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Local Radiation-Driven Instabilities in Post-Main Sequence Massive Stars

Late in their evolution, massive stars may undergo periods of violent instability and mass loss, but the mechanism responsible for these episodes has not been identified. We study one potential contributor: the development of local radiation-driven instabilities in the outer layers of main sequence (MS) and post-MS massive stars. We construct a sequence of massive stellar evolution models and investigate where they are subject to local radiative instabilities, both in the presence of magnetic fields and without them,and at a range of metallicities. We find that these types of instabilities do not occur in solar-metallicity MS stars up to 100\,M$_\odot$, but they set in immediately post-MS for stars heavier than $\sim 25$\,M$_\odot$. Once an instability appears, it involves a significant amount of mass in the star's upper layers (up to $\sim 1$ per cent of the initial stellar mass), suggesting that radiation-driven instabilities are a potentially viable mechanism for dynamic mass loss. We find that the presence of magnetic fields at strengths low enough not to disturb the hydrostatic balance of the star does not alter these results. Stars with sub-Solar metallicity also show instability, but their instabilities involve less mass and appear later in the star's evolution.