Using hot-subdwarf binaries to constrain binary interaction theory

Joris Vos (University of Valparaiso, Chile) - 13.11.17

Abstract:

Hot subdwarf-B (sdB) binaries are evolved core-He burning stars with very thin hydrogen envelopes. The only way to form these stars is through binary interaction, making them very useful objects to study binary evolution.

sdB stars in wide binaries (P = 2-3 years) are formed through stable Roche-lobe overflow (RLOF) when the sdB progenitor is at the tip of the red giant branch. These binaries are found to have eccentric orbits even though current binary evolution theory predicts these systems to circularise before the onset of RLOF. This observation is not limited to sdB binaries, also post-AGB binaries and central binaries in planetary nebula show signs of eccentric orbits. Furthermore a link between rotational velocity and accreted mass, and an interesting correlation between mass-ratio and orbital period linked to the stability of RLOF are found.

By analyzing the orbital and spectral parameters of wide sdBs, we constructed a test sample for binary interaction and evolution theory. As wide sdB+MS binaries are double lined binaries, these systems can provide strong restrictions on binary population synthesis studies that can not be achieved by other types of post mass-loss systems.