First Results from the CHARA Array. III. Oblateness, Rotational Velocity, and Gravity Darkening of Alderamin

We present observations of the A7 IV-V star Alderamin (α Cep, HR 8162, HD 203280) from the Georgia State University CHARA Array. These infrared interferometric angular size measurements indicate a noncircular projected disk brightness distribution for this known rapid rotator. The interferometric observations are modeled as arising from an elongated rigid atmosphere, with apparent polar and equatorial radii of r_p=0.6753^+0.0119_-0.0135 and r_e=0.8767^+0.0293_-0.0183 mas, respectively, for a difference of 201+/-32 μas, and with an axial ratio of r_e/r_p=1.298+/-0.051. Using the Hipparcos distance of 14.96+/-0.11 pc, these angular measures translate to 2.18+/-0.05 and 2.82+/-0.10 R_solar. The inclination of Alderamin to the line of sight indicated by this modeling is effectively edge-on (i=88.2^+1.8_-13.3 deg). The star has a true rotational velocity of 283+/-10 km s^-1 (~83% of breakup velocity) and a polar temperature of roughly 8400 K. Significantly, a necessary aspect of this modeling is a determination of the gravity-darkening coefficient, which at a value of β=0.084^+0.026_-0.049 is consistent with a convective photosphere, as expected for an A7 IV-V star. Our detailed characterization of this object allows us to investigate various scenarios for the angular momentum history of Alderamin and the appropriateness of certain stellar evolution models.