Atomic Diffusion and Pulsation in Post-Common-Envelope Binary Stars

Abstract

I study the evolution of peculiar low-mass, evolved stars. Two groups of interest are the core-helium-burning hot subdwarfs and the shell-hydrogen-burning low-mass pre-white dwarfs. Stars in both groups have had previous interactions with a companion in order to remove most of their outer layers to leave the stars we see today. A detailed understanding of the past evolution of these stars provides important information about the interaction between close binary companions. This in itself is important, as approximately half of all stars in the Milky Way are in a binary or multiple star system. Additionally, hot subdwarfs show a variety of unusual surface compositions, with some stars showing lead and zirconium on their surface in amounts thousands of times more plentiful than seen in the Sun. It has been proposed that this is the result of radiative levitation, whereby outgoing radiation from the star can provide an upward force on certain elements, depending on their atomic structure.

The open-source MESA stellar evolution code was used to compute models of hot subdwarf stars and low-mass white dwarfs. These models were produced by using a large mass-loss rate to strip the outer layers from a red giant model, leaving either a low-mass core-helium burning model (subdwarf) or a model with an inert helium core and a thin hydrogen-burning envelope (pre-white dwarf) depending on the mass of the red giant core. Models were produced both with and without the effects of radiative levitation to determine the impact that this has on the evolution and surface composition of these stars. These were the first self-consistent models of hot subdwarfs from Main Sequence star through to the Horizontal Branch.

It is shown that radiative levitation is important in the phase of evolution immediately following the ejection of the common envelope, something which had not previously been considered. Comparison with the abundances of the observed population of hot subdwarfs showed that while the qualitative abundance patterns agree, the efficiency of radiative levitation is too strong in the simulations. This implies that other mixing processes must be invoked in order to accurately reproduce the observed abundances. In the case of heavy metals such as lead and zirconium, the amount of atomic data presently available is insufficient to calculate radiative accelerations for these elements.

In the case of the low-mass white dwarfs, the interest here focused on the discovery by Pietrukowicz et al. (2017) and Kupfer et al. (2019) of large-amplitude pulsating variable stars which show surface temperatures similar to hot subdwarfs. These stars are currently known as Blue Large-Amplitude Pulsators (BLAPs). The proposed structure of these stars is a low-mass pre-white dwarf, formed in a similar manner to a hot subdwarf. I used the GYRE stellar oscillation code to analyse the structure of my models and determined that low-mass pre-white dwarfs can pulsate when they have a temperature and surface gravity equivalent to the observed pulsators, with the appropriate pulsation period. The inclusion of radiative levitation in the models was found to be necessary in order to drive the pulsations. This indicates that the driving mechanism is the kappa-mechanism as a result of iron group opacity, the same mechanism which drives hot subdwarf pulsators. This represents the first conclusive identification of the evolutionary status of BLAPs along with the driving mechanism responsible for the pulsations. I have shown that a large instability region exists which unites the two currently observed groups of pulsator, and I predict that further such objects should be detected in ongoing and future large-scale sky surveys.