First Results from the CHARA Array. VII. Long-Baseline Interferometric Measurements of Vega Consistent with a Pole-On, Rapidly Rotating Star

We have obtained high-precision interferometric measurements of Vega with the CHARA Array and FLUOR beam combiner in the K’ band at projected baselines between 103m and 273m. The measured visibility amplitudes beyond the first lobe are significantly weaker than expected for a slowly rotating star characterized by a single effective temperature and surface gravity. Our measurements, when compared to synthetic visibilities and synthetic spectrophotometry from a Roche-von Zeipel gravitydarkened model atmosphere, provide strong evidence for the model of Vega as a rapidly rotating star viewed very nearly pole-on. Our model of Vega’s projected surface consists of two-dimensional intensity maps constructed from a library of model atmospheres which follow pole-to-equator gradients of effective temperature and surface gravity over the rotationally distorted stellar surface. Our best fitting model, in good agreement with both our interferometric data and archival spectrophotometric data, indicates that Vega is rotating at ∼91% of its angular break-up rate with an equatorial velocity of 275 km s −1 . Together with the measured v sin i, this velocity yields an inclination for the rotation axis of 5 ◦ . For this model the pole-to-equator effective temperature difference is 2250 K, a value much larger than previously derived from spectral line analyses. A polar effective temperature of 10150 K is derived from a fit to ultraviolet and optical spectrophotometry. The synthetic and observed spectral energy distributions are in reasonable agreement longward of 140 nm where they agree to 5% or better. Shortward of 140 nm, the model is up to 10 times brighter than observed. The far-UV flux discrepancy suggests a breakdown of von Zeipel’s Teff ∝ g 1/4 relation. The derived equatorial Teff of 7900 K indicates Vega’s equatorial atmosphere may be convective and provides a possible explanation for the discrepancy. The model has a luminosity of ∼37 L⊙, a value 35% lower than Vega’s apparent luminosity based on its bolometric flux and parallax, assuming a slowly rotating star. The model luminosity is consistent with the mean absolute magnitude of A0V stars from the W(H) − MV calibration. Our model predicts the spectral energy distribution of Vega as viewed from its equatorial plane; a model which may be employed in radiative models for the surrounding debris disk. Subject headings: methods: numerical — stars: atmospheres — stars: fundamental parameters (radii, temperature) — stars: rotation — stars:individual (Vega) — techniques: interferometric