Apsidal Motion in Massive Binaries: CPD-41° 7742, an Extreme Case?

We study the apsidal motion in close eccentric massive binaries. We focus on CPD-41° 7742, located in the very young and rich open cluster NGC 6231. Measuring the rate of apsidal motion in such a binary system gives insight into the internal structure and evolutionary state of the stars composing it.
Independent studies of CPD-41° 7742 in the past showed large discrepancies in the longitude of periastron of the orbit, hinting at the presence of apsidal motion (i.e. slow precession of the line of apsides with time). We perform a consistent analysis of all observational data explicitly accounting for the apsidal motion. We make use of the extensive set of spectroscopic and photometric observations of the binary to infer fundamental parameters of the stars and of the binary. The age estimates are in good agreement with estimates obtained for other massive binaries in NGC 6231. This study confirms the need for enhanced mixing inside the stellar evolution models of the most massive stars to reproduce the observational stellar properties. This points toward larger convective cores than usually considered.