Apsidal Motion in The Massive Binary HD 152248

The eccentric massive binary HD 152248, which hosts two O7.5 III(f) stars, is the most emblematic eclipsing O-star binary 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.
From a set of optical spectra of HD 152248, we reconstruct the individual spectra of the stars and establish their radial velocities using a disentangling code. Combining radial velocity measurements spanning seven decades, we show that the system displays an apsidal motion at a rate of 1.750°/yr. We further analyse the reconstructed spectra with the CMFGEN model atmosphere code to determine stellar and wind properties of the system. The optical light curve of the binary is analysed with the Nightfall binary star code to constrain the Roche lobe filling factors of both stars to a value of 0.86 and derive an orbital inclination of 68.6°. Absolute masses of 28.9 and 29.1 solar masses are derived for the primary and secondary star respectively and mean stellar radii of 14.2 solar radii are obtained for both stars.