The Applegate mechanism in Post-Common-Envelope Binaries: Investigating the role of rotation

TL;DR

This study investigates whether the Applegate mechanism, driven by stellar magnetic activity and rotation, can explain observed eclipse timing variations in post-common-envelope binaries, using stellar evolution models and dynamo theory.

Contribution

It demonstrates the energetic feasibility of the Applegate mechanism in certain PCEBs and highlights the importance of stellar rotation rate in its effectiveness.

Findings

Applegate mechanism feasible in 5 PCEB systems

Higher stellar rotation rates correlate with mechanism feasibility

Physical to critical rotation ratio is a key indicator

Abstract

Eclipsing time variations (ETVs) are observed in many close binary systems. In particular, for several post-common-envelope binaries (PCEBs) that consist of a white dwarf and a main sequence star, the O-C diagram suggests that real or apparent orbital period variations are driven by Jupiter-mass planets or as a result of magnetic activity, the so-called Applegate mechanism. The latter explains orbital period variations as a result of changes in the stellar quadrupole moment due to magnetic activity. We explore the feasibility of driving ETVs via the Applegate mechanism for a sample of PCEB systems, including a range of different rotations. Using the MESA code we evolve 12 stars with different masses and rotation rates. We apply a simple dynamo model to their radial profiles to investigate on which scale the predicted activity cycle matches the observed modulation period, and quantify the uncertainty, and further calculate the required energies to drive que Applegate mechanism. We show that the Applegate mechanism is energetically feasible in 5 PCEB systems, and note that these are the systems with the highest rotation rate compared to the critical rotation rate of the main-sequence star. The results suggest that the ratio of physical to critical rotation in the main sequence star is an important indicator for the feasibility of Applegate's mechanism, but exploring larger samples will be necessary to probe this hypothesis.