Mixed modes in red giants: a window on stellar evolution

TL;DR

This paper uses mixed mode oscillations in red giants observed by Kepler to map stellar evolution stages, providing precise seismic markers that inform models of stellar interior processes across various evolutionary phases.

Contribution

It introduces a method to measure asymptotic period spacings in over 1170 stars, enabling detailed seismic tracking of stellar evolution from main sequence to giant phases.

Findings

Seismic markers clearly distinguish evolutionary stages.

Quantitative definitions characterize transitions like subgiant to red giant.

Seismic data allows model-independent insights into stellar interior changes.

Abstract

The detection of oscillations with a mixed character in subgiants and red giants allows us to probe the physical conditions in their cores. With these mixed modes, we aim at determining seismic markers of stellar evolution. Kepler asteroseismic data were selected to map various evolutionary stages and stellar masses. Seismic evolutionary tracks were then drawn with the combination of the frequency and period spacings. We measured the asymptotic period spacing for more than 1170 stars at various evolutionary stages. This allows us to monitor stellar evolution from the main sequence to the asymptotic giant branch and draw seismic evolutionary tracks. We present clear quantified asteroseismic definitions that characterize the change in the evolutionary stages, in particular the transition from the subgiant stage to the early red giant branch, and the end of the horizontal branch.The seismic information is so precise that clear conclusions can be drawn independently of evolution models. The quantitative seismic information can now be used for stellar modeling, especially for studying the energy transport in the helium-burning core or for specifying the inner properties of stars entering the red or asymptotic giant branches. Modeling will also allow us to study stars that are identified to be in the helium-subflash stage, high-mass stars either arriving or quitting the secondary clump, or stars that could be in the blue-loop stage.