Asteroseismology and Magnetic Cycles

TL;DR

This paper investigates how stellar magnetic activity cycles influence oscillation frequencies in solar-like stars, focusing on the indirect effects of starspots on internal stratification and their contribution to observed frequency variations.

Contribution

It provides a quantitative estimate of the impact of starspot-induced stratification changes on oscillation frequencies, highlighting that this effect is much smaller than observed solar cycle shifts.

Findings

Frequency shifts from stratification changes are about two orders of magnitude smaller than observed shifts.

Indirect effects of starspots on oscillation frequencies are minimal compared to the total observed variations.

The study isolates the indirect stratification effect, excluding direct magnetic field influences.

Abstract

Small cyclic variations in the frequencies of acoustic modes are expected to be a common phenomenon in solar-like pulsators, as a result of stellar magnetic activity cycles. The frequency variations observed throughout the solar and stellar cycles contain information about structural changes that take place inside the stars as well as about variations in magnetic field structure and intensity. The task of inferring and disentangling that information is, however, not a trivial one. In the sun and solar-like pulsators, the direct effect of the magnetic field on the oscillations might be significantly important in regions of strong magnetic field (such as solar- / stellar-spots), where the Lorentz force can be comparable to the gas-pressure gradient. Our aim is to determine the sun- / stellar-spots effect on the oscillation frequencies and attempt to understand if this effect contributes strongly to the frequency changes observed along the magnetic cycle. The total contribution of the spots to the frequency shifts results from a combination of direct and indirect effects of the magnetic field on the oscillations. In this first work we considered only the indirect effect associated with changes in the stratification within the starspot. Based on the solution of the wave equation and the variational principle we estimated the impact of these stratification changes on the oscillation frequencies of global modes in the sun and found that the induced frequency shifts are about two orders of magnitude smaller than the frequency shifts observed over the solar cycle.