[Paper Review] Be star outbursts: transport of angular momentum by waves

C. Neiner, S. Mathis+2 more|arXiv (Cornell University)|Nov 11, 2013

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper proposes that stochastic excitation of gravito-inertial (g) waves in the convective cores of rapidly rotating B stars transports angular momentum to the surface, driving them to critical rotation and triggering Be outbursts. CoRoT observations of HD 49330 show g-mode amplitude surges before and during outbursts, while p-modes vanish, confirming that wave-driven angular momentum transport enables matter ejection and disk formation in Be stars.

ABSTRACT

The Be phenomenon, that is the ejection of matter from Be stars into a circumstellar disk, has been a long lasting mystery. In the last few years, the CoRoT (Convection, Rotation and planetary Transits) satellite brought clear evidence that Be outbursts are directly correlated with pulsations. We found that it may be the transport of angular momentum by waves or pulsation modes that brings the already rapid stellar rotation to its critical value at the surface, and allows the star to eject material. The recent discovery of stochastically excited gravito-inertial modes by CoRoT in a hot Be star strengthens this scenario. We present the CoRoT observations and modeling of several Be stars and describe the new picture of the Be phenomenon which arose from these results.

Motivation & Objective

• To resolve the long-standing mystery of what triggers Be star outbursts and circumstellar disk formation.
• To investigate the role of pulsations and wave-driven angular momentum transport in enabling rapid surface rotation necessary for ejection.
• To test whether stochastically excited gravito-inertial waves in convective cores can explain the observed correlation between pulsation modes and outburst phases.
• To determine how rotation enhances the amplitude of stochastic g-waves, enabling detectable angular momentum transport.
• To unify the Be phenomenon across spectral types by linking it to pulsation-driven angular momentum transport in rapidly rotating stars.

Proposed method

• Analyzing high-precision CoRoT photometric light curves of the Be star HD 49330 over ~136 days to identify and characterize pulsation frequencies.
• Cross-referencing photometric frequencies with spectroscopic line profile variations to confirm the photospheric origin of pulsation modes.
• Using seismic modeling with the Tohoku pulsation code to compute angular momentum transport rates due to pulsations, particularly focusing on g-mode excitation and damping.
• Evaluating the impact of stochastic excitation in convective zones on g-wave amplitudes and their angular momentum transport efficiency.
• Comparing the angular momentum deposition rates in the outer layers of HD 49330, showing a strong peak just below the surface due to pulsational damping.
• Extending the model to other Be stars by generalizing the scenario based on rotation rate, pulsation type, and core convection.

Experimental results

Research questions

• RQ1 Can stochastic excitation of gravito-inertial waves in the convective core of a B star explain the onset of Be outbursts?
• RQ2 How do the amplitudes and frequencies of g-modes correlate with the phases of outbursts in HD 49330?
• RQ3 To what extent does rapid rotation enhance the amplitude of stochastically excited g-waves, enabling detectable angular momentum transport?
• RQ4 Why do only rapidly rotating B stars become Be stars, despite all pulsating B stars having some form of oscillations?
• RQ5 What is the relative contribution of different pulsation modes (p-modes vs. g-modes) to surface angular momentum transport in Be stars?

Key findings

• CoRoT observations of HD 49330 revealed over 300 frequencies, including 30 independent pulsation modes, with p-modes decreasing in amplitude before and during outbursts.
• Groups of g-mode frequencies appeared just before the outburst, peaked during it, and disappeared afterward, indicating a transient wave-driven mechanism.
• The surface layers of HD 49330 reached critical rotation during the outburst, likely due to angular momentum deposition by waves, causing the p-mode cavity to break and triggering ejection.
• Seismic modeling shows a drastic increase in local angular momentum change rate in the outermost few percent of the stellar radius, confirming strong net angular momentum deposition in surface layers.
• Stochastic g-waves excited in turbulent convective regions can transport angular momentum as effectively as κ-mechanism-driven waves, with transport efficiency increasing with wave amplitude and decreasing with frequency.
• Rapid rotation enhances the amplitude of stochastic g-waves, enabling them to transport sufficient angular momentum to push the surface to critical rotation, explaining why only fast rotators become Be stars.