scholarly journals Gravity and limb-darkening coefficients for compact stars: DA, DB, and DBA eclipsing white dwarfs

2020 ◽  
Vol 634 ◽  
pp. A93 ◽  
Author(s):  
A. Claret ◽  
E. Cukanovaite ◽  
K. Burdge ◽  
P.-E. Tremblay ◽  
S. Parsons ◽  
...  
Keyword(s):  
White Dwarfs ◽  
Least Squares Method ◽  
Stellar Atmospheres ◽  
Eclipsing Binaries ◽  
Compact Stars ◽  
Stellar Disk ◽  
Chemical Compositions ◽  
Specific Intensity ◽  
Effective Temperatures ◽  
Limb Darkening

Context. The distribution of the specific intensity over the stellar disk is an essential tool for modeling the light curves in eclipsing binaries, planetary transits, and stellar diameters through interferometric techniques, line profiles in rotating stars, gravitational microlensing, etc. However, the available theoretical calculations are mostly restricted to stars on the main sequence or the giant branch, and very few calculations are available for compact stars. Aims. The main objective of the present work is to extend these investigations by computing the gravity and limb-darkening coefficients for white dwarf atmosphere models with hydrogen, helium, or mixed compositions (types DA, DB, and DBA). Methods. We computed gravity and limb-darkening coefficients for DA, DB, and DBA white dwarfs atmosphere models, covering the transmission curves of the Sloan, UBVRI, Kepler, TESS, and Gaia photometric systems. Specific calculations for the HiPERCAM instrument were also carried out. For all calculations of the limb-darkening coefficients we used the least-squares method. Concerning the effects of tidal and rotational distortions, we also computed for the first time the gravity-darkening coefficients y(λ) for white dwarfs using the same models of stellar atmospheres as in the case of limb-darkening. A more general differential equation was introduced to derive these quantities, including the partial derivative (∂ln Io(λ)/∂ln g)Teff. Results. Six laws were adopted to describe the specific intensity distribution: linear, quadratic, square root, logarithmic, power-2, and a more general one with four coefficients. The computations are presented for the chemical compositions log[H/He] = −10.0 (DB), −2.0 (DBA) and He/H = 0 (DA), with log g varying between 5.0 and 9.5 and effective temperatures between 3750 and 100 000 K. For effective temperatures higher than 40 000 K, the models were also computed adopting nonlocal thermal equilibirum (DA). The adopted mixing-length parameters are ML2/α = 0.8 (DA case) and 1.25 (DB and DBA). The results are presented in the form of 112 tables. Additional calculations, such as for other photometric systems and/or different values of log[H/He], log g, and Teff can be performed upon request.

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 Fundamental Parameters and Models of Stellar Atmospheres

1985 ◽  
Vol 111 ◽  
pp. 303-329
Author(s):  
Bengt Gustafsson ◽  
Uffe Graae-Jørgensen
Keyword(s):  
Model Atmosphere ◽  
Stellar Atmospheres ◽  
Chemical Compositions ◽  
Fundamental Parameters ◽  
Different Types ◽  
Effective Temperatures

The use of photometric and spectroscopic criteria, calibrated by model-atmosphere calculations, for determining effective temperatures, surface gravities and chemical compositions of stars is illustrated and commented on. The accuracy that can be obtained today in such calibrations is discussed, as well as possible ways of improving this accuracy further for different types of stars.

scholarly journals Search For Cool White Dwarf Pulsators

2000 ◽  
Vol 176 ◽  
pp. 525-526
Author(s):  
Atsuko Nitta ◽  
A. Mukadam ◽  
D. E. Winget ◽  
A. Kanaan ◽  
S. J. Kleinman ◽  
...  
Keyword(s):  
Lower Limit ◽  
White Dwarf ◽  
White Dwarfs ◽  
Galactic Disk ◽  
Chemical Compositions ◽  
Physical Processes ◽  
Effective Temperatures

AbstractWe are searching for pulsations in cool (< 6000 K) white dwarfs (WDs), hoping to apply asteroseismological techniques to improve our understanding of their structure and the physical processes inside them. This information is important as we use cool WDs to estimate the lower limit of the age of the Galactic disk. Within a spectroscopic and photometric survey of 110 cool WDs by Bergeron, Ruiz, & Legget, we find 28 candidates with appropriate effective temperatures, masses, and chemical compositions for possible pulsations in nonradial g modes with periods similar to those we observe in DAVs. So far, we have observed 4 candidates, but have found no evidence of large variation.

