Suppression of quadrupole and octupole modes in red giants observed by Kepler

TL;DR

This study confirms that strong internal magnetic fields in red giants suppress certain oscillation modes, with observational evidence matching theoretical predictions about mode visibility changes during stellar evolution.

Contribution

It provides observational validation for the theory that magnetic greenhouse effects cause mode suppression in red giants, analyzing mode visibility across different degrees and evolutionary stages.

Findings

Quadrupole mode visibility reduces by up to 49% in less evolved stars.

No detectable suppression of octupole modes observed.

Mode lifetimes influence measured visibilities along the red giant branch.

Abstract

The asteroseismology of red giant stars has continued to yield surprises since the onset of high-precision photometry from space-based observations. An exciting new theoretical result shows that the previously observed suppression of dipole oscillation modes in red giants can be used to detect strong magnetic fields in the stellar cores. A fundamental facet of the theory is that nearly all the mode energy leaking into the core is trapped by the magnetic greenhouse effect. This results in clear predictions for how the mode visibility changes as a star evolves up the red giant branch, and how that depends on stellar mass, spherical degree, and mode lifetime. Here, we investigate the validity of these predictions with a focus on the visibility of different spherical degrees. We find that mode suppression weakens for higher degree modes with an average reduction in the quadrupole mode visibility of up to 49% for the least evolved stars in our sample, and no detectable suppression of octupole modes, in agreement with the theoretical predictions. We furthermore find evidence for the influence of increasing mode lifetimes on the measured visibilities along the red giant branch, in agreement with previous independent observations. These results support the theory that strong internal magnetic fields are responsible for the observed suppression of non-radial modes in red giants. We also find preliminary evidence that stars with suppressed dipole modes on average have slightly lower metallicity than normal stars.