The birth of Be star disks

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Abstract
Context. Classical Be stars exhibit mass ejection events that feed their viscous decretion disks. Recent TESS space photometry and simultaneous spectroscopy have revealed that these flickers are localised, short-lived, and associated with near-Keplerian rotating material close to the stellar surface. Aims. We constrain the geometrical and dynamical conditions required for a localised surface ejection to generate a Keplerian decretion disk, and we predict the corresponding photometric, spectroscopic, and polarimetric observables. Methods. We performed 3D smoothed particle hydrodynamic (SPH) simulations of asymmetric, eruption-like mass ejections from a rotating equatorial sector of a B-type star. We systematically varied the size and angular velocity of the injection volume, the viscosity of the material, and the injection radius. The SPH outputs were post-processed with the radiative transfer code HDUST to obtain synthetic observables. We scaled the density of each model to approximately match the observed properties of a reference flicker for the Be star f Car (B3Vne). Results. A mildly super-Keplerian rotation of the injection volume, a high viscosity, and a mass-loss rate of the order 10 −6 M ⊙ yr −1 str −1 are required for the ejected material to remain in orbit and form a small Keplerian disk. The resulting photometric, polarimetric, and H α variations reproduce the observed behaviour of the reference flicker. The simulations confirm that during mass ejection the disk is strongly asymmetric and dynamically evolving, and circularises within a few days after the end of the flicker, as observed. Models with a mass ejection angle that is too wide or an angular velocity that is too high fail to reproduce the observed light curve and line profile behaviour. The models are consistent with mass ejection happening very close to the stellar equator. Conclusions. Localised, short-duration, mildly super-Keplerian ejections combined with high viscosity and high mass-loss rates can account for the short-timescale variability of the circumstellar environment of Be stars. Be disks can be formed from such outbursts, and realistic 3D injection geometries are essential to connect surface dynamics to disk build-up within the framework of the viscous decretion disk model.

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