Observational Insights into Post-Main-Sequence Rotation and Magnetism: 20 Years of Science, from the Subgiant to the White Dwarf Phase

Abstract

Rotation and magnetism are important phenomena to consider when trying to understand stars. By shaping the structure and chemical mixing in the interior of stars, they can induce significant changes in both their fundamental and observable properties. Despite this, our theoretical picture of how rotation and magnetism evolve throughout the stellar lifecycle remains incomplete. Gaps in our understanding are particularly stark for evolved stars, where slow rotation rates and weak magnetic fields make observational studies difficult. In response to this issue, the last 20 years have seen instrumentation to probe magnetism and rotation progressing in leaps and bounds. Through these new tools we have found that evolved stars rotate in ways that we cannot predict and have magnetic fields with properties that we did not expect. In the following, I will provide a brief overview of some recent observational results and their implications for our understanding of stars. I will focus on advances in asteroseismology and high-precision spectropolarimetry, as these techniques have advanced significantly over the last few decades. In keeping with the theme of Cool Stars, I'll cover stars with low to intermediate masses and their evolution from the subgiant to the white dwarf phase.