Origin of eclipsing time variations: Contributions of different modes of the dynamo-generated magnetic field

Abstract

The possibility to detect circumbinary planets and to study stellar magnetic fields through eclipsing time variations (ETVs) in binary stars has sparked an increase of interest in this area of research. We revisit the connection between stellar magnetic fields and the gravitational quadrupole moment $Q_{xx}$ and compare different dynamo-generated ETV models with our simulations. We present magnetohydrodynamical simulations of solar mass stars with rotation periods of 8.3, 1.2, and 0.8 days and perform a detailed analysis of the magnetic and quadrupole moment using spherical harmonic decomposition. The extrema of $Q_{xx}$ are associated with changes in the magnetic field structure. This is evident in the simulation with a rotation period of 1.2 days. Its magnetic field has a more complex behavior than in the other models, as the large-scale nonaxisymmetric field dominates throughout the simulation and the axisymmetric component is predominantly hemispheric. This triggers variations in the density field that follow the magnetic field asymmetry with respect to the equator, affecting the $zz$ component of the inertia tensor, and thus modulating $Q_{xx}$. The magnetic field of the two other runs are less variable in time and more symmetric with respect to the equator, such that the variations in the density are weaker, and therefore only small variations in $Q_{xx}$ are seen. If interpreted via the classical Applegate mechanism (tidal locking), the quadrupole moment variations obtained in the current simulations are about two orders of magnitude below those deduced from observations of post-common-envelope binaries. However, if no tidal locking is assumed, our results are compatible with the observed ETVs.

arXiv comment15 pages, 16 figures. Accepted for publication in A&A

References in corpus (21)

• Solar-type dynamo behaviour in fully convective stars without a tachocline
• From solar-like to anti-solar differential rotation in cool stars
• The Pencil Code, a modular MPI code for partial differential equations and particles: multipurpose and multiuser-maintained
• Effects of enhanced stratification on equatorward dynamo wave propagation
• Detection of a period decrease in NN Ser with ULTRACAM: evidence for strong magnetic braking or an unseen companion?
• Tidal Synchronization and Differential Rotation of Kepler Eclipsing Binaries
• Origin of apparent period variations in eclipsing post-common-envelope binaries
• Confirmation of bistable stellar differential rotation profiles
• Convective dynamos in spherical wedge geometry
• Eclipsing time variations in close binary systems: Planetary hypothesis vs. Applegate mechanism
• Transition from axi- to nonaxisymmetric dynamo modes in spherical convection models of solar-like stars
• Second generation planet formation in NN Serpentis?
• The First Science Results from SPHERE: Disproving the Predicted Brown Dwarf around V471 Tau
• The large-scale magnetic field of Proxima Centauri near activity maximum
• Azimuthal dynamo wave in spherical shell convection
• Common dynamo scaling in slowly rotating young and evolved stars
• The V471 Tauri System: A Multi-datatype Probe
• The physics of the Applegate mechanism: Eclipsing time variations from magnetic activity
• Star-in-a-box simulations of fully convective stars
• Single-hemisphere dynamos in M-dwarf stars
• The Applegate mechanism in Post-Common-Envelope Binaries: Investigating the role of rotation

Cited by in corpus (3)

• Origin of eclipsing time variations in Post-Common-Envelope binaries: role of the centrifugal force
• Eclipse timing variations in 7 post-common envelope binaries: an update on recent circumbinary models
• Effects of the centrifugal force in stellar dynamo simulations