2,991 research outputs found
Spherically symmetric model stellar atmospheres and limb darkening II: limb-darkening laws, gravity-darkening coefficients and angular diameter corrections for FGK dwarf stars
Limb darkening is a fundamental ingredient for interpreting observations of
planetary transits, eclipsing binaries, optical/infrared interferometry and
microlensing events. However, this modeling traditionally represents limb
darkening by a simple law having one or two coefficients that have been derived
from plane-parallel model stellar atmospheres, which has been done by many
researchers. More recently, researchers have gone beyond plane-parallel models
and considered other geometries. We previously studied the limb-darkening
coefficients from spherically symmetric and plane-parallel model stellar
atmospheres for cool giant and supergiant stars, and in this investigation we
apply the same techniques to FGK dwarf stars. We present limb-darkening
coefficients, gravity-darkening coefficients and interferometric angular
diameter corrections from Atlas and SAtlas model stellar atmospheres. We find
that sphericity is important even for dwarf model atmospheres, leading to
significant differences in the predicted coefficients.Comment: 9 pages, 8 figures. Accepted for publication in A&
Limb Darkening and Planetary Transits II: Intensity profile correction factors for a grid of model stellar atmospheres
The ability to observe extrasolar planets transiting their stars has
profoundly changed our understanding of these planetary systems. However, these
measurements depend on how well we understand the properties of the host star,
such as radius, luminosity and limb darkening. Traditionally, limb darkening is
treated as a parameterization in the analysis, but these simple
parameterizations are not accurate representations of actual center-to-limb
intensity variations (CLIV) to the precision needed for interpreting these
transit observations. This effect leads to systematic errors for the measured
planetary radii and corresponding measured spectral features. We compute
synthetic planetary transits using model stellar atmosphere CLIV and
corresponding best-fit limb-darkening laws for a grid spherically symmetric
model stellar atmospheres. From these light curves we measure the differences
in flux as a function of the star's effective temperature, gravity, mass, and
the inclination of the planet's orbit.Comment: 10 pages, 8 figures, submitted to AAS journals. Comments welcom
Indicators of Mass in Spherical Stellar Atmospheres
Mass is the most important stellar parameter, but it is not directly
observable for a single star. Spherical model stellar atmospheres are
explicitly characterized by their luminosity (), mass () and
radius (), and observations can now determine directly and
. We computed spherical model atmospheres for red giants and for red
supergiants holding and constant at characteristic values
for each type of star but varying , and we searched the predicted flux
spectra and surface-brightness distributions for features that changed with
mass. For both stellar classes we found similar signatures of the star's mass
in both the surface-brightness distribution and the flux spectrum. The spectral
features have been use previously to determine , and now that
the luminosity and radius of a non-binary red giant or red supergiant can be
observed, spherical model stellar atmospheres can be used to determine the
star's mass from currently achievable spectroscopy. The surface-brightness
variations with mass are slightly smaller than can be resolved by current
stellar imaging, but they offer the advantage of being less sensitive to the
detailed chemical composition of the atmosphere.Comment: 24 pages, 9 figure
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
Wigner crystallization in transition metal dichalcogenides: A new approach to correlation energy
We introduce a new approach for the correlation energy of one- and two-valley
two-dimensional electron gas (2DEG) systems. Our approach is based on a random
phase approximation at high densities and a classical approach at low
densities, with interpolation between the two limits. This approach gives
excellent agreement with available Quantum Monte Carlo (QMC) calculations. We
employ the two-valley 2DEG model to describe the electron correlations in
monolayer transition metal dichalcogenides (TMDs). The zero-temperature
transition from a Fermi liquid to a quantum Wigner crystal phase in monolayer
TMDs is obtained using density-functional theory within the local-density
approximation. Consistent with QMC, we find that electrons crystallize at
in one-valley 2DEG. For two-valleys, we predict Wigner
crystallization at , indicating that valley degeneracy has little
effect on the critical , in contrast to an earlier claim.Comment: 5 pages, 3 figure
SAtlas: Spherical Versions of the Atlas Stellar Atmosphere Program
Context: The current stellar atmosphere programs still cannot match some
fundamental observations of the brightest stars, and with new techniques, such
as optical interferometry, providing new data for these stars, additional
development of stellar atmosphere codes is required. Aims: To modify the
open-source model atmosphere program Atlas to treat spherical geometry,
creating a test-bed stellar atmosphere code for stars with extended
atmospheres. Methods: The plane-parallel Atlas has been changed by introducing
the necessary spherical modifications in the pressure structure, in the
radiative transfer and in the temperature correction. Results: Several test
models show that the spherical program matches the plane-parallel models in the
high surface gravity regime, and matches spherical models computed by Phoenix
and by MARCS in the low gravity case.Comment: 10 pages, 10 figures, Accepted for publication in A&
Limb Darkening and Planetary Transits: Testing Center-to-limb Intensity Variations and Limb-Darkening Directly from Model Stellar Atmospheres
The transit method, employed by MOST, \emph{Kepler}, and various ground-based
surveys has enabled the characterization of extrasolar planets to unprecedented
precision. These results are precise enough to begin to measure planet
atmosphere composition, planetary oblateness, star spots, and other phenomena
at the level of a few hundred parts-per-million. However, these results depend
on our understanding of stellar limb darkening, that is, the intensity
distribution across the stellar disk that is sequentially blocked as the planet
transits. Typically, stellar limb darkening is assumed to be a simple
parameterization with two coefficients that are derived from stellar atmosphere
models or fit directly. In this work, we revisit this assumption and compute
synthetic planetary transit light curves directly from model stellar atmosphere
center-to-limb intensity variations (CLIV) using the plane-parallel
\textsc{Atlas} and spherically symmetric \textsc{SAtlas} codes. We compare
these light curves to those constructed using best-fit limb-darkening
parameterizations. We find that adopting parametric stellar limb-darkening laws
lead to systematic differences from the more geometrically realistic model
stellar atmosphere CLIV of about 50 -- 100 ppm at the transit center and up to
300 ppm at ingress/egress. While these errors are small they are systematic,
and appear to limit the precision necessary to measure secondary effects. Our
results may also have a significant impact on transit spectra.Comment: 12 pages, 14 figures, accepted for publication in ApJ after revision
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