Apsidal motion in the massive binary HD 152248

Context. The eccentric massive binary HD 152248 (also known as V1007 Sco), which hosts two O7.5 III-II(f) stars, is the most emblematic eclipsing O-star binary in the very young and rich open cluster NGC 6231. Its properties render the system an interesting target for studying tidally induced apsidal motion.Aims. 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.Methods. A large set of optical spectra was used to reconstruct the spectra of the individual binary components and establish their radial velocities using a disentangling code. Radial velocities measured over seven decades were used to establish the rate of apsidal motion. We furthermore analysed the reconstructed spectra with the CMFGEN model atmosphere code to determine stellar and wind properties of the system. Optical photometry was analysed with the Nightfall binary star code. A complete photometric and radial velocity model was constructed in PHOEBE 2 to determine robust uncertainties.Results. We find a rate of apsidal motion of (1.843(-0.083)(+0.064))degrees 1 . 843 - 0.083 + 0.064 degrees yr(-1). The photometric data indicate an orbital inclination of (67.6(-0.1)(+0.2))degrees 67 . 6 - 0.1 + 0.2 degrees and Roche-lobe filling factors of both stars of about 0.86. Absolute masses of 29.5(-0.4)(+0.5)M(circle dot) 29 . 5 - 0.4 + 0.5 M circle dot and mean stellar radii of 15.07(-0.12)(+0.08)R(circle dot) 15 . 07 - 0.12 + 0.08 R circle dot are derived for both stars. We infer an observational value for the internal structure constant of both stars of 0.00100.0001.Conclusions. Our in-depth analysis of the massive binary HD 152248 and the redetermination of its fundamental parameters can serve as a basis for the construction of stellar evolution models to determine theoretical rates of apsidal motion to be compared with the observational one. In addition, the system hosts two twin stars, which offers a unique opportunity to obtain direct insight into the internal structure of the stars.