Sub-milliarcsecond precision spectro-astrometry of Be stars

Context. The origin of the disks around Be stars is still not known. Further progress requires a proper parametrization of their structure, both spatially and kinematically. This is challenging as the disks are very small. Aims. Here we assess whether a novel method is capable of providing these data. Methods. We obtained spectro-astrometry around the Pa β line of two bright Be stars, α  Col and ζ  Tau, to search for disk signatures. The data, with a pixel-to-pixel precision of the centroid position of 0.3..0.4 milliarcsecond is the most accurate such data to date. Artefacts at the 0.85 mas level are present in the data, but these are readily identified as they were non-repeatable in our redundant datasets. This does illustrate the need of taking multiple data to avoid spurious detections. Results. The data are compared with model simulations of the spectro-astrometric signatures due to rotating disks around Be stars. The upper limits we find for the disk radii correspond to disk sizes of a few dozen stellar radii if they rotate Keplerian. This is very close to observationally measured and theoretically expected disk sizes, and this paper therefore demonstrates that spectro-astrometry, of which we present the first such attempt, has the potential to resolve the disks around Be stars.