# Magneto-hydrodynamical origin of eclipsing time variations in post-common-envelope binaries for solar mass secondaries

## TL;DR

This study uses magneto-hydrodynamical simulations to explore how magnetic activity in solar-mass secondaries of binary systems can cause observed eclipsing time variations, offering a potential explanation beyond third-body effects.

## Contribution

The paper presents the first MHD simulations of magnetic dynamo effects on quadrupole moments in solar-mass stars with different rotation rates, linking magnetic activity to eclipse timing variations.

## Key findings

- Slow rotator shows quasi-periodic magnetic and quadrupole changes causing ~0.025 s O-C variations.
- Rapid rotator exhibits complex magnetic behavior with ~0.13 s O-C variations.
- Simulations suggest magnetic activity could explain observed eclipse timing variations in systems like V471 Tau.

## Abstract

Eclipsing time variations have been observed for a wide range of binary systems, including post-common-envelope binaries. A frequently proposed explanation, apart from the possibility of having a third body, is the effect of magnetic activity, which may alter the internal structure of the secondary star, particularly its quadrupole moment, and thereby cause quasi-periodic oscillations. Here we present two compressible non-ideal magneto-hydrodynamical (MHD) simulations of the magnetic dynamo in a solar mass star, one of them with three times the solar rotation rate ("slow rotator"), the other one with twenty times the solar rotation rate ("rapid rotator"), to account for the high rotational velocities in close binary systems. For the slow rotator, we find that both the magnetic field and the stellar quadrupole moment change in a quasi-periodic manner, leading to O-C (observed - corrected times of the eclipse) variations of ~0.025 s. For the rapid rotator, the behavior of the magnetic field as well as the quadrupole moment changes become considerably more complex, due to the less coherent dynamo solution. The resulting O-C variations are of the order 0.13 s. The observed system V471~Tau shows two modes of eclipsing time variations, with amplitudes of 151 s and 20 s, respectively. However, the current simulations may not capture all relevant effects due to the neglect of the centrifugal force and self-gravity. Considering the model limitations and that the rotation of V471 Tau is still a factor of 2.5 faster than our rapid rotator, it may be conceivable to reach the observed magnitudes.