mu}2, although {\mu}2
is so close to the edge of the disk that it is missed using plane-parallel
atmospheres. We also find that the spherically- symmetric models appear to
agree better with published microlensing observations relative to
plane-parallel models.Comment: 8 pages, 8 figures, figures 4 and 6 have lower resolution. A&A in
pres
Using limb darkening to measure fundamental parameters of stars
Context. Limb darkening is an important tool for understanding stellar
atmospheres, but most observations measuring limb darkening assume various
parameterizations that yield no significant information about the structure of
stellar atmospheres. Aims. We use a specific limb-darkening relation to study
how the best-fit coefficients relate to fundamental stellar parameters from
spherically symmetric model stellar atmospheres. Methods. Using a grid of
spherically symmetric Atlas model atmospheres, we compute limb-darkening
coefficients, and develop a novel method to predict fundamental stellar
parameters. Results. We find our proposed method predicts the mass of stellar
atmosphere models given only the radius and limb-darkening coefficients,
suggesting that microlensing, interferometric, transit and eclipse observations
can constrain stellar masses. Conclusions. This novel method demonstrates that
limb-darkening parameterizations contain important information about the
structure of stellar atmospheres, with the potential to be a valuable tool for
measuring stellar masses.Comment: 8 pages, 6 figures, 2 tables, A&A accepte
Extended-Source Effect and Chromaticity in Two-Point-Mass Microlensing
We explore the sensitivity of two-point-mass gravitational microlensing to
the extended nature of the source star, as well as the related sensitivity to
its limb darkening. We demonstrate that the sensitive region, usually
considered to be limited to a source-diameter-wide band along the caustic, is
strongly expanded near cusps, most prominently along their outer axis. In the
case of multi-component caustics, facing cusps may form a region with a
non-negligible extended-source effect spanning the gap between them. We
demonstrate that for smaller sources the size of the sensitive region extending
from a cusp measured in units of source radii increases, scaling as the inverse
cube root of the radius. We study the extent of different sensitivity contours
and show that for a microlensed Galactic bulge giant the probability of
encountering at least a 1% extended-source effect is higher than the
probability of caustic crossing by 40-60% when averaged over a typical range of
lens-component separations, with the actual value depending on the mass ratio
of the components. We derive analytical expressions for the extended-source
effect and chromaticity for a source positioned off the caustic. These formulae
are more generally applicable to any gravitational lens with a sufficiently
small source. Using exactly computed amplifications we test the often used
linear-fold caustic approximation and show that it may lead to errors on the
level of a few percent even in near-ideal caustic-crossing events. Finally, we
discuss several interesting cases of observed binary and planetary microlensing
events and point out the importance of our results for the measurement of
stellar limb darkening from microlensing light curves.Comment: 25 pages, 16 figures; accepted by The Astrophysical Journal.
Discussion of probabilities and source-size dependence extended, figures
adde
Effect of NLTE model atmospheres on photometric amplitudes and phases of early B-type pulsating stars
We study all possible sources of inaccuracy in theoretical values of the
photometric observables, i.e. amplitude ratios and phase differences, of early
B-type main sequence pulsators. Here, we discuss effects of parameters coming
from both models of stellar atmospheres and linear nonadiabatic theory of
stellar pulsation. In particular, we evaluate for the first time the effect of
the departure from the LTE approximation. The atmospheric input comes from
line-blanketed, LTE and NLTE plane-parallel, hydrostatic models. To compute the
limb-darkening coefficients for NLTE models, we use the Least-Square Method
taking into account the accuracy of the flux conservation. We present effects
of NLTE atmospheres, chemical composition and opacities on theoretical values
of the photometric observables of early B-type pulsators. To this end, we
compute tables with the passband fluxes, flux derivatives over effective
temperature and gravity as well as the non-linear limb-darkening coefficients
in 12 most often used passbands, i.e. in the Str\"omgern system, , and in
the Johnson-Cousins-Glass system, . We make these tables public
available at the Wroc{\l}aw HELAS Web page, http://helas.astro.uni.wroc.pl.Comment: 13 pages, 2 tables, 17 figues submitted to A&
Resolving Stellar Atmospheres I: The H alpha line and comparisons to microlensing observations
We present work on H alpha spectral line characteristics in PHOENIX stellar
model atmospheres and their comparison to microlensing observations. We examine
in detail the H alpha equivalent width (EW) and the line shape characteristics
for effective temperatures of 4500K< Teff < 5600K where H alpha is a strong
spectral feature. We find that H alpha EW in models calculated under the
assumption of local thermodynamic equilibrium (LTE) is up to 15% smaller than
in models without this assumption, non-LTE models (NLTE) and that line shapes
vary significantly for the two model types. A comparison with available high
quality microlensing data, capable of tracing H alpha absorption across the
face of one G5III giant, shows that the LTE model that fits the EW best is
about 100K hotter than and the best-fitting NLTE model has a similar Teff as
predicted by the spectral type analysis of the observed star but agree within
the uncertainties of the observationally derived temperature. Neither LTE nor
NLTE models fit the line shape well. We suspect unmodelled chromospheric
emission. Line shape diagnostics suggest lower gravities than derived for the
star and are unacceptable low in the case of the LTE models. We show that EW
alone is insufficient for comparison to stellar model atmospheres, but combined
with a new shape parameter we define is promising. In stellar parameter ranges
where the H alpha line is strong, a NLTE approach of modeling stellar
atmospheres is not only beneficial but mandatory.Comment: 11 pages, 9 figures, accepted to Astronomy & Astrophysic
Gravitational microlensing as a test of stellar model atmospheres
We present calculations illustrating the potential of gravitational
microlensing to discriminate between classical models of stellar surface
brightness profiles and the recently computed ``Next Generation'' models of
Hauschildt et al. These spherically-symmetric models include a much improved
treatment of molecular lines in the outer atmospheres of cool giants -- stars
which are very typical sources in Galactic bulge microlensing events. We show
that the microlensing signatures of intensively monitored point and fold
caustic crossing events are readily able to distinguish between NextGen and the
classical models, provided a photometric accuracy of 0.01 magnitudes is
reached. This accuracy is now routinely achieved by alert networks, and hence
current observations can discriminate between such model atmospheres, providing
a unique insight on stellar photospheres.Comment: 4 pages, 4 figures, Astronomy & Astrophysics (Letters), vol. 388, L1
(2002
Can microlensing fold caustics reveal a second stellar limb-darkening coefficient?
Dense high-precision photometry of microlensed stars during a fold-caustic
passage can be used to reveal their intensity profiles from which the
temperature of the stellar atmosphere as function of fractional radius can be
derived. While the capabilities of current microlensing follow-up campaigns
such as PLANET allow a precise measurement of linear limb-darkening
coefficients, the residual signal of a second coefficient characterizing
square-root limb darkening is ~ 25 times smaller which prevents a proper
determination except for unlikely cases of very high caustic-peak-to-outside
relative magnifications with no adequate event being observed so far or for
source stars passing over a cusp singularity. Although the presence of limb
darkening can be well established from the data, a reliable measurement of the
index of an underlying power-law cannot be obtained.Comment: 4 pages with 4 EPS figures embedded, LaTeX2e using mn2e.cls. Final
version, minor changes. This is a preprint of an Article accepted for
publication in Monthly Notices of the Royal Astronomical Society, (C) 2004
The Royal Astronomical Societ
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