It's not just a phase: oblique pulsations in magnetic red giants and other stochastic oscillators
Rui, Nicholas; Fuller, Jim; Ong, Joel
- Publisher
- Zenodo
- Language
- en
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 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 strongly magnetic red giant), 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.
Full text
“it’s not just a phase!” oblique pulsations in magnetic red giants and other stochastic oscillators Nicholas Z. Rui,Jim Fuller,Joel Ong TASC9/KASC16 Workshop ⟣Tuesday, 8 July 2025 1
Li et al. 2022 Bugnet et al. 2021 •magnetism: seismic measurements in ~dozens of red giant interiors • key feature: magnetic “asymmetries” in seismic rotational splittings seismic magnetometry of red giants see talks by: Daniel Lecoanet, Sébastien Deheuvels, Lukas Einramhof, Jérôme Ballot, Lucas Barrault, etc.
anatomy of a seismic rotational splitting • modes characterized by quantum numbers ℓ (scale) and m (azimuth) • spherical symmetry → (2ℓ+1)-fold degeneracy in m (arbitrary z axis) •degeneracy typically broken by rotation (rotation splitting)
anatomy of a seismic rotational splitting δωrot = -m (1 - Cnℓ)Ωrot 1) Doppler: rotation modifies the apparent speed of traveling waves 2) Coriolis: fictitious force in rotating form acts as a restorative force (g modes) 1 from: blog.matten.com from: torch-harmonics 2
animations from Fuller et al. 2025 (tidally tilted pulsations) “standard case” Coriolis > magnetism •Doppler : shifts frequency •signal stays sinusoidal “oblique case” magnetism > Coriolis •Doppler : shift depends on m •signal undergoes beating
6 roAp star HD 60435 (Kurtz et al. 2025) •prototype WD pulsator GD 358 (Montgomery et al. 2025) •prototype MS pulsator β Cephei (Shibahashi & Aerts 2000) •BLAPs OGLE-BLAP-001 and ZGP-BLAP-08 (Pigulski et al. 2024) see also Andrzej Pigulski’s talk examples of oblique pulsators a single oblique pulsation mode
current asymmetric magnetic RGs are “aligned” 7 •standard case : magnetism causes “asymmetries” •oblique case : magnetism generates extra “modes”
stochasticity breaks the degeneracy 8
9 stochastic variables oblique pulsations generate perfect relative coherence •oblique pulsator signals are nonstationary: •the PSD does not fully describe their statistics
