Angular Diameters Of Stars Research Articles

SPICA, Stellar Parameters and Images with a Cophased Array: a 6T visible combiner for the CHARA array.

High angular resolution studies of stars in the optical domain have highly progressed in recent years. After the results obtained with the visible instrument Visible spEctroGraph and polArimeter (VEGA) on the Center for High Angular Resolution Astronomy (CHARA) array and the recent developments on adaptive optics and fibered interferometry, we have started the design and study of a new six-telescope visible combiner with single-mode fibers. It is designed as a low spectral resolution instrument for the measurement of the angular diameter of stars to make a major step forward in terms of magnitude and precision with respect to the present situation. For a large sample of bright stars, a medium spectral resolution mode will allow unprecedented spectral imaging of stellar surfaces and environments for higher accuracy on stellar/planetary parameters. To reach the ultimate performance of the instrument in terms of limiting magnitude (Rmag≃8 for diameter measurements and Rmag≃4 to 5 for imaging), Stellar Parameters and Images with a Cophased Array (SPICA) includes the development of a dedicated fringe tracking system in the H band to reach "long" (200 ms to 30 s) exposures of the fringe signal in the visible.

Determining parameters of cool giant stars by modeling spectrophotometric and interferometric observations using the SAtlas program

Context. Optical interferometry is a powerful tool for observing the intensity structure and angular diameter of stars. When combined with spectroscopy and/or spectrophotometry, interferometry provides a powerful constraint for model stellar atmospheres. Aims. The purpose of this work is to test the robustness of the spherically symmetric version of the Atlas stellar atmosphere program, SAtlas, using interferometric and spectrophotometric observations. Methods. Cubes (three dimensional grids) of model stellar atmospheres, with dimensions of luminosity, mass, and radius, are computed to fit observations for three evolved giant stars, ψ Phoenicis, γ Sagittae, and α Ceti. The best-fit parameters are compared with previous results. Results. The best-fit angular diameters and values of χ 2 are consistent with predictions using Phoenix and plane-parallel Atlas models. The predicted effective temperatures, using SAtlas, are about 100 to 200 K lower, and the predicted luminosities are also lower due to the differences in effective temperatures. Conclusions. It is shown that the SAtlas program is a robust tool for computing models of extended stellar atmospheres that are consistent with observations. The best-fit parameters are consistent with predictions using Phoenix models, and the fit to the interferometric data for ψ Phe differs slightly, although both agree within the uncertainty of the interferometric observations.

Determining field correlations produced by stars from the study of spectral changes in double slit experiment

Experiments are performed to determine the coherence properties of wave fields, produced by the broadband stellar sources on the earth surface, from the study of spectral changes produced on interference in the Young's double slit experiment. The spectral degree of coherence obtained experimentally in this study for four bright stars in the wavelength range from 325 nm to 660 nm is in close agreement with the value that is expected theoretically from the known angular diameter of the stars. It is shown that the spectral degree of coherence obtained experimentally by this spectral interferometric technique could be used to determine the angular diameter of stars.

Effective temperatures of solar type stars

For 29 stars of G0-G5 spectral types with color indices 0.60 \leq B\ndash V \le 0.68 and parallaxes obtained by Hipparcos satellite effective temperatures, angular diameters and radii are determined by means of the method of infrared fluxes (IRFM) on the basis of JHKLM photometry. Effective temperatures obtained by iterations in the process of combined determination of the T_{\rm eff} and angular diameter of star (T_{1}) and using modification of IRFM, so-called observation R -factor, (T_{2}) are compared. The mean differences of these two effective temperatures don't exceed 1% for all the IR bands. For the majority of investigated stars (23 from 29) bolometric magnitudes are less than for the Sun. The analysis of five parameters of solar-type stars investigated in this paper, such a effective temperature, bolometric magnitude, radius, metallicity and infrared color indices, shows that 18 Sco seems to be the most accetable candidate for solar twins.

