On the evolution and simulation of strange-mode instabilities

Abstract

Sufficiently luminous objects, such as massive stars, suffer from strange mode instabilities with growth rates in the dynamical range. The boundary of the domain in the Hertzsprung-Russel diagram above which all stellar models are unstable, coincides with the observed Humphreys-Davidson (HD) limit. Moreover, the range of unstable models covers the stellar parameters for which the LBV phenomenon is observed.The high growth rates of the instabilities indicate a connection to the observed mass loss of the corresponding objects. To test this conjecture, stellar envelope models in the vicinity of the HD limit are constructed. Their linear stability properties are investigated and the evolution of the identified instabilities is followed into the non-linear regime by direct numerical simulation. The instabilities lead to stellar pulsations with velocity amplitudes that reach 50 per cent of the escape velocity. Furthermore, they transfer mass into the outer parts of the stellar envelope.For the considered models, shocks are captured in the hydrogen ionisation zone and start to oscillate on very short timescales. The origin of these shock oscillations is identified by a linear stability analysis and the construction of analytical models. For an adequate numerical treatment of this new phenomenon a domain decomposition procedure is presented. The observed mass flow leads to a reduced grid resolution in the region of the envelope that drives the pulsation. To deal with this problem, a grid reconstruction procedure is developed.