1. Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794-3800, USA
2. European Southern Observatory (ESO), Casilla 19001, Santiago 19, Chile
3. US Naval Observatory, 3450 Massachusetts Avenue NW, Washington DC, 20392-5420, USA
4. Naval Research Laboratory, Code 7215, 4555 Overlook Avenue SW, Washington DC 20375, USA
5. US Naval Observatory, Flagstaff Station, 10391 West Naval Observatory Road, Flagstaff, Arizona 86001-8521, USA
6. Seabrook Engineering, 9310 Dubarry Road, Seabrook, Maryland 20706, USA
7. Interferometrics, Inc., 13454 Sunrise Valley Drive, Suite 240, Herndon, Virginia 20171, USA

Correspondence to: D. M. Peterson¹ Correspondence and requests for materials should be addressed to D.M.P. (Email: dpeterson@astro.sunysb.edu).

Abstract

Vega, the second brightest star in the northern hemisphere, serves as a primary spectral type standard¹. Although its spectrum is dominated by broad hydrogen lines, the narrower lines of the heavy elements suggested slow to moderate rotation, giving confidence that the ground-based calibration of its visible spectrum could be safely extrapolated into the ultraviolet and near-infrared (through atmosphere models²), where it also serves as the primary photometric calibrator. But there have been problems: the star is too bright compared to its peers³ and it has unusually shaped absorption line profiles, leading some^{4, 5} to suggest that it is a distorted, rapidly rotating star seen pole-on. Here we report optical interferometric observations that show that Vega has the asymmetric brightness distribution of the bright, slightly offset polar axis of a star rotating at 93 per cent of its breakup speed. In addition to explaining the unusual brightness and line shape peculiarities, this result leads to the prediction of an excess of near-infrared emission compared to the visible, in agreement with observations^{6, 7}. The large temperature differences predicted across its surface call into question composition determinations, adding uncertainty to Vega's age and opening the possibility that its debris disk⁸ could be substantially older than previously thought^{9, 10}.

1. Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794-3800, USA
2. European Southern Observatory (ESO), Casilla 19001, Santiago 19, Chile
3. US Naval Observatory, 3450 Massachusetts Avenue NW, Washington DC, 20392-5420, USA
4. Naval Research Laboratory, Code 7215, 4555 Overlook Avenue SW, Washington DC 20375, USA
5. US Naval Observatory, Flagstaff Station, 10391 West Naval Observatory Road, Flagstaff, Arizona 86001-8521, USA
6. Seabrook Engineering, 9310 Dubarry Road, Seabrook, Maryland 20706, USA
7. Interferometrics, Inc., 13454 Sunrise Valley Drive, Suite 240, Herndon, Virginia 20171, USA

Correspondence to: D. M. Peterson¹ Correspondence and requests for materials should be addressed to D.M.P. (Email: dpeterson@astro.sunysb.edu).

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