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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 Vegawith the CHARA Array and FLUOR beam combiner in the K' bandat projected baselines between 103 and 273 m. The measured visibilityamplitudes beyond the first lobe are significantly weaker than expectedfor a slowly rotating star characterized by a single effectivetemperature and surface gravity. Our measurements, when compared tosynthetic visibilities and synthetic spectrophotometry from a Roche-vonZeipel gravity-darkened model atmosphere, provide strong evidence forthe model of Vega as a rapidly rotating star viewed very nearly pole-on.Our best-fitting model indicates that Vega is rotating at ~91% of itsangular break-up rate with an equatorial velocity of 275 kms-1. Together with the measured vsini, this velocity yieldsan inclination for the rotation axis of 5deg. For this modelthe pole-to-equator effective temperature difference is ~2250 K, a valuemuch larger than previously derived from spectral line analyses. A polareffective temperature of 10,150 K is derived from a fit to ultravioletand optical spectrophotometry. The synthetic and observed spectralenergy distributions are in reasonable agreement longward of 140 nm,where they agree to 5% or better. Shortward of 140 nm, the model is upto 10 times brighter than observed. The model has a luminosity of ~37Lsolar, a value 35% lower than Vega's apparent luminositybased on its bolometric flux and parallax, assuming a slowly rotatingstar. Our model predicts the spectral energy distribution of Vega asviewed from its equatorial plane, and it may be employed in radiativemodels for the surrounding debris disk.
| Spectral Classification of Stars Noted on Case Objective Prism Plates. II
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