Photometric and Spectrophotometric Data Required for the SUSI Programme

The Sydney University Stellar Interferometer, SUSI, a modern version of a Michelson stellar interferometer, will be capable of measuring the angular diameters of stars down to V ≈ 8.0, for all spectral types. The resolution of the 640 m baseline — <0.1 milliarcsec at optical wavelengths — includes reasonable sample sizes of all spectral types and luminosity classes to this magnitude limit.The angular diameter data require complementary photometric, spectrophotometric and also spectroscopic data for the determination of fundamental parameters of single and multiple stars. This paper discusses the accuracy required of these data for some of the main problems to be tackled by SUSI. A selection of the programmes planned for this unique instrument is listed below, where the need for complementary data is also indicated.• The measurement of the changes of angular diameters of Cepheids and other pulsating stars such as Mira variables. Comparison with the linear changes determined from radial velocity curves will enable their distances and luminosities to be determined by an essentially geometric method.• The measurement of the angular sizes of the orbits and of one or both components of spectroscopic binary stars. This effectively makes them ‘visual’ binaries so that the orbital inclination, i, may be determined. When these data are combined with the velocity curve solutions which include (mass)sin3i and (semi – majoraxis)sin i, we may determine masses and linear radii of one or both components and also the distances to the systems. Also, the light curve of an eclipsing binary provides information on the ellipticities and radii of the components and on the eccentricity of the orbit — information which may be used to aid the analysis of the SUSI data. In addition, the determination of the light curve (some bright systems have not been observed for up to one or two decades) is the quickest method of finding the current phase of its orbit.• The establishment of the total-flux-based temperature scale of all spectral types and luminosity classes from O to M. This requires accurate photometry and spectrophotometry over as wide a wavelength range as possible; it therefore also requires observations from space observatories, as was also necessary for the analysis using the Narrabri intensity interferometer data (Code et al., 1976).• The measurement of diameters of stars with shells or extended atmospheres, such as Be and Wolf Rayet stars, at different wavelengths — in particular, in the emission lines.

The McDonald photoelectric occultation observation program

Since 1968 an assiduous program of photoelectric observation of occultations of stars by the Moon has been pursued at McDonald Observatory. A total of about 600 events has been observed of which 254 have been published and a second list is in preparation. Timings derived from the reductions have errors of the order of 1 ms corresponding to a positional uncertainty in the lunar limb of typically 80 cm. Results are routinely communicated to the ephemerides offices. In about 140 events the fringe pattern due to diffraction by the lunar limb is sufficiently well defined to permit a determination of the slope near the point of occultation. A statistical discussion of these data is given. In all except a few cases, the slopes are numerically less than 15°, though 7 cases with slopes between 30° and 40° have been found. The relations between numbers of observations and contact angle, and between errors of slope determination and contact angle are discussed. The distribution of slope data with contact angle seems adequately explained. When slope data are collected by position angle consistently large and consistently small values show a tendency to group in a pattern suggesting a connection with observed large scale features on the lunar limb. The influence of lunar limb irregularities of a scale of a few meters on observed diffraction patterns and inferred timings is discussed. Multichannel observations should be of value in removing ambiguities. The use of occultation observations for the discovery of multiple stars and for the measurement of angular diameters of stars is mentioned. Future developments proposed for the project are considered. The project has involved contributions by a considerable number of individuals from the staff and student body at Austin, Texas, from the staff at McDonald Observatory, from visiting scientists and from the Laser Ranging Group. These contributions are acknowledged in the paper. The work has been supported by NSF Grants GP-21204 and GP-32263X.

The Determination of Angular Diameters of Stars

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.

Wideband multiplier correlator

Apparatus to measure the correlation between two wideband (10–100MHz) noise signals has been developed for the stellar intensity interferometer at the Narrabri observatory. This instrument is used to measure the angular diameters of stars in the range 0.01–0.0001″ of arc, with an accuracy within about 5%. To achieve this precision with the low signal/noise ratio of an intensity interferometer, it is necessary to integrate and average the data for periods up to 100h, which may involve observations spread over several weeks. The paper describes the novel techniques which have been used to obtain the necessary efficiency and stability in the electronic correlator.

