The peculiar fast-rotating star 51 Ophiuchi probed by VEGA/CHARA

Context. Stellar rotation is a key in our understanding of both mass-loss and evolution of intermediate and massive stars. It can lead to anisotropic mass-loss in the form of radiative wind or an excretion disk.
Aims: We wished to spatially resolve the photosphere and gaseous environment of 51 Oph, a peculiar star with a very high vsini of 267 km s^-1 and an evolutionary status that remains unsettled. It has been classified by different authors as a Herbig, a β Pic, or a classical Be star.
Methods: We used the VEGA visible beam combiner installed on the CHARA array that reaches a submilliarcsecond resolution. Observation were centered on the Hα emission line.
Results: We derived, for the first time, the extension and flattening of 51 Oph photosphere. We found a major axis of θ_eq = 8.08 ± 0.70 R_⊙ and a minor axis of θ_pol = 5.66 ± 0.23 R_⊙. This high photosphere distortion shows that the star is rotating close to its critical velocity. Finally, using spectro-interferometric measurements in the Hα line, we constrained the circumstellar environment geometry and kinematics and showed that the emission is produced in a 5.2 ± 2 R_⋆ disk in Keplerian rotation.
Conclusions: From the visible point of view, 51 Oph presents all the features of a classical Be star: near critical-rotation and double-peaked Hα line in emission produced in a gaseous disk in Keplerian rotation. However, this does not explain the presence of dust as seen in the mid-infrared and millimeter spectra, and the evolutionary status of 51 Oph remains unsettled.