Abstract
Context. HD 49330 is an early Be star that underwent an outburst during its five-month observation with the CoRoT satellite. An analysis of its light curve revealed several independent p and g pulsation modes, in addition to showing that the amplitude of the modes is directly correlated with the outburst. Aims. We modelled the results obtained with CoRoT to understand the link between pulsational parameters and the outburst of this Be star. Methods. We modelled the flattening of the structure of the star due to rapid rotation in two ways: Chandrasekhar-Milne’s expansion and 2D structure computed with ROTORC. We then modelled κ-driven pulsations. We also adapted the formalism of the excitation and amplitude of stochastically excited gravito-inertial modes to rapidly rotating stars, and we modelled those pulsations as well. Results. We find that while pulsation p modes are indeed excited by the κ mechanism, the observed g modes are, rather, a result of stochastic excitation. In contrast, g and r waves are stochastically excited in the convective core and transport angular momentum to the surface, increasing its rotation rate. This destabilises the external layers of the star, which then emits transient stochastically excited g waves. These transient waves produce most of the low-frequency signal detected in the CoRoT data and ignite the outburst. During this unstable phase, p modes disappear at the surface because their cavity is broken. Following the outburst and ejection of the surface layer, relaxation occurs, making the transient g waves disappear and p modes reappear. Conclusions. This work includes the first coherent model of stochastically excited gravito-inertial pulsation modes in a rapidly rotating Be star. It provides an explanation for the correlation between the variation in the amplitude of frequencies detected in the CoRoT data and the occurrence of an outburst. This scenario could apply to other pulsating Be stars, providing an explanation to the long-standing questions surrounding Be outbursts and disks.
Highlights
Be stars are late-O to early-A stars surrounded by a decretion disk fed by discrete mass-loss events
Be stars are known to host pulsations excited by the κ mechanism, similar to those observed in β Cephei and SPB stars (e.g. Neiner et al 2009)
From the models presented above we conclude that (1) nonadiabatic treatment is needed, in particular to study the modes below 2Ω and to know whether the modes are excited; and (2) a 2D structure model is required to be able to reproduce p modes for which the rotational deformation has a strong impact
Summary
Be stars are late-O to early-A stars surrounded by a decretion disk fed by discrete mass-loss events (see the review by Rivinius et al 2013). Be excited in numerical simulations, to efficiently transport angular momentum in hot stars (Rogers et al 2013), and to provide predictions for rotation profiles that match the seismic observations (Rogers 2015). During the CoRoT observation, an outburst of several hundredths of magnitude occurred Outbursts, and their subsequent dimming, are known to occur frequently in this star. The correlation between the changes in the pulsation parameters and the outburst is clear, the mechanism linking the two phenomena still needs to be investigated This can be done thanks to seismic modelling and this investigation is the purpose of this paper. We draw conclusions about the treatment and interpretation of seismic observations of early Be stars and about the impact of stochastic g waves on the ignition of Be outbursts in Sect.
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