The life cycles of Be viscous decretion discs: The case of {\omega} CMa

TL;DR

This study uses four decades of photometric data and hydrodynamic modeling to analyze the viscous decretion discs of the Be star {} CMa, revealing variable viscosity, ongoing angular momentum loss, and discrepancies with stellar evolution predictions.

Contribution

It demonstrates that the viscous decretion disc model accurately describes the disc evolution and provides new measurements of viscosity and angular momentum loss rates in {} CMa.

Findings

Viscosity parameter {} varies from 0.1 to 1.0 across cycles.

Disc dissipation involves ongoing angular momentum loss, not zero flux.

Measured angular momentum loss rates are over ten times smaller than stellar evolution predictions.

Abstract

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