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G. van Belle (Jet Proplusion Laboratory), D. R. Ciardi (University of Florida), R. R. Thompson (Jet Proplusion Laboratory/Univ. of Wyoming), R. L. Akeson (IPAC/Caltech), E. A. Lada (University of Florida)
We present infrared interferometric angular size measurements for the A7IV-V star Altair which indicate a non-circular projected disk brightness distribution. Given the known rapid rotation of this star, we model the data as arising from an elliptically shaped photosphere, with major and minor axes of 2a=3.461±0.038 milliarcseconds (mas) and 2b=3.037±0.069 mas, respectively, for a difference of 424±79 microarcseconds (\muas) between 2a and 2b. The axial ratio is a/b =1.140±0.029 with the major axis lying at a position angle of 65±9\deg. Assuming that the apparent oblateness of the photosphere is due to the star's rapid rotation, an estimate of v \sin i = 224±28 km s-1 can be derived that is independent of spectroscopic techniques. Also derived are values for the mean effective temperature TEFF=7680±90K and the mean linear radius \overline{R}=1.794±0.047 R\odot. Altair is the first main sequence star for which observations of an oblate photosphere have been directly observed, and the first star for which v \sin i has been established from observations of the star's photospheric geometry.
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