Probing the properties of Be star discs with spectroastrometry and NLTE radiative transfer modelling: beta CMi

Abstract: While the presence of discs around classical Be stars is well established, their origin is still uncertain. To understand what processes result in the creation of these discs and how angular momentum is transported within them, their physical properties must be constrained. This requires comparing high spatial and spectral resolution data with detailed radiative transfer modelling. We present a high spectral resolution, R~80,000, sub milli-arcsecond precision, spectroastrometric study of the circumstellar disc around the Be star beta CMi. The data are confronted with three-dimensional, NLTE radiative transfer calculations to directly constrain the properties of the disc. Furthermore, we compare the data to disc models featuring two velocity laws; Keperian, the prediction of the viscous disc model, and angular momentum conserving rotation. It is shown that the observations of beta CMi can only be reproduced using Keplerian rotation. The agreement between the model and the observed SED, polarisation and spectroastrometric signature of beta CMi confirms that the discs around Be stars are well modelled as viscous decretion discs.