Can angular momentum loss cause the period change of NN Ser?

Abstract

NN Ser is a non mass-transferring pre-cataclysmic variable containing a white dwarf with a mass of $\sim 0.5 M_{\odot}$ and an M dwarf secondary star with a mass of $\sim 0.2 M_{\odot}$. Based on the data detected by the high-speed CCD camera ULTRACAM, it was observed that the orbital period of NN Ser is decreasing, which may be caused by a genuine angular momentum loss or the presence of a third body. However, neither gravitational radiation and magnetic braking can ideally account for the period change of NN Ser. In this Letter, we attempt to examine a feasible mechanism which can drain the angular momentum from NN Ser. We propose that a fossil circumbinary disk (CB disk) around the binary may have been established at the end of the common envelope phase, and the tidal torques caused by the gravitational interaction between the disk and the binary can efficiently extract the orbital angular momentum from the system. We find that only if M dwarf has an ultra-high wind loss rates of $\sim 10^{-10} M_{\odot} \rm yr^{-1}$, and a large fraction ($\delta\sim 10 %$) of wind loss is fed into the CB disk, the loss rates of angular momentum via the CB disk can interpret the period change observed in NN Ser. Such a wind loss rate and $\delta$-value seem to be incredible. Hence it seems that the presence of a third body in a long orbit around the binary might account for the changing period of NN Ser.