scholarly journals A new non-linear limb-darkening law for LTE stellar atmosphere models III

2004 ◽  
Vol 428 (3) ◽  
pp. 1001-1005 ◽  
Author(s):  
A. Claret
Keyword(s):  
Stellar Atmosphere ◽  
Non Linear ◽  
Limb Darkening

scholarly journals Direct Observations of the Heterogeneity of Supergiant Disks

1977 ◽  
Vol 4 (2) ◽  
pp. 405-405
Author(s):  
J. W. Harvey ◽  
C. R. Lynds ◽  
S. P. Worden
Keyword(s):  
Signal To Noise Ratio ◽  
Atmospheric Dispersion ◽  
Stellar Atmosphere ◽  
High Magnification ◽  
Specific Technique ◽  
Signal To Noise ◽  
Speckle Imaging ◽  
Direct Observations ◽  
Good Signal ◽  
Limb Darkening

Resolved images of the disks of the largest stars observed with the largest telescopes can be constructed using the class of techniques called speckle imaging. The observations must be made with narrow passbands (~ 10 nm), short exposures (~ 20 ms) compensation for atmospheric dispersion, high magnification and good signal-to-noise ratio. One specific technique applied to a Ori (Lynds et al., 1976) shows slight but apparently real differences in the images of the disk corresponding to low and high opacity in the stellar atmosphere which we interpret as due to temperature differences. There are also significant differences in the star’s diameter and/or limb darkening at the two different opacity wavelengths.

scholarly journals Interferometric Fringe Visibility Null as a Function of Spatial Frequency: A Probe of Stellar Atmospheres

2019 ◽  
Vol 08 (04) ◽  
pp. 1950012
Author(s):  
J. T. Armstrong ◽  
A. M. Jorgensen ◽  
D. Mozurkewich ◽  
H. R. Neilson ◽  
E. K. Baines ◽  
...  
Keyword(s):  
Spatial Frequency ◽  
Angular Size ◽  
Brightness Distribution ◽  
Stellar Atmosphere ◽  
Stellar Atmospheres ◽  
Stellar Disk ◽  
Baseline Length ◽  
Fringe Visibility ◽  
Multiple Wavelengths ◽  
Limb Darkening

We introduce an observational tool based on visibility nulls in optical spectro-interferometry fringe data to probe the structure of stellar atmospheres. In a preliminary demonstration, we use both Navy Precision Optical Interferometer (NPOI) data and stellar atmosphere models to show that this tool can be used, for example, to investigate limb darkening. Using bootstrapping with either multiple linked baselines or multiple wavelengths in optical and infrared spectro-interferometric observations of stars makes it possible to measure the spatial frequency [Formula: see text] at which the real part of the fringe visibility [Formula: see text] vanishes. That spatial frequency is determined by [Formula: see text], where [Formula: see text] is the projected baseline length, and [Formula: see text] is the wavelength at which the null is observed. Since [Formula: see text] changes with the Earth’s rotation, [Formula: see text] also changes. If [Formula: see text] is constant with wavelength, [Formula: see text] varies in direct proportion to [Formula: see text]. Any departure from that proportionality indicates that the brightness distribution across the stellar disk varies with wavelength via variations in limb darkening, in the angular size of the disk, or both. In this paper, we introduce the use of variations of [Formula: see text] with [Formula: see text] as a means of probing the structure of stellar atmospheres. Using the equivalent uniform disk diameter [Formula: see text], given by [Formula: see text], as a convenient and intuitive parameterization of [Formula: see text], we demonstrate this concept by using model atmospheres to calculate the brightness distribution for [Formula: see text] Ophiuchi and to predict [Formula: see text], and then comparing the predictions to coherently averaged data from observations taken with the NPOI.

scholarly journals The stellar atmosphere physical system II. An operative sequential algorithm to solve the stellar atmosphere problem

2019 ◽  
pp. 1-11
Author(s):  
Lucio Crivellari
Keyword(s):  
Radiative Transfer ◽  
Physical System ◽  
Stellar Atmosphere ◽  
Sequential Algorithm ◽  
Transfer Equations ◽  
Non Linear ◽  
Local Problems ◽  
Equilibrium Constraint ◽  
Non Local ◽  
Radiative Transfer Equations

In this paper, the second and the last of the series, we present a sequential algorithm to solve the stellar atmosphere problem that may serve as a paradigm for the solution of more general non-linear and non-local problems. The Iteration Factors (IF) Method is applied to achieve a solution of the radiative transfer equations, consistent with the radiative equilibrium constraint.

scholarly journals Forward modelling of Kepler-band variability due to faculae and spots

2021 ◽  
Author(s):  
Luke J Johnson ◽  
Charlotte M Norris ◽  
Yvonne C Unruh ◽  
Sami K Solanki ◽  
Natalie Krivova ◽  
...  
Keyword(s):  
Stellar Atmosphere ◽  
Magnetic Activity ◽  
Stellar Atmospheres ◽  
Area Coverage ◽  
Magnetic Flux Density ◽  
Model Parameters ◽  
Spot Area ◽  
Solar Type ◽  
Limb Darkening ◽  
Spot Temperature

