Apsidal Motion in Massive Eccentric Binaries in NGC 6231

The young and rich open cluster NGC 6231 hosts a substancial population of O-type binary stars. We study several of these eccentric short-period massive eclipsing binaries and assess their fundamental parameters (e.g. masses, radii, effective temperatures, luminosities). To this aim, we use both spectroscopic and photometric observations of the systems.
The properties of these systems make them interesting targets to study tidally induced apsidal motion. The analysis of apsidal motion offers a powerful means to obtain information about the stellar interior, which is otherwise difficult to get. Indeed, since the rate of apsidal motion in a binary system is proportional to the internal structure constants (a global measure of the density contrast between the stellar core and external layers) of the stars composing it, its value gives direct insight into the internal structure and evolutionary state of these stars.
Stellar evolution models are constructed based on the observationally determined fundamental parameters and a theoretical rate of apsidal motion is inferred. The results are striking: adopting standard stellar evolution models yields a theoretical rate of apsidal motion much larger than the observational value. This discrepancy results from the standard models predicting too low an efficiency of internal mixing and thus too low a density contrast. By enforcing the theoretical rates of apsidal motion to match the observational values, enhanced mixing is required, through a large overshooting parameter and/or additional turbulent/rotational mixing. Our analysis leads to the conclusion that the convective cores in those massive stars must be more extended than anticipated from standard models.