The rotational velocity of the sdOB primary of the eclipsing binary system LB 3459 (AA Dor)

Abstract

We present an analysis of the rotational velocity of the primary of LB 3459 based on 107 new high-resolution and high- ESO VLT UVES spectra. 105 of them cover a complete orbital period (0.26 d) of this binary system. We have determined an orbital period of , a radial velocity amplitude of , and . From simulations of the He II λ 4686 Å line profile (based on NLTE model atmosphere calculations), we derive . We present an animation which shows the orbital movement of the binary system, its synthetic lightcurve, and compares the phase-dependent variation of the predicted with the observed He II λ 4686 Å line profile. The radius of the cool component is almost the same size like Jupiter but its mass is about 70 times higher than Jupiter's mass. Thus, from its present mass (), the secondary of LB 3459 lies formally within the brown-dwarf mass range (). It might be a former planet which has survived the previous common-envelope phase and even has gained mass.