Abstract Variability observed in photometric lightcurves of late-type stars (on timescales longer than a day) is a dominant noise source in exoplanet surveys and results predominantly from surface manifestations of stellar magnetic activity, namely faculae and spots. The implementation of faculae in lightcurve models is an open problem, with scaling typically based on spectra equivalent to hot stellar atmospheres or assuming a solar-derived facular contrast. We modelled rotational (single period) lightcurves of active G2, K0, M0 and M2 stars, with Sun-like surface distributions and realistic limb-dependent contrasts for faculae and spots. The sensitivity of lightcurve variability to changes in model parameters such as stellar inclination, feature area coverage, spot temperature, facular region magnetic flux density and active band latitudes is explored. For our lightcurve modelling approach we used actress, a geometrically accurate model for stellar variability. actress generates 2-sphere maps representing stellar surfaces and populates them with user-prescribed spot and facular region distributions. From this, lightcurves can be calculated at any inclination. Quiet star limb darkening and limb-dependent facular contrasts were derived from MURaM 3D magnetoconvection simulations using ATLAS9. 1D stellar atmosphere models were used for the spot contrasts. We applied actress in Monte Carlo simulations, calculating lightcurve variability amplitudes in the Kepler band. We found that, for a given spectral type and stellar inclination, spot temperature and spot area coverage have the largest effect on variability of all simulation parameters. For a spot coverage of $1{{\ \rm per\ cent}}$, the typical variability of a solar-type star is around 2 parts-per-thousand. The presence of faculae clearly affects the mean brightness and lightcurve shape, but has relatively little influence on the variability.

scholarly journals Model-free inverse method for transit imaging of stellar surfaces

2019 ◽  
Vol 630 ◽  
pp. A122 ◽  
Author(s):  
Erik Aronson ◽  
Nikolai Piskunov
Keyword(s):  
Light Curve ◽  
Synthetic Data ◽  
Brightness Distribution ◽  
Stellar Atmosphere ◽  
Light Curves ◽  
Stellar Surface ◽  
Model Free ◽  
Very Large Telescope ◽  
Limb Darkening ◽  
Low Dispersion

Context. We present a model-free method for mapping surface brightness variations. Aims. We aim to develop a method that is not dependent on either stellar atmosphere models or limb-darkening equation. This method is optimized for exoplanet transit surveys such that a large database of stellar spot coverage can be created. Methods. The method uses light curves from several transit events of the same system. These light curves are phase-folded and median-combined to for a high-quality light curve without temporal local brightness variations. Stellar specific intensities are extracted from this light curve using a model-free method. We search individual light curves for departures from the median-combined light curve. Such departures are interpreted as brightness variations on the stellar surface. A map of brightness variations on the stellar surface is produced by finding the brightness distribution that can produce a synthetic light curve that fits observations well. No assumptions about the size, shape, or contrast of brightness variations are made. Results. We successfully reproduce maps of stellar disks from both synthetic data and archive observations from FORS2, the visual and near UV FOcal Reducer and low dispersion Spectrograph for the Very Large Telescope (VLT).

scholarly journals A new method to compute limb-darkening coefficients for stellar atmosphere models with spherical symmetry: the space missions TESS, Kepler, CoRoT, and MOST

2018 ◽  
Vol 618 ◽  
pp. A20 ◽  
Author(s):  
Antonio Claret
Keyword(s):  
Spherical Symmetry ◽  
Least Squares Method ◽  
Stellar Atmosphere ◽  
Space Missions ◽  
Stellar Atmospheres ◽  
Eclipsing Binaries ◽  
New Method ◽  
Test Models ◽  
Local Gravity ◽  
Limb Darkening

Aims. One of the biggest problems we can encounter while dealing with the limb-darkening coefficients for stellar atmospheric models with spherical symmetry is the difficulty of adjusting both the limb and the central parts simultaneously. In particular, the regions near the drop-offs are not well reproduced for most models, depending on Teff, log g, or wavelength. Even if the law with four terms is used, these disagreements still persist. Here we introduce a new method that considerably improves the description of both the limb and the central parts and that will allow users to test models of stellar atmospheres with spherical symmetry more accurately in environments such as exoplanetary transits, eclipsing binaries, etc. Methods. The method introduced here is simple. Instead of considering all the μ points in the adjustment, as is traditional, we consider only the points until the drop-off (μcri) of each model. From this point, we impose a condition I(μ)/I(1) = 0. All calculations were performed by adopting the least-squares method. Results. The resulting coefficients using this new method reproduce the intensity distribution of the PHOENIX spherical models (COND and DRIFT) quite well for the photometric systems of the space missions TESS, Kepler, CoRoT, and MOST. The calculations cover the following ranges of local gravity and effective temperatures: 2.5 ≤ log g ≤ 6.0 and 1500 K ≤ Teff ≤ 12 000 K. The new spherical coefficients can easily be adapted to the most commonly used light curve synthesis codes.

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