AE Aquarii: how cataclysmic variables descend from supersoft binaries

Schenker, K.; King, A.R.; Kolb, U.; Wynn, G.A. and Zhang, Z. (2002). AE Aquarii: how cataclysmic variables descend from supersoft binaries. Monthly Notices of the Royal Astronomical Society, 337(3) pp. 1105–1112.

DOI: https://doi.org/10.1046/j.1365-8711.2002.05999.x

Abstract

AE Aquarii (AE Aqr) is a propeller system. It has the shortest spin period among cataclysmic variables (CVs), and this is increasing on a 107 yr time-scale. Its ultraviolet spectrum shows very strong carbon depletion versus nitrogen, and its secondary mass indicates a star far from the zero-age main sequence. We show that these properties strongly suggest that AE Aqr has descended from a supersoft X-ray binary. We calculate the evolution of systems descending through this channel, and show that many of them end as AM CVn systems. The short spin-down time-scale of AE Aqr requires a high birth rate for such systems, implying that a substantial fraction of cataclysmic variables must have formed in this way. A simple estimate suggests that this fraction could be of the order of one-third of current CVs. We emphasize the importance of measurements of the C/N abundance ratio in CVs, particularly via the C iv 1550/N v 1238 ratio, in determining how large the observed fraction is.