Where are all the strongly magnetic AM CVn Stars?

Abstract

AMCVn stars are not only uncommon binary systems but also among the most important for different areas in astrophysics. They allow us to calibrate low-frequency gravitational wave detec tors. However, their formation and evolution are still not well understood as some processes, such as magnetic braking, common envelope evolution, and mass transfer stability, are currently largely unconstrained. A directly related subject that remains an enigma is the absence of strong magnetic f ields in AMCVn, as none has been found in observations so far, which is in stark contrast to other samples of white dwarf binary stars. In this thesis, we used the BSE code for evolving an initial population of 1240857 binaries obtained from a Monte Carlo simulation and combined these simulations with predictions of the crystallization and rotation-driven dynamo proposed scenario for magnetic field generation in white dwarfs. The studied AMCVn formation channels were the WD/WD and the WD/He channel, each one from a single common envelope phase (St-CE) or two common envelope phases (CE-CE), while the CV formation channel could not analyzed due to limitations of BSE. The results have revealed that the WD/WD channel is highly predominant over the WD/He channel in the formation of AMCVn systems, making up ≈ 80 − 100% of the predicted binaries. Among the systems forming AMCVn through the WD/WD channel the CE-CE path dominates with ≈ 79−100% over the St-CE path. Combining the binary evolution calculations with magnetic f ield generation according to the crystallization and rotation driven dynamo, we find that at least one-third of AMCVn are predicted to host a strongly magnetic white dwarf. This result is in disagreement with the observations. We propose two possible scenarios that could be the reason of this contradiction. First, the CV formation channel which we ignored might be the dominating over the WD/WD and the WD/He channels, based also on recent postulates, and supposing that AMCVnsystems coming from that channel may not form magnetic fields. As a second possibility, as the white dwarfs in AMCVn accrete helium, an originally formed magnetic field might end up being completely buried by the accreted material, a scenario that has been proposed for accreting neutron stars in binaries.