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Learn about measuring star radii through direct methods like interferometry, occultations, and eclipsing binaries, and determining temperatures using bolometric flux and model photospheres.
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Measuring Radii and Temperatures of Stars • Definitions (again…) • Direct measurement of radii • Speckle • Interferometry • Occultations • Eclipsing binaries • Photometric determinations of radii • Bolometric flux • Surface brightness • Absolute flux • Determining temperatures • Absolute flux • Model photospheres • Colors • Balmer jump • Hydrogen lines • Metal lines R = radius r = distance R/r=angular diameter
Stellar Diameters • Angular diameters typically measured in milli-arcseconds (mas) • Angular diameter (in radians) given by physical diameter divided by distance The diameter of Aldebaran is ~40 RSUN. Its distance is about 19 pc. The angular diameter of Aldebaran is … (work in cgs or MKS units or work in AU and use the definition of a parsec) What would the angular diameter of the Sun be at 10 pc?
Speckle Diameters • The diffraction limit of 4-m class telescopes is ~0.02” at 4000A, comparable to the diameter of some stars • The seeing disk of a large telescope is made up of the rapid combination of multiple, diffraction-limited images • 2-d Fourier transform of short exposures will recover the intrinsic image diameter • Only a few stars have large enough angular diameters. • Speckle mostly used for binary separations
Interferometry • 7.3-m interferometer originally developed by Michelson • Measured diameters for only 7 K & M giants • Until recently, only a few dozen stars had interferometric diameters
Other Methods • Occultations • Moon used as knife-edge • Diffraction pattern recorded as flux vs. time • Precision ~ 0.5 mas • A few hundred determined • Eclipsing binaries • Photometry gives ratio of radii to semi-major axes • Velocities give semi-major axes (i=90)
Photometric Methods – Bolometric Flux • Must know bolometric flux of star • Distance • Temperature • Bolometric correction • Calibrated with • Stellar models • Nearby stars with direct measurements
Surface Brightness • To avoid uncertainties in Teff and BC • Determine PV as a function of B-V PV(B-V)=logTeff – 0.1BC • PV(B-V) is known as the “surface brightness function” • Calibrate with directly measured diameters
Absolute Flux • Determine the apparent monochromatic flux at some wavelength, Fn • From a model that fits the spectral energy distribution, compute the flux at the star’s surface, Fn • From the ratio of Fn/Fn, compute the radius • The infrared flux method is just this method applied in the infrared.
Hipparcos! • The European Hipparcos satellite determined milli-arcsec parallaxes for more than 100,000 stars. • Distances are no longer the major source of uncertainty in radius determinations for many stars • Zillions of stars within range of the Keck interferometer (3 mas at 2m) • USNO & CHARA interferometers < 1 mas • Surface structure • Pulsations • Circumstellar material