Can the Anomalous Magnetic Braking of Ap/Bp Stars Explain the Orbital Decay of Algol-type Binaries?

Abstract

Several Algol-type binaries were detected to be experiencing rapid orbital decay, which is in contradiction with the conservative mass transfer model.} In this work, we investigate whether anomalous magnetic braking (MB) of intermediate-mass Ap/Bp stars, characterized by surface magnetic fields of $\sim10^2 \mathendash 10^4~\rm G$, could drive orbital decay through magnetic wind-field coupling. Using the stellar evolution code {\ttfamily MESA}, we simulate the evolution of several \add{main-sequence binaries containing Ap/Bp stars}, with typical initial parameters \add{of Algol binaries}. Our models indicate that the anomalous MB mechanism could induce orbital decay in long timescales (hundreds of Myr to several Gyr), \add{reproducing several basic Algol parameters such as the effective temperatures and luminosities of donor stars. However, the predicted orbital period decay rates are much lower than those observed in several Algol systems. We analyze the limitations of the anomalous MB model and discuss alternative mechanisms that could account for the long- or short-term orbital period variations observed in Algol systems, including a surrounding circumbinary disk, stellar expansion, the Applegate mechanism, and the light travel-time effect. Long-term observations are still required to distinguish between these mechanisms in the future.

12 pages, 9 figures, to appear in ApJ

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