Monitoring evolved stars for binarity with the hermes spectrograph

¹ Institute of Astronomy, KU Leuven, Belgium
² Institute of Astronomy and Astrophysics, ULB, Belgium
³ Sternberg Astronomical Institute, Russia

Abstract

Binarity is often invoked to explain peculiarities that can not be explained by the standard theory of stellar evolution. Detecting orbital motion via the Doppler effect is the best method to test binarity when direct imaging is not possible. However, when the orbital period exceeds length of a typical observing run, monitoring often becomes problematic. Placing a high-throughput spectrograph on a small semi-robotic telescope allowed us to carry out a radial velocity survey of various types of peculiar evolved stars. In this review we highlight some findings after the first four years of observations. Thus, we detect eccentric binaries among hot subdwarfs, barium, S stars, and post-AGB stars with disks, which are not predicted by the standard binary interaction theory. In disk objects, in addition, we find signs of the on-going mass transfer to the companion, and an intriguing line splitting, which we attribute to the scattered light of the primary.