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 elongated rigid atmosphere .
To first order , an ellipse may be fit to our interferometric diameter measurements , with major and minor axes of 2 a = 3.461 \pm 0.038 milliarcseconds ( mas ) and 2 b = 3.037 \pm 0.069 mas , respectively , for a difference of 424 \pm 79 microarcseconds ( \mu as ) between 2 a and 2 b , and with an axial ratio of a / b = 1.140 \pm 0.029 .
Assuming that the apparent oblateness of the photosphere is due to the star ’ s rapid rotation , a more rigorous evaluation of the observation data in the context of a rigidly rotating Roche model shows that an estimate of v \sin i = 210 \pm 13 km s ^ { -1 } can be derived that is independent of spectroscopic techniques .
Also derived are values for the mean effective temperature , the mean linear radius , and an observational constraint upon the the relationship between rotation velocity and stellar inclination is established .
Altair is the first main sequence star for which direct observations of an oblate photosphere have been reported , and the first star for which v \sin i has been established from observations of the star ’ s photospheric geometry .