Diameter and limb-darkening measures for Alpha Orionis

Abstract

Previously reported speckle interferometric measurements of the angular diameter of Alpha Ori are widely scattered and apparently inconsistent. Stellar atmosphere models predict a stellar disk to be limb-darkened differently for different spectral features, and that less limb-darkening will be observed at longer wavelengths. Upon careful analysis of calibrated speckle observations obtained at various wavelengths with the KPNO 4 m telescope, a single diameter parameter is found to be inadequate to explain all of the features of the measurements. There is evidence for a central stellar component of 42.1(+ or - 1.1) mas diameter with extended structure surrounding it. Limb-darkening effects are suggested by attempts to fit cosine model image profiles to the data. These fits have significant residuals at large radii suggesting a faint envelope extending from 1 to 5 stellar radii.