Disk and wind kinematics in MWC 349 A

Context. Recombination-line maser emission arising from MWC 349 A offers a unique possibility to study the disk kinematics and the origin of ionized outflows driven by massive stars. Aims. We aim to constrain the disk inclination and its kinematics as well as the main parameters of the outflow launching processes. Methods. We used the IRAM interferometer to measure the relative positions of the H30 alpha centroid emission as a function of the radial velocity with an accuracy of similar to 2 mas (2.4 AU) for the strongest maser features and similar to 5 mas (6 AU) for the weaker line wings. Results. In addition to the east-west velocity gradient expected for a rotating disk, our data reveal for the first time the complex velocity gradients perpendicular to the disk that are related to the ejection of the ionized gas from the disk. Conclusions. From the comparison of the data with non-LTE 3D radiative transfer model predictions of the H30a line we conclude that the kinematics in the outer parts of the disk is represented by pure Keplerian rotation. We constrain the wind launching radius to less than 25 AU, much smaller than the gravitational radius of similar to 150 AU. The ionized outflow seems to be launched from the disk surface because it is rotating in the same sense than the disk. Disk wind models seem to explain the inferred kinematics.