Magnetohydrodynamical origin of eclipsing time variations in post-common-envelope binaries for solar mass secondaries

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 magnetohydrodynamical simulations of the magnetic dynamo in a solar mass star, one of them with three times the solar rotation rate ('slow rotator'), and the other one with 20 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 minus corrected times of the eclipse) variations of similar to 0.025 s. For the rapid rotator, the behaviour of the magnetic field as well as the quadrupole moment changes becomes considerably more complex, due to the less coherent dynamo solution. The resulting O-C variations are of the order of 0.13 s. The observed system V471 Tau shows two modes of eclipsing time variations, with amplitudes of 151 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.