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An active core
An active core
In massive stars, the advanced burning (carbon-, oxygen- and silicon burning) provides a high luminosity from the core. This triggers a vigorous convective layer outside the burning core. Convective motion created by floating and sinking fluid parcel transports a significant amount of energy. Some waves can escape from the cavity and reach the surface as acoustic waves. Because of the density gradient, these waves strengthen into shocks and deposit their energy in the envelope. The envelope has a much lower binding energy, so even a small fraction of energy escapes, it is sufficient to trigger large surface movement. Some of them form the circumstellar medium around the star.
A simplified diagram in how convection in the core heats the envelope, and creates pre-supernova mass loss
Wave propagation in Super-AGB stars
Wave propagation in Super-AGB stars
The snapshot of a 15 solar mass star being excited by wave energy. Besides expansion, the envelope begins to show aspherical structure
I use my multi-D hydro code to compute how the energy finishes its propagation and deposit itself in the envelope as thermal energy. Within a year, the extended envelope becomes even puffer. Meanwhile the core remains moderately changed.
Mass loss of AGB as circumstellar medium
Mass loss of AGB as circumstellar medium
Time evolution of the total escaped mass for different energy (and duration)
When the excited envelope expands rapidly, part of it can excape from the gravitational bound of the star. They can travel for some period of time before the star collapses and explodes. These matter become the circumstellar medium (CSM).
CSM is an important component for interpreting the early time variation in the luminosity after the star explodes.
In this model, the CSM mass has a wide range from ~0.01 solar mass to as high as the full envelope mass (~6 solar mass)
Mass-Loss Statistics
Mass-Loss Statistics
The overall distribution of wave-energy escaped during the late-phase stellar evolution.
By computing a large array of stellar evolutionary models, we can derive the general trend of how this mechanism triggers the mass loss for both H-rich (crosses) and H-poor (circles; stripped-envelope) stars.
Overall results show that H-rich star can harvest much more wave energy than the H-poor ones. This suggests that, even though the H-poor star shows a bare core, the lower energy means there is less expected surface motion triggered by this process.
The metallicity (Z) does not play an important role here. Exceptional models exists for H-rich M=45 solar mass. This model exhibits the shell-merger event, meaning that some of the actively burning C-shell merges with the He-shell by convective overshooting. As a result, the He, which has a much lower ignition temperature, acts as a fresh fuel to enhance the nuclear reactions and the energy generation.
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