The relationship between γ Cassiopeiae's X-ray emission and its circumstellar environment II. Geometry and kinematics of the disk from MIRC and VEGA instruments on the CHARA Array

Context. gamma Cas is thought to be the prototype of classical Be stars and is the most studied object among this group. However, as for all Be stars, the origin and the physics of its circumstellar disk responsible for the observed near IR-excess, emission lines, and peculiar X-ray emission is still being debated. Aims. We constrain the geometry and kinematics of its circumstellar disk from the highest spatial resolution ever achieved on this star. This investigation is a part of a large multi-technique observing campaign to obtain the most complete picture of gamma Cas which emphasizes the relation of the circumstellar environment to the star's X-ray flux. Methods. We present new observations in the near infrared (MIRC) and in the visible (VEGA) obtained with the CHARA interferometer. The VEGA instrument allows us to not only obtain a global disk geometry but also spectrally dispersed visibility modulus and phases within the H alpha emission line, which enables us to study the kinematics within gamma Cas's disk. Results. We obtain a disk extension in the nearby Ha continuum of 1.72 stellar diameter and 1.86 stellar diameter in the H band at 1.65 mu m assuming a Gaussian disk model but also compatible with an elliptical ring model with a minor internal diameter of 1.38 stellar diameter in H. For the first time we demonstrate that the rotation mapped by the emission in the Ha line within the disk of gamma Cas and up to 10 R-* is Keplerian. Conclusions. These observations have pushed the size of the disk to greater proportions. gamma Cas was also confirmed to be a nearly critical rotator. The disk imaging gives neither indication of a 1-arm spiral feature nor evidence of a secondary star reinforcing the interpretation that the secondary is certainly a low-mass and low-luminosity star or a degenerate companion.