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

T. E. Nordgren (University of Redlands), N. Ohishi (NAOJ), D.J. Hutter (USNO - NPOI)

Using the long-basline optical interferometer, NPOI, we confirm the oblateness of the rapidly rotating A7IV-V star, Altair. Our observations of Altair yield an elliptical disk in agreement with those first reported by the Palomar Testbed Interferometer in 2001. In addition, two features of the data indicate a surface brightness distribution for Altair inconsistent with both uniform-disk and limb-darkened disk models. The first feature is that measured squared visibility amplitudes at the first minimum do not reach 0.0 but rather remain at ~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 or non-180 degrees at all spectral channels, which requires an asymmetric surface brightness distribution. Neither of these features are present in concurrent observations of the slow rotating, yet similar in magnitude, angular size, and spectral type star, Vega.

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Bulletin of the American Astronomical Society, 36 #2
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