It's not just a phase: oblique pulsations in magnetic red giants and other stochastic oscillators

Abstract: Magnetic fields play a significant role in stellar evolution. In the last few years, asteroseismology has enabled the measurement of strong magnetic fields $10^4$--$10^6\,\mathrm{G}$ in the cores of dozens of red giants, and is the only known way to directly measure internal stellar magnetic fields. However, current data are still interpreted assuming that these fields are too weak or too axisymmetric to affect the orientation of the pulsations (i.e., make the pulsations ``oblique''), rendering stronger field strengths beyond the reach of existing asteroseismic searches. We show that, even when an oblique pulsator is also stochastic (such as in a red giant with a strong non-axisymmetric magnetic field), geometric effects will cause the signal to contain frequency components which remain in perfect relative phase with each other. This perfect phase relationship persists even over timescales in which stochasticity erases absolute phase information. This perfect relative coherence is a distinctive observational signature of oblique pulsation that does not require a model for mode frequencies to search for. However, due to its dependence on phase, this effect will not be evident in the power spectral density alone, and phase information should be retained in order to detect it. Coherence-based searches for oblique pulsations may pave the way to measurements of magnetic fields of currently inaccessible strengths in red giants, as well as some main-sequence and compact pulsators.