The Determination of Angular Diameters of Stars

1971 ◽  
Vol 2 ◽  
pp. 713-720
Author(s):  
John Davis
Keyword(s):  
Effective Temperature ◽  
Binary Systems ◽  
Spectral Energy Distribution ◽  
Stellar Atmospheres ◽  
Angular Diameter ◽  
Effective Temperatures ◽  
The Absolute ◽  
Limb Darkening ◽  
Angular Diameters

Ideally the determination of the angular diameter of a star would include the measurement of the distribution of intensity across the stellar disc. However, direct methods of measuring angular diameters have so far lacked adequate ‘signal to noise’ ratio to measure the intensity distribution and it has been the custom, in the first instance, to express the measured angular diameter in terms of the angular diameter of the equivalent uniform disc (θUD). Subsequent use of the angular diameter involves the assumption of a limb-darkening law and the application of an appropriate correction to θUD to find the ‘true’ angular diameter (θLD) of the star (e.g. Hanbury Brown et al., 1967). In this article we will discuss the determination of θUD for single stars and we will not refer further to the more difficult problems of determining intensity distributions involving limb-darkening and rotational effects and of measuring the angular parameters of binary systems.By itself the angular diameter of a star has no intrinsic value but when it is combined with other observational data it enables basic physical properties of the star to be determined. It is then possible to make a direct comparison of the observed properties of the star with the predictions of theoretical models of stellar atmospheres and interiors. For example, the combination of an angular diameter with the absolute monochromatic flux received from the star (ƒν), corrected for interstellar extinction, yields the absolute emergent flux at the stellar surface (). If the spectral energy distribution for the star is known it can be calibrated absolutely by and hence the effective temperature (Te) of the star can be found (this is equivalent to knowing the bolometric correction for the star and using it with the angular diameter to find Te). In addition to leading to the determination of Te, the absolute surface flux distribution may be compared directly with the predicted flux distributions for theoretical model stellar atmospheres (e.g. Davis and Webb, 1970). For O and early B. type stars a large fraction of the emergent flux is in the far ultra-violet and the effective temperatures cannot be determined from the, at present, incomplete empirical flux curves. In these cases it is possible to obtain an estimate of the effective temperatures by using the values of to calibrate a grid of model atmospheres which have Te as a parameter. In this way, by measuring the angular diameters of stars of different spectral types, it is possible to establish an effective temperature scale.

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 Doppler beaming factors for white dwarfs, main sequence stars, and giant stars

2020 ◽  
Vol 641 ◽  
pp. A157
Author(s):  
A. Claret ◽  
E. Cukanovaite ◽  
K. Burdge ◽  
P.-E. Tremblay ◽  
S. Parsons ◽  
...  
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Main Sequence ◽  
Theoretical Calculations ◽  
Three Dimensional ◽  
Black Body ◽  
3D Models ◽  
Giant Stars ◽  
Effective Temperatures ◽  
Limb Darkening

Context. Systematic theoretical calculations of Doppler beaming factors are scarce in the literature, particularly in the case of white dwarfs. Additionally, there are no specific calculations for the limb-darkening coefficients of 3D white dwarf models. Aims. The objective of this research is to provide the astronomical community with Doppler beaming calculations for a wide range of effective temperatures, local gravities, and hydrogen/metal content for white dwarfs as well as stars on both the main sequence and the giant branch. In addition, we present the theoretical calculations of the limb-darkening coefficients for 3D white dwarf models for the first time. Methods. We computed Doppler beaming factors for DA, DB, and DBA white dwarf models, as well as for main sequence and giant stars covering the transmission curves of the Sloan, UBVRI, HiPERCAM, Kepler, TESS, and Gaia photometric systems. The calculations of the limb-darkening coefficients for 3D models were carried out using the least-squares method for these photometric systems. Results. The input physics of the white dwarf models for which we have computed the Doppler beaming factors are: chemical compositions log [H/He] = −10.0 (DB), −2.0 (DBA), and He/H = 0 (DA), with log g varying between 5.0 and 9.5 and effective temperatures in the range 3750–100 000 K. The beaming factors were also calculated assuming non-local thermodynamic equilibrium for the case of DA white dwarfs with Teff >  40 000 K. For the mixing-length parameters, we adopted ML2/α = 0.8 (DA case) and 1.25 (DB and DBA). The Doppler beaming factors for main sequence and giant stars were computed using the ATLAS9 version, characterized by metallicities ranging from [−2.5, 0.2] solar abundances, with log g varying between 0 and 5.0 and effective temperatures between 3500 and 50 000 K. The adopted microturbulent velocity for these models was 2.0 km s−1. The limb-darkening coefficients were computed for three-dimensional DA and DB white dwarf models calculated with the CO5BOLD radiation-hydrodynamics code. The parameter range covered by the three-dimensional DA models spans log g values between 7.0 and 9.0 and Teff between 6000 and 15 000 K, while He/H = 0. The three-dimensional DB models cover a similar parameter range of log g between 7.5 and 9.0 and Teff between 12 000 and 34 000 K, while logH/He = −10.0. We adopted six laws for the computation of the limb-darkening coefficients: linear, quadratic, square root, logarithmic, power-2, and a general one with four coefficients. Conclusions. The beaming factor calculations, which use realistic models of stellar atmospheres, show that the black body approximation is not accurate, particularly for the filters u, u′, U, g, g′, and B. The black body approach is only valid for high effective temperatures and/or long effective wavelengths. Therefore, for more accurate analyses of light curves, we recommend the use of the beaming factors presented in this paper. Concerning limb-darkening, the distribution of specific intensities for 3D models indicates that, in general, these models are less bright toward the limb than their 1D counterparts, which implies steeper profiles. To describe these intensities better, we recommend the use of the four-term law (also for 1D models) given the level of precision that is being achieved with Earth-based instruments and space missions such as Kepler and TESS (and PLATO in the future).

