The physics of the Applegate mechanism: Eclipsing time variations from magnetic activity

TL;DR

This paper provides a detailed physical analysis of the Applegate mechanism, showing it likely cannot explain observed eclipse timing variations in most close binary systems, especially RS-CVn types, but may be relevant for certain low-mass PCEBs.

Contribution

It introduces the most detailed model to date for the Applegate mechanism, incorporating kinetic and magnetic fluctuations to assess its viability in explaining timing variations.

Findings

Binary period variations are 10-100 times lower than observed in RS-CVn systems.

The mechanism is more plausible in low-mass post-common-envelope binaries with specific parameters.

Supports previous conclusions that the Applegate mechanism alone cannot explain all observed timing variations.

Abstract

Since its proposal in 1992, the Applegate mechanism has been discussed as a potential intrinsical mechanism to explain transit timing variations in various kinds of close binary systems. Most analytical arguments presented so far focused on the energetic feasibility of the mechanism, while applying rather crude one- or two-zone prescriptions to describe the exchange of angular momentum within the star. In this paper, we present the most detailed approach to date to describe the physics giving rise to the modulation period from kinetic and magnetic fluctuations. Assuming moderate levels of stellar parameter fluctuations, we find that the resulting binary period variations are one or two orders of magnitude lower than the observed values in RS-CVn like systems, supporting the conclusion of existing theoretical work that the Applegate mechanism may not suffice to produce the observed variations in these systems. The most promising Applegate candidates are low-mass post-common-envelope binaries (PCEBs) with binary separations $\lesssim 1~\mathrm{R}_\odot$ and secondary masses in the range of $0.30~\mathrm{M}_\odot$ and $0.36~\mathrm{M}_\odot$.