Stellar Angular Diameter Measurements Research Articles

Direct measurement of stellar angular diameters by the VERITAS Cherenkov telescopes

The angular size of a star is a critical factor in determining its basic properties. Direct measurement of stellar angular diameters is difficult: at interstellar distances stars are generally too small to resolve by any individual imaging telescope. This fundamental limitation can be overcome by studying the diffraction pattern in the shadow cast when an asteroid occults a star, but only when the photometric uncertainty is smaller than the noise added by atmospheric scintillation. Atmospheric Cherenkov telescopes used for particle astrophysics observations have not generally been exploited for optical astronomy due to the modest optical quality of the mirror surface. However, their large mirror area makes them well suited for such high-time-resolution precision photometry measurements. Here we report two occultations of stars observed by the VERITAS Cherenkov telescopes with millisecond sampling, from which we are able to provide a direct measurement of the occulted stars' angular diameter at the $\leq0.1$ milliarcsecond scale. This is a resolution never achieved before with optical measurements and represents an order of magnitude improvement over the equivalent lunar occultation method. We compare the resulting stellar radius with empirically derived estimates from temperature and brightness measurements, confirming the latter can be biased for stars with ambiguous stellar classifications.

Angular Diameters of O- and B-type Stars

AbstractWe are observing a sample of 10 O-type stars and 60 B-type stars to determine angular diameters using the Center for High Angular Resolution Astronomy (CHARA) Array, the foremost optical long baseline interferometer in the world. Our goal is to establish accurate stellar parameters to test modern theories of stellar evolution that include rotation. We will combine our stellar angular diameter measurements with flux and line measurements from spectroscopy, projected rotational velocities, and distances to determine radius, effective temperature, luminosity, equatorial rotational velocity, and evolutionary mass. Knowing these properties will allow us to place the stars in a Hertzsprung-Russell diagram and obtain estimates for the age and evolutionary state.

Towards stellar effective temperatures and diameters at 1 per cent accuracy for future surveys

The apparent size of stars is a crucial benchmark for fundamental stellar properties such as effective temperatures, radii and surface gravities. While interferometric measurements of stellar angular diameters are the most direct method to gauge these, they are still limited to relatively nearby and bright stars, which are saturated in most of the modern photometric surveys. This dichotomy prevents us from safely extending well-calibrated relations to the faint stars targeted in large spectroscopic and photometric surveys. Here, we alleviate this obstacle by presenting South African Astronomical Observatory near-infrared JHK observations of 55 stars: 16 of them have interferometric angular diameters and the rest are in common with the 2 Micron All Sky Survey (2MASS, unsaturated) data set, allowing us to tie the effective temperatures and angular diameters derived via the infrared flux method to the interferometric scale. We extend the test to recent interferometric measurements of unsaturated 2MASS stars, including giants, and the metal-poor benchmark target HD122563. With a critical evaluation of the systematics involved, we conclude that a 1 per cent accuracy in fundamental stellar parameters is usually within reach. Caution, however, must be used when indirectly testing a Teff scale via colour relations as well as when assessing the reliability of interferometric measurements, especially at submilliarcsec level. As a result, rather different effective temperature scales can be compatible with a given subset of interferometric data. We highlight some caveats to be aware of in such a quest and suggest a simple method to check against systematics in fundamental measurements. A new diagnostic combination seismic radii with astrometric distances is also presented.

STELLAR DIAMETERS AND TEMPERATURES. IV. PREDICTING STELLAR ANGULAR DIAMETERS

The number of stellar angular diameter measurements has greatly increased over the past few years due to innovations and developments in the field of long baseline optical interferometry (LBOI). We use a collection of high-precision angular diameter measurements for nearby, main-sequence stars to develop empirical relations that allow the prediction of stellar angular sizes as a function of observed photometric color. These relations are presented for a combination of 48 broad-band color indices. We empirically show for the first time a dependence on metallicity to these relations using Johnson $(B-V)$ and Sloan $(g-r)$ colors. Our relations are capable of predicting diameters with a random error of less than 5% and represent the most robust and empirical determinations to stellar angular sizes to date.

The effective temperature scale of giant stars (F0–K5)

We have applied the InfraRed Flux Method (IRFM) to a sample of approximately 500 giant stars in order to derive their effective temperatures with an internal mean accuracy of about 1.5% and a maximum uncertainty in the zero point of the order of 0.9%. For the application of the IRFM, we have used a homogeneous grid of theoretical model atmosphere flux distributions developed by Kurucz (1993). The atmospheric parameters of the stars roughly cover the ranges: 3500 K 8000 K; -3.0 ; 0.5 . The monochromatic infrared fluxes at the continuum are based on recent photometry with errors that satisfy the accuracy requirements of the work. We have derived the bolometric correction of giant stars by using a new calibration which takes the effect of metallicity into account. Direct spectroscopic determinations of metallicity have been adopted where available, although estimates based on photometric calibrations have been considered for some stars lacking spectroscopic ones. The adopted infrared absolute flux calibration, based on direct optical measurements of stellar angular diameters, puts the effective temperatures determined in this work in the same scale as those obtained by direct methods. We have derived up to four temperatures, TJ, TH, TK and , for each star using the monochromatic fluxes at different infrared wavelengths in the photometric bands J, H, K and . They show good consistency over 4000 K, and there is no appreciable trend with wavelength, metallicity and/or temperature. We provide a detailed description of the steps followed for the application of the IRFM, as well as the sources of error and their effect on final temperatures. We also provide a comparison of the results with previous work.

