Non-radial oscillations in M-giant semi-regular variables: Stellar models and Kepler observations

TL;DR

This study combines stellar modeling and Kepler data to analyze non-radial oscillation patterns in M-giant semi-regular variables, revealing triplet frequency structures and new mode ridges.

Contribution

It demonstrates the evolution of non-radial mode patterns in red giants and confirms triplet structures in Kepler observations, advancing asteroseismology of evolved stars.

Findings

Non-radial modes form triplet patterns near the red giant tip.

Kepler data shows dominant dipole triplet patterns in luminous M-giants.

New ridges of non-radial modes below the fundamental frequency are identified.

Abstract

The success of asteroseismology relies heavily on our ability to identify the frequency patterns of stellar oscillation modes. For stars like the Sun this is relatively easy because the mode frequencies follow a regular pattern described by a well-founded asymptotic relation. When a solar like star evolves off the main sequence and onto the red giant branch its structure changes dramatically resulting in changes in the frequency pattern of the modes. We follow the evolution of the adiabatic frequency pattern from the main sequence to near the tip of the red giant branch for a series of models. We find a significant departure from the asymptotic relation for the non-radial modes near the red giant branch tip, resulting in a triplet frequency pattern. To support our investigation we analyze almost four years of Kepler data of the most luminous stars in the field (late K and early M type) and find that their frequency spectra indeed show a triplet pattern dominated by dipole modes even for the most luminous stars in our sample. Our identification explains previous results from ground-based observations reporting fine structure in the Petersen diagram and sub ridges in the period-luminosity diagram. Finally, we find `new ridges' of non-radial modes with frequencies below the fundamental mode in our model calculations, and we speculate they are related to f modes.