scholarly journals Computing Limb‐darkening Coefficients from Stellar Atmosphere Models

2007 ◽  
Vol 656 (1) ◽  
pp. 483-492 ◽  
Author(s):  
David Heyrovsky
Keyword(s):  
Stellar Atmosphere ◽  
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 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.

scholarly journals The Effective Temperature Determination

1995 ◽  
Vol 10 ◽  
pp. 395-398
Author(s):  
Roger Cayrel
Keyword(s):  
Effective Temperature ◽  
Stellar Atmosphere ◽  
Angular Diameter ◽  
Stellar Disk ◽  
Basic Method ◽  
Stellar Diameters ◽  
F Stars ◽  
Subgiant Stars ◽  
Limb Darkening ◽  
Current Accuracy

The content of my oral contribution has been reduced here to a guide to the recent litterature on the subject.The effective temperature of a star can be obtained directly by measuring the total flux received on earth from it, and its angular diameter. In principle interferometrie methods could determine both the limb-darkening law of the stellar disk and the angular diameter of the disk. In practice, in order to obtain a decent accuracy on the angular diameter, interferometrie methods have so far borrowed the limb-darkening law from stellar atmosphere theory, in order to concentrate all the observational information on a single parameter. The total number of stellar diameters and effective temperatures obtained by this basic method remains very small (about 30 stars) and concerns exclusively giant or subgiant stars. The reader is invited to look at the following references and those therein (Di Benedetto 1993, Mozurkewich et al. 1991, Alonso et al. 1994). Smalley and Dworetsky (A&A in press) have reinvestigated the old Code et al. (1976) calibration with more recent spatial UV data, for B, A, and F stars, and have found no significant changes. The accuracy of the basic method is at the best of the order of 1.5 %, and there is a deep need for more data, and for doubling the current accuracy. Actually there is a strong effort for developping interferometry both from the ground (ESO VLTI, USNO astrometric interferometer) andfrom space (OSI project, Colavita et al. 1993; GAIA project at ESA). So there is hope for a not to far future.

Polarimetric Observations of Red Variables: A Study of Gas and Particle Interactions

1979 ◽  
Vol 46 ◽  
pp. 386-408 ◽  
Author(s):  
G. V. Coyne ◽  
I. S. McLean
Keyword(s):  
Wavelength Dependence ◽  
Stellar Atmosphere ◽  
Molecular Absorption ◽  
Absorption Bands ◽  
Absorption And Emission ◽  
Mira Variables ◽  
Stellar Photosphere ◽  
Regular Variable ◽  
Red Supergiants ◽  
Balmer Lines

AbstractIn recent years the wavelength, dependence of the polarization in a number of Mira variables, semi-regular variables and red supergiants has been measured with resolutions between 0.3 and 300 A over the range 3300 to 11000 A. Variations are seen across molecular absorption bands, especially TiO bands, and across atomic absorption and emission lines, especially the Balmer lines. In most cases one can ignore or it is possible to eliminate the effects due to interstellar polarization, so that one can study the polarization mechanisms operating in the stellar atmosphere and environment. The stars Omicron Ceti. (Mira), V CVn (semi-regular variable) and Mu Cephei (M2 la), in addition to other stars similar to them, will be discussed in some detail.Models to explain the observed polarization consider that the continuum flux is polarized either by electron, molecular and/or grain scattering or by temperature variations and/or geometrical asymmetries over the stellar photosphere. This polarized radiation is affected by atomic and molecular absorption and emission processes at various geometric depths in the stellar atmosphere and envelope. High resolution spectropolarimetry promises, therefore, to be a power-rul tool for studying stratification effects in these stars.

scholarly journals The Ariel 0.6 - 7.8 μm stellar limb-darkening coefficients

2021 ◽  
Author(s):  
Giuseppe Morello ◽  
Camilla Danielski ◽  
Subhajit Sarkar
Keyword(s):  
Limb Darkening
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