Stellar angular diameter measurements by interferometry

Stellar angular diameter measurements have been made with a range of interferometric techniques including speckle, aperture masking and long baseline optical/infrared interferometry. The current status of these measurements are summarised in terms of the range of spectral types and luminosity classes measured, the accuracies achieved, the wavelengths used for observations, and the reliability of the results. A number of major long-baseline interferometers are coming on-line, or are under development, and their potential is assessed in terms of wavelength cover, accuracy, angular resolution, and the range of spectral type and luminosity class cover.

Lunar occultation measurements of stellar angular diameters

Offering an angular resolution which has remained unattained by any other technique for decades, lunar occultations have traditionally been the most productive method for the measurement of stellar angular diameters. Unlike interferometric methods, which are limited in resolution by the size of the aperture or of the baseline between apertures, in a lunar occultation the key to high angular resolution is the phenomenon of diffraction by a straight edge, that occurs at the Moon's limb in a turbulence-free environment. For the reader not familiar with the physics and technical aspects of the lunar occultation (LO) technique, it is sufficient here to show in Fig. 1 some practical examples of occultation lightcurves for sources with different angular diameters. It can be noted that the contrast of the fringes is maximum for a point-like source; it then decreases with the angular diameter, and eventually reaches the regime of a monotonic drop in the signal-as predicted by simple geometrical optics- when the angular extent of the source is large. In practice, the LO method is well suited to measure angular diameters in the range 1 to 50 milliarcseconds (mas). There is no real limitation concerning the wavelength of observation, although at present the near-IR is the region of choice for several different reasons (Richichi 1994).

Stellar Diameters, Limb Darkening, Extended Atmospheres, and Shells: Observations with the MKIII Interferometer

The resolution of close binaries and direct measurements of stellar angular diameters were the first achievements of astronomical interferometry (Michelson and Pease 1921). Fringe tracking interferometers are now capable of producing visibility measurements which are sufficiently sensitive and well-calibrated to make more sophisticated measurements possible. Results from current instruments include measurements of limb darkening, of the wavelength-dependence of stellar diameters, and of non-spherical stars, and observations with narrow spectral bands. This paper summarizes recent results from the MkIII interferometer, concentrating on single stars and their envelopes. More detailed descriptions of the instrument and the data reduction procedures are given by Mozurkewich et al. (1991).

Determination DES Diametres Stellaires Par Occultations. Technique de Reduction

Lunar occultations provide measurements of stellar angular diameters, leading thus to a determination of their effective temperatures. Basically, the data reduction process relies on two methods: model fitting and deconvolution. “Integrated deconvolution”, presented here, is a derived method of deconvolution and its results can compare with the two methods. The purpose is to determine the variation of the uncovered surface of the source throughout the occultation instead of extracting the brightness profile of the source. The main advantage provided is that computing the signal derivative is no longer necessary. In addition to saving a lot of computing time, the method affords a good estimate of the apparent speed, precise dating, and the angular separation of double stars down to 0.002 arcsec. After being tested against the usual methods, “integrated deconvolution” is now currently used. Some results are presented here.

Measuring Stars with High Angular Resolution: Current Status and Future Prospects

The current state of knowledge of angular diameters of stars is reviewed and, based on this review and the requirements for the determination of surface fluxes, effective temperatures, radii and masses, targets of sensitivity, angular resolution and accuracy for future programs of stellar angular diameter measurements are established. Long baseline interferometry is the only technique with the potential to meet all the targets. The necessary improvements in sensitivity, angular resolution and accuracy are promised by the approach adopted in the modern Michelson stellar interferometer under development at the University of Sydney and the prototype instrument, which is currently nearing completion, is briefly described to illustrate how the atmospheric and mechanical problems which have inhibited the development of amplitude interferometry may be overcome using modern technology. This program together with the developments taking place at CERGA lead to the conclusion that the prospects for contributions by high angular resolution measurements to the determination of fundamental stellar quantities during the next decade are excellent.

Lunar occultation stellar angular diameter measurements. III

view Abstract Citations (18) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Lunar occultation stellar angular diameter measurements. III. Beavers, W. I. ; Cadmus, R. R. ; Eitter, J. J. Abstract Additional stellar angular diameter measurements obtained by two-color photoelectric lunar occultation observations are reported. Within this set of 21 candidate events, nine yield measurable angular diameters, five are interpreted to be of unresolved stars, and seven proved to be too noisy for reliable analysis. Publication: The Astronomical Journal Pub Date: May 1982 DOI: 10.1086/113161 Bibcode: 1982AJ.....87..818B Keywords: Astrometry; Diameters; Lunar Occultation; Stellar Spectrophotometry; Stellar Structure; Electrophotometers; Point Sources; MOON; OCCULTATIONS; DIAMETERS; PHOTOELECTRIC OBSERVATIONS; ANALYSIS; DATA; PROCEDURE; COMPARISONS; Astronomy; Moon full text sources ADS | data products SIMBAD (17) Related Materials (2) Part 1: 1980AJ.....85.1505B Part 2: 1981AJ.....86.1404B

The Scope of the Lunar Occulation for Measurement of Stellar Angular Diameters

AbstractThis review of the lunar occultation technique summarizes results of 94 diameter measurements for 62 stars. The range of angular diameters measured, the coverage of spectral types, and the wavelengths studied are illustrated and discussedo Comparison with other techniques, magnitude limits and possible future developments are also considered.

Measurement of Stellar Angular Diameters by Speckle Interferometry

Interference fringes have recently been obtained with two small telescopes operated as a Michelson stellar interferometer. With the availability of photon-counting television cameras, it now becomes possible to build synthetic aperture arrays of large telescopes working in the speckle interferometry mode. Details of the two-telescope prototype are given, as well as preliminary plans for the construction of a full-size array.