We have obtained high-precision interferometric measurements of the A0
V standard star Vega with the Center for High Angular Resolution
Astronomy (CHARA) Array and the Fiber Linked Unit for Optical
Recombination (FLUOR) beam combiner in the K' band at projected
baselines between 103 m and 273 m. The measured squared visibility
amplitudes beyond the first lobe are significantly weaker than
expected for a slowly rotating star and provide strong evidence for
the model of Vega as a rapidly rotating star viewed very nearly pole
on. We have constructed a Roche-von Zeipel gravity-darkened model
atmosphere which is in generally good agreement with both our
interferometric data and archival spectrophotometry. Our model
indicates Vega is rotating at 91% of its angular break-up rate with
an equatorial velocity of
275 km
. We find a polar effective
temperature of
10150 K and a pole-to-equator effective
temperature difference of
2300 K. Our model suggests that
Vega's cool equatorial atmosphere may have significant convective flux
and predicts a significantly cooler spectral energy distribution for
Vega as seen by its surrounding debris disk.