scholarly journals Comparison of Limb-Darkening Laws from Plane-Parallel and Spherically-Symmetric Model Stellar Atmospheres

2011 ◽  
Vol 7 (S282) ◽  
pp. 243-246
Author(s):  
Hilding R. Neilson
Keyword(s):  
Stellar Atmospheres ◽  
Stellar Disk ◽  
Symmetric Model ◽  
Spherically Symmetric ◽  
Linear Quadratic ◽  
Plane Parallel ◽  
Curve Fit ◽  
Parallel Models ◽  
Spherically Symmetric Model ◽  
Limb Darkening

AbstractLimb-darkening is a fundamental constraint for modeling eclipsing binary and planetary transit light curves. As observations, for example from Kepler, CoRot, and Most, become more precise then a greater understanding of limb-darkening is necessary. However, limb-darkening is typically modeled as simple parameterizations fit to plane-parallel model stellar atmospheres that ignores stellar atmospheric extension. In this work, I compute linear, quadratic and four-parameter limb-darkening laws from grids of plane-parallel and spherically-symmetric model stellar atmospheres in a temperature and gravity range representing stars evolving on the Red Giant branch. The limb-darkening relations for each geometry are compared and are found to fit plane-parallel models much better than the spherically-symmetric models. Assuming that limb-darkening from spherically-symmetry model atmospheres are more physically representative of actual stellar limb-darkening than plane-parallel models, then these limb-darkening laws will not fit the limb of a stellar disk leading to errors in a light curve fit. This error will increase with a star's atmospheric extension.

Effective Temperatures of White Dwarfs

White Dwarfs ◽  
1971 ◽  
pp. 67-76 ◽  
Author(s):  
J. B. Oke ◽  
H. L. Shipman
Keyword(s):  
White Dwarfs ◽  
Effective Temperatures

scholarly journals Synthesis of Classification Dispersion Spectra from Stellar Atmospheres Theory and from High Dispersion Spectra

1976 ◽  
Vol 72 ◽  
pp. 17-18
Author(s):  
A. W. Irwin ◽  
C. T. Bolton ◽  
R. F. Garrison
Keyword(s):  
Spectral Synthesis ◽  
Stellar Atmospheres ◽  
Oscillator Strengths ◽  
Continuum Models ◽  
High Dispersion ◽  
University Of Toronto ◽  
Spectrum Synthesis ◽  
Effective Temperatures ◽  
The University ◽  
Temperature Surface

The ATLAS programme has been corrected and modified for use at low effective temperatures. A grid of unblanketed model atmospheres has been generated for the region G5-K5, V-II. A spectrum synthesis programme has been written for the calculation of flux vs wavelength for selected regions of the spectrum. Temperature distributions from the grid of continuum models and from published blanketed models will be used along with published oscillator strengths as input data for the spectral synthesis programme.Spectrograms of MK standards have been taken at 12 Å mm−1 and 120 Å mm−1. The University of Toronto PDS microdensitometer system has been programmed to allow comparison between the synthesized spectra and the observed spectra to determine the effects of effective temperature, surface gravity, microturbulence, and abundance on classification criteria.

scholarly journals 29. Stellar Spectra

1970 ◽  
Vol 14 (1) ◽  
pp. 319-329
Author(s):  
M. W. Feast ◽  
Y. Fujita ◽  
M. K. V. Bappu ◽  
G. Herbig ◽  
L. Houziaux ◽  
...  
Keyword(s):  
Mass Loss ◽  
Working Group ◽  
White Dwarfs ◽  
Vice President ◽  
Stellar Atmospheres ◽  
High Dispersion ◽  
Stellar Rotation ◽  
Stellar Spectra ◽  
Working Groups ◽  
Main Activity

Material for this report was collected by the President, Vice-President and Members of the Organizing Committee. The President is, however, responsible for the form in which the report now appears. A number of special abbreviations in the references are explained in the report of Committee 27a. In addition, 3rd Harvard = 3rd Harvard-Smithsonian Conference on Stellar Atmospheres (1968). The field of Commission 29 overlaps particularly with those of 9, 27a, 36, 44 and 45 whose reports should be consulted. Since the last IAU meeting 29 has co-sponsored the following meetings: IAU Colloquium No. 4 on Stellar Rotation (Columbus, Ohio, September 1969); IAU Symposium No. 36, Ultraviolet Stellar Spectra and Related Ground-Based Observations (Lunteren, June, 1969); Second Trieste Colloquium, Mass Loss from Stars (September, 1968). We are also co-sponsoring IAU Symposium No. 42 on White Dwarfs to be held in Scotland (August, 1970). The thanks of the commission are due to their representatives on the organizing committees of these meetings. Reports from some working groups are appended. The working group with Commission 44 has not felt it necessary to submit a report (its main activity was the organization of Symposium No. 36). Miss Underhill (Chairman) recommends that the working group on Tracings of High Dispersion Stellar Spectra be dissolved.

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