Asymmetric Surface Brightness Distribution of Altair Observed with the Navy Prototype Optical Interferometer

Abstract

An asymmetric surface brightness distribution of the rapidly rotating A7IV-V star, Altair, has been measured by the Navy Prototype Optical Interferometer (NPOI). The observations were recorded simultaneously using a triangle of three long baselines of 30m, 37m, and 64m, on 19 spectral channels, covering the wavelength range of 520nm to 850nm. The outstanding characteristics of these observations are (a) high resolution with the minimum fringe spacing of 1.7mas, easily resolving the 3-milliarcsecond (mas) stellar disk, and (b) the measurement of closure phase which is a sensitive indicator to the asymmetry of the brightness distribution of the source. Uniform disk diameters fit to the measured squared visibility amplitudes confirms the Altair's oblate shape due to its rapid rotation. The measured observables of Altair showed two features which are inconsistent with both the uniform-disk and limb-darkened disk models, while the measured observable of the comparison star, Vega, are consistent with the limb-darkened disk model. The first feature is that measured squared visibility amplitudes at the first minimum do not reach 0.0 but rather remain at about 0.02, indicating the existence of a small bright region on the stellar disk. The other is that the measured closure phases show non-zero/180 degrees at all spectral channels, which requires an asymmetric surface brightness distribution. We fitted the measured observables to a model with a bright spot on a limb-darkened disk and found the observations are well reproduced by a bright spot, which has relative intensity of 4.7%, on a 3.38 mas limb-darkened stellar disk. Rapid rotation of Altair indicates that this bright region is a pole, which is brighter than other part of the star owing to gravity darkening.