Spectro-interferometric observations of a sample of Be stars: Setting limits to the geometry and kinematics of stable Be disks★

Be stars are rapid rotators surrounded by a gaseous disk envelope whose origin is still under debate. This envelope is responsible for observed emission lines and large infrared excess. To progress in the understanding of the physical processes involved in the disk formation, we estimate the disk parameters for a sample of Be stars and search for correlations between these parameters and stellar properties. We performed spectro-interferometric observations of 26 Be stars in the region of the Br$\gamma$ line to study the kinematical properties of their disks through the Doppler effect. Observations were performed at the Paranal observatory with the VLTI/AMBER interferometer. This instrument provides high spectral and high spatial resolutions. We modeled 18 Be stars with emission in the Br$\gamma$ line. The disk kinematic is described by a quasi-Keplerian rotation law, with the exception of HD28497 that presents a one-arm density-wave structure. Using a combined sample, we derived a mean value for the velocity ratio V/Vc=0.75, and found that rotation axes are probably randomly distributed in the sky. Disk sizes in the line component model are in the range of 2-13 stellar radii and do not correlate with the effective temperature or spectral type. However, we found that the maximum size of a stable disk correlates with the rotation velocity at the inner part of the disk and the stellar mass. We found that, on average, the Be stars of our combined sample do not rotate at their critical velocity. However, the centrifugal force and mass of the star defines an upper limit size for a stable disk configuration. For a given rotation, high-mass Be stars tend to have more compact disks than their low-mass counterparts. It would be interesting to follow up the evolution of the disk size in variable stars to better understand the formation and dissipation processes of their circumstellar disks.