3,923 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&
High Temperature Superfluidity in Double Bilayer Graphene
Exciton bound states in solids between electrons and holes are predicted to
form a superfluid at high temperatures. We show that by employing atomically
thin crystals such as a pair of adjacent bilayer graphene sheets, equilibrium
superfluidity of electron-hole pairs should be achievable for the first time.
The transition temperatures are well above liquid helium temperatures. Because
the sample parameters needed for the device have already been attained in
similar graphene devices, our work suggests a new route towards realizing
high-temperature superfluidity in existing quality graphene samples.Comment: 6 pages, 4 figures, effect of screening on superfluidity include
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
Long-term polarization observations of Mira variable stars suggest asymmetric structures
Mira and semi-regular variable stars have been studied for centuries but
continue to be enigmatic. One unsolved mystery is the presence of polarization
from these stars. In particular, we present 40 years of polarization
measurements for the prototype o Ceti and V CVn and find very different
phenomena for each star. The polarization fraction and position angle for Mira
is found to be small and highly variable. On the other hand, the polarization
fraction for V CVn is large and variable, from 2 - 7 %, and its position angle
is approximately constant, suggesting a long-term asymmetric structure. We
suggest a number of potential scenarios to explain these observations.Comment: 2 pages, 1 figure, poster presented at IAU Symposium 301, Precision
Asteroseismology, August 2013, Wroclaw, Polan
A comparison of plastic collapse and limit loads for single mitred pipe bends under in-plane bending
This paper presents a comparison of the plastic collapse loads from experimental in-plane bending tests on three 90 degree single un-reinforced mitred pipe bends, with the results from various 3D solid finite element models. The bending load applied reduced the bend angle and in turn, the resulting cross-sectional ovalisation led to a recognised weakening mechanism, which is only observable by testing or by including large displacement effects in the plastic finite element solution. A small displacement limit solution with an elastic-perfectly-plastic material model overestimated the collapse load by 40%. The plastic collapse finite element solution produced excellent agreement with experiment
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
- …