On the Evolution of the Nova-like Variable AE Aquarii

Abstract

It is shown here that the peculiar properties of AE Aqr can can be accounted for if the mass transfer from an evolved 0.7M_⊙ secondary K4-5 star (q_i ≈ 0.8, i.e. < 1) initiated when the orbital period of the binary was P_orb,i ≈ 8.5 hours and the white dwarf period P_*,i ≈ 1 hour. This resulted in a significant amount of orbital angular momentum being accreted by the white dwarf in an initial discless spin-up phase towards P_* ≈ 0.1 P_orb,i. This destabilized the mass transfer, resulting in a run-away mass transfer from the secondary that lasted for approximately 10⁴ years, with the orbital period evolving to P_orb ≈ 11 hours until a critical mass ratio of q_crit = 0.73 had been reached. In this phase the mass transfer from the secondary occurred at a rapid rate of approximately Ṁ₂ ≈ 10²⁰ g s^-1, resulting in an accretion disc which spun-up the white dwarf to a period of approximately P_* ≈ 33 s. For all q ≤ q_crit = 0.73 the mass transfer proceeded on the thermal time scale of the secondary star, i.e. at a much slower rate, resulting in the binary converging and forcing AE Aqr into the propeller phase. Applying stellar wind theory, this allow an estimate of the polar magnetic field of the secondary star, which is of the order of B_° ≈ (1600 – 2000) G. It has been shown here that the duration of mass transfer phase q = q_crit → 0.67 (now) lasted for approximately t_Ṁ2 ~ 10⁷ years, similar to the spin-down time scale of the white dwarf, t_sd = P_*/P_* ≈ 10⁷ years. The propeller ejection of matter in the current phase results in the dissipation of mhd power of L_mhd ≈ 10³⁴ erg s^-1, probably channeled into mass ejection and non-thermal activity. This explains the non-thermal outbursts that are observed in radio wavelengths, and occasionally also in TeV energies, from AE Aqr.