Diffraction by an Irregular Screen of Limited Extent

The problem considered is the nature of the diffraction pattern produced by an irregular diffracting screen of limited extent. In particular, the deductions which may be made about the screen from measurements of the correlation of the wave disturbances at separated points in the diffraction pattern are discussed. It is shown that such measurements can give information about either the size of the irregularities in the screen or about its overall extent depending on the distance from the screen at which the measurements are made. Close to the screen, the measured auto-correlation function of the diffraction pattern is shown to be the same as that of the small scale irregular structure in the screen. Observations of this function can, therefore, be used to determine the size of the irregularities in the screen. Far from the screen this is no longer true, and the auto-correlation function is determined by the angular distribution of power over the screen as seen from the observing point; it is in fact equal to the Fourier transform of this angular power distribution. Measurements made at a large distance, therefore, give no information about the small scale structure of the screen, but can be used to determine the distribution of power over it. The distance from the screen at which the transition takes place may be calculated by multiplying the size of the small irregularities by the overall length of the screen, and dividing by the wavelength of the radiation. If nothing is known about the nature of the screen or its distance, it is impossible to decide upon the correct interpretation of the measurements. However, in most experiments enough is known to enable the correct interpretation to be made. Several examples are discussed including measurements of the angular diameter of stars, measurements of the small scale structure of the ionospheric layers, and the problem of reflection from irregular ionized trails produced by meteors.

Angular diameters of stars from occultations by the moon.

view Abstract Citations (3) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Angular diameters of stars from occultations by the moon. Whifford, A. E. Abstract The occultations of four stars were observed with the 100-inch telescope of the Mount Wilson Observatory in the summer of 1946. The intensity variations as the diffraction pattern swept over the telescope were recorded by means of a multiplier phototube, cathode ray oscilloscope and moving film camera. The multiplier tube gave a much better signal-to-noise ratio than that of the 1938 apparatus1 and permitted use of fainter stars. The stars, with their calculated angular diameters, were: Star Mag. Spectrum Aug. Diam. p Virginis 4.2 gMi g Virginis 4.3 gK2 .005 191B Ophiuchi 6.3 gKi .002 44b Ophiuchi 4.3 A9 The diffraction maxima and minima for the two larger stars were less pronounced than those calculated for a point source; for the two smaller stars no significant departure from a point-source pattern was expected and none was observed. Preliminary calculations based on resolving the stars into equivalent doubles shows agreement with theory within the limits of observational error. In the range from 0'.'005 to o'.'015 it seems possible to determine the angular diameters of stars within 10 per cent. This corresponds for a Ko star to an uncertainty in the color temperature of about 1000K. I.Ap. J. 89, 472, 1939. Washburn Observatory, Madison, Wis. Publication: The Astronomical Journal Pub Date: 1947 DOI: 10.1086/106023 Bibcode: 1947AJ.....52R.131W full text sources ADS |

A Treatise on Light

THE fact that this book has had a new impression or new edition every two years since 1919 is sufficient evidence of its use to teachers and students and of their appreciation of it. The author is to be congratulated in that he has not burdened the student by the introduction of accounts of recent advances in kindred branches of the subject. His final chapter might well have been omitted, the one valuable addition on the angular diameter of stars being inserted in the chapter on interference. The rest is out of place hi the book and in any case could not fail to be inadequate.

On the Simplest Form of the Stellar Interferometer for Determining the Angular Diameters of Stars by Means of Elliptically Polarized Light

On the Simplest Form of the Stellar Interferometer for Determining the Angular Diameters of Stars by Means of Elliptically Polarized Light

On a New Method of Determining the Angular Diameters of Stars by Means of Elliptically Polarized Light

On a New Method of Determining the Angular Diameters of Stars by Means of Elliptically Polarized Light