ASYMMETRIC SURFACE BRIGHTNESS DISTRIBUTION OF ALTAIR OBSERVED WITH THE NAVY PROTOTYPE OPTICAL INTERFEROMETER

An asymmetric surface brightness distribution of the rapidly rotating A7 IV-V star Altair has been measured by the Navy Prototype Optical Interferometer (NPOI). The observations were recorded simultaneously using a triangle with long baselines of 30, 37, and 64 m, on 19 spectral channels, covering the wavelength range of 520-850 nm. The outstanding characteristics of these observations are (1) high resolution with the minimum fringe spacing of 1.7 mas, easily resolving the 3 mas stellar disk, and (2) the measurement of the closure phase, which is a sensitive indicator of the asymmetry of the brightness distribution of the source. Uniform-disk diameters fitted to the measured squared visibility amplitudes confirm Altair's oblate shape due to its rapid rotation. The measured observables of Altair showed two features that are inconsistent with both the uniform-disk and limb-darkened-disk models, while the measured observables 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 zero but rather remain at ≈0.002, indicating the existence of a small bright region on the stellar disk. The other is that the measured closure phases are neither 0° nor 180° 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 that 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.