Magneto-hydrodynamical 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. We here 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 ∼ 0.635 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 23 s.The observed system V471 Tau shows two modes of eclipsing time variations, with amplitudes of 151 s and 20 s, respectively. While our simulations do not fully explain the observations, they provide the first demonstration through 3D MHD simulations of the importance of the MHD dynamo on stellar structure.

Eclipse Timing Modeling of Three Post-Common Envelope Binaries: Hybrid Solutions

“…Qian et al (2008); Lee et al (2009); Wittenmyer et al (2012); Nasiroglu et al (2017). However, detailed stellar models of the angular momentum exchange between a finite shell and the core of the secondary star show that the required energy to produce the quadrupole moment variations is too high in most PCEB systems, including all three of those described here (Völschow, M. et al 2016;Navarrete, F. H. et al 2018;Navarrete et al 2019). However, recently (Lanza 2020) has proposed a new model based on the angular momentum exchange between the spin of the magnetically active secondary and the binary orbital motion.…”

“…Several recent papers have analyzed modifications to the original Applegate model using more complex 2-shell models, generalizing the magnetohydrodynamic effects of internal magnetic fields, and considering different types of dynamo models (e.g., Lanza et al 1998;Lanza 2006;Navarrete, F. H. et al 2018;Völschow, M. et al 2018;Navarrete et al 2019). Here we consider the recent model of Lanza (2020) which is based on angular momentum exchange between the spin of the active component and the orbital motion.…”

Challenges in 2D Stellar Modeling

“…The effects on internal energy transport are particularly pronounced in the convective regions, and this has been suggested to be the origin of the radius inflation problem in M dwarfs (Jackson et al, 2018). One promising line of inquiry uses post-commonenvelope binaries as probes of the magnetic field (Navarrete et al, 2019). In this case, it appears that the magnetic field can have a measurable effect on the stellar structure.…”

First measurement of the total gravitational quadrupole moment of a black widow companion

“…by stellar magnetic cycles as proposed in Applegate (1992) and later refined by e.g. Lanza & Rodonò (1999); Lanza (2006); Völschow et al (2018);Navarrete et al (2019). If quadrupole-moment changes are responsible for the observed orbital period variations, then it is possible to design a dynamical model of the binary that includes this contribution.…”