The relationship between 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

Cas is thought to be the prototype of classical Be starsand is the most studied object among this group. However, as for all Bestars, the origin and the physics of its circumstellar disk responsiblefor the observed near IR-excess, emission lines, and peculiar X-rayemission is still being debated. Aims: We constrain the geometryand kinematics of its circumstellar disk from the highest spatialresolution ever achieved on this star. This investigation is a part of alarge multi-technique observing campaign to obtain the most completepicture of ¿ Cas which emphasizes the relation of thecircumstellar environment to the star's X-ray flux. Methods: Wepresent new observations in the near infrared (MIRC) and in the visible(VEGA) obtained with the CHARA interferometer. The VEGA instrumentallows us to not only obtain a global disk geometry but also spectrallydispersed visibility modulus and phases within the H¿ emissionline, which enables us to study the kinematics within ¿ Cas'sdisk. Results: We obtain a disk extension in the nearby H¿continuum of 1.72 stellar diameter and 1.86 stellar diameter in the Hband at 1.65 ¿m assuming a Gaussian disk model but also compatiblewith an elliptical ring model with a minor internal diameter of 1.38stellar diameter in H. For the first time we demonstrate that therotation mapped by the emission in the H¿ line within the disk of¿ Cas and up to 10 R¿ is Keplerian. Conclusions: These observations have pushed the size of the disk togreater proportions. ¿ Cas was also confirmed to be a nearlycritical rotator. The disk imaging gives neither indication of a 1-armspiral feature nor evidence of a secondary star reinforcing theinterpretation that the secondary is certainly a low-mass andlow-luminosity star or a degenerate companion.