The life cycles of Be viscous decretion discs: The case of ω CMa

Abstract

We analyzed V-band photometry of the Be star {\\omega} CMa, obtained during\nthe last four decades, during which the star went through four complete cycles\nof disc formation and dissipation. The data were simulated by hydrodynamic\nmodels based on a time-dependent implementation of the viscous decretion disc\n(VDD) paradigm, in which a disc around a fast-spinning Be star is formed by\nmaterial ejected by the star and driven to progressively larger orbits by means\nof viscous torques. Our simulations offer a good description of the photometric\nvariability during phases of disc formation and dissipation, which suggests\nthat the VDD model adequately describes the structural evolution of the disc.\nFurthermore, our analysis allowed us to determine the viscosity parameter\n{\\alpha}, as well as the net mass and angular momentum (AM) loss rates. We find\nthat {\\alpha} is variable, ranging from 0.1 to 1.0, not only from cycle to\ncycle but also within a given cycle. Additionally, build-up phases usually have\nlarger values of {\\alpha} than the dissipation phases. Furthermore, during\ndissipation the outward AM flux is not necessarily zero, meaning that {\\omega}\nCMa does not experience a true quiescence but, instead, switches between a high\nto a low AM loss rate during which the disc quickly assumes an overall lower\ndensity but never zero. We confront the average AM loss rate with predictions\nfrom stellar evolution models for fast-rotating stars, and find that our\nmeasurements are smaller by more than one order of magnitude.\n