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 Effective Temperatures of Stars with “Standard” Angular Diameters

1985 ◽  
Vol 111 ◽  
pp. 465-467
Author(s):  
I. N. Glushneva
Keyword(s):  
Effective Temperature ◽  
Astronomical Institute ◽  
Joint Determination ◽  
Sternberg Astronomical Institute ◽  
Effective Temperatures ◽  
Angular Diameters

For 12 stars from the list of stars with “standard” angular diameters (Fracassini et al. 1983), effective temperatures, bolometric corrections, radii and luminosities were determined. These stars are included in the stellar spectrophotometric catalog of the Sternberg Astronomical Institute and three of them were used as spectrophotometric standards. A comparison was made of Teff obtained directly using angular diameters from the list of Fracassini et al. (1983) and by means of joint determination of Teff and θ (Blackwell and Shallis 1977). For 7 stars the differences in Teff values don't exceed 1–1.5% and the maximum discrepancies are about 6% for BS 2294, 2943 and 4% for the spectrophotometric standard α Aql (BS 7557). Effective temperature values of α Lyr obtained by these two methods are in the agreement within 0.5%.

Estimation of Mass-loss Rates for Some Galactic Wolf Rayet Stars

1991 ◽  
Vol 9 (2) ◽  
pp. 275-276
Author(s):  
Jong Ok Woo
Keyword(s):  
Mass Loss ◽  
Stellar Wind ◽  
Empirical Relation ◽  
Terminal Velocity ◽  
Effective Temperatures ◽  
The Absolute ◽  
Absolute Measurements ◽  
Loss Rates ◽  
Angular Diameters

AbstractThe absolute measurements of flux emitted in the visible continua of some Galactic Wolf-Rayet stars were carried out by means of a two-channel scanner. The measurements lead to the determination of stellar angular diameters which enable us to compute log L*/L⊙. The stellar wind terminal velocity, V∞ was obtained from the empirical relation of the effective temperatures by Underbill (1983) and V∞, adopted from the work of Willis (1982). Also, we derived the rate of mass loss for the WR stars from the formula Ṁ = є(Teff)L/(V∞c) by using the obtained effective temperatures, luminosity and V∞ in this work. The values of the rates range from 1.4 × 10−5 to 5.8 × 10−5M⊙yr−1.

Angular Diameters and Effective Temperatures of Some Bright Stars

1967 ◽  
Vol 1 (1) ◽  
pp. 7-8
Author(s):  
R. Hanbury Brown
Keyword(s):  
Michelson Interferometer ◽  
Angular Diameter ◽  
Frequency Range ◽  
Fringe Visibility ◽  
Stellar Interferometer ◽  
Effective Temperatures ◽  
Angular Diameters

The Stellar Interferometer at Narrabri Observatory consists of two large reflectors which focus the light from a star on to two photoelectric detectors. The output currents from these detectors contain fluctuations which correspond to fluctuations in the starlight itself. The fluctuations from the two detectors are amplified in the frequency range 10-100 Mc/s and their correlation is measured by a linear multiplier. This correlation is measured as a function of the separation between the two detectors. It can be shown theoretically that the correlation at any given baseline is proportional to the square of the fringe visibility which would be observed by a classical Michelson interferometer. It is therefore possible from observations of the correlation at different baselines to find the angular diameter of a star.

scholarly journals The Gravity-Darkening of the Main-Sequence Components of Spectral Types A, F and G in Detached Close Binary Systems

1989 ◽  
Vol 107 ◽  
pp. 353-353
Author(s):  
Y. Nakamura ◽  
M. Kitamura
Keyword(s):  
Structural Change ◽  
Binary Systems ◽  
Main Sequence ◽  
Stellar Atmospheres ◽  
Close Binary ◽  
Close Binary Systems ◽  
Correction Process ◽  
Sequence Components ◽  
Effective Temperatures ◽  
Ellipticity Effect

Abstract.From analysis of the photometric ellipticity effect in seven well-understood detached close binary systems, empirical values of the exponent α of gravity-darkening have been practically deduced for eleven main-sequence components of spectral types A, F and G which should cover the range of structural change (from radiative to convective) in stellar atmospheres. The result indicate that values of the exponent diminish gradually with decreasing effective temperatures from α ~ 1.0 for radiative atmospheres with T > 8500 K to α =0.2 ~ 0.3 for convective atmospheres with T < 6500 K, in spite of some uncertainty in the reflection correction process.

The Absolute Flux Distribution of Sirius

1970 ◽  
Vol 1 (8) ◽  
pp. 378-379 ◽  
Author(s):  
J. Davis ◽  
R. J. Webb
Keyword(s):  
Effective Temperature ◽  
Flux Distribution ◽  
Model Atmosphere ◽  
Angular Diameter ◽  
Sufficient Data ◽  
Absolute Flux ◽  
Stringent Test ◽  
Net Flux ◽  
The Absolute

The angular diameter of a star, combined with observed stellar fluxes in absolute units, allows the absolute fluxes emitted at the surface of the star to be calculated and thus provides a stringent test of model atmosphere predictions. Furthermore, if the flux distribution is observed at all wavelengths contributing significantly to the net flux, an empirical effective temperature can be found. Sufficient data are now available for Sirius (αCMa) to be studied in this way.

scholarly journals The Kinematics of Globular Cluster NGC 288

2001 ◽  
Vol 183 ◽  
pp. 333-334
Author(s):  
Chan-Kao Chang ◽  
Alfred B. Chen ◽  
Wean-Shun Tsay ◽  
Wen-Ping Chen ◽  
Phillip K. Lu
Keyword(s):  
Energy Distribution ◽  
Globular Cluster ◽  
Effective Temperature ◽  
Proper Motion ◽  
Spectral Energy Distribution ◽  
Spectral Energy ◽  
Galactic Rotation ◽  
Distribution Fitting ◽  
The Mean ◽  
Effective Temperatures

AbstractThe mean radial velocity of NGC 288 (accuracy 5.5 km/s) is determined to be −56.3 ± 20.1 km/s which, when combined with the mean proper motion (Guo, 1995), yields a peculiar velocity with respect to the LSR of (u,v,w) = (29.7 ± 18.1, −258.6 ± 18.3,62.3 ± 20.3) km/s. This implies that NGC 288 moves in a retrograde sense with the Galactic rotation. We also derived the effective temperatures for stars in our sample and, as a corroborative effort, compared with those estimated previously from the BATC data (Tsai 1998) by spectral energy distribution fitting. We demonstrate that the BATC/SED fitting is an appropriate and efficient way to estimate the effective temperature of a star.

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 Energy Distributions and Fundamental Parameters of 6 A Stars

1993 ◽  
Vol 137 ◽  
pp. 156-158
Author(s):  
R. Monier
Keyword(s):  
Effective Temperature ◽  
Surface Gravity ◽  
Infrared Photometry ◽  
Flux Method ◽  
Energy Distributions ◽  
Fundamental Parameters ◽  
Effective Temperatures ◽  
Best Fit ◽  
Angular Diameters

AbstractThe energy distributions (EDs) of 6 peculiar (3 Aps and 3 λ Bootis) stars are presented from 1200Ȧ up to about 12000Ȧ. For those stars having infrared photometry, the integrated flux from 1200& to 22000Å has been calculated and the Infrared Flux Method (IRFM, Blackwell and Shallis, 1977) applied to derive effective temperatures and angular diameters. For all stars, the effective temperature Teff, the surface gravity logg and metallicity [M/H] were also derived by matching the EDs to a grid of model atmospheres. A chisquare techniques is used to sort out the best fit to the observed EDs.

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