Apsidal motion in massive eccentric binaries in NGC 6231

Abstract

Context. The measurement of the apsidal motion in close eccentric massive binary systems provides essential information to probe the internal structure of the stars that compose the system. Aims. Following the determination of the fundamental stellar and binary parameters, we make use of the tidally induced apsidal motion to infer constraints on the internal structure of the stars composing the binary system HD 152219. Methods. The extensive set of spectroscopic, photometric, and radial velocity observations allowed us to constrain the fundamental parameters of the stars together with the rate of apsidal motion of the system. Stellar structure and evolution models were further built with the Clés code testing different prescriptions for the internal mixing occurring inside the stars. The effect of stellar rotation axis misalignment with respect to the normal to the orbital plane on our interpretation of the apsidal motion in terms of internal structure constants is investigated. Results. Made of an O9.5 III primary star (M1 = 18.64 ± 0.47 M⊙, R1 = 9.40−0.15+0.14 R⊙, Teff,1 = 30 900 ± 1000 K, Lbol,1 = (7.26 ± 0.97)×104 L⊙) and a B1-2 V-III secondary star (M2 = 7.70 ± 0.12 M⊙, R2 = 3.69 ± 0.06 R⊙, Teff,2 = 21 697 ± 1000 K, Lbol,2 = (2.73 ± 0.51)×103 L⊙), the binary system HD 152219 displays apsidal motion at a rate of (1.198 ± 0.300)° yr−1. The weighted-average mean of the internal structure constant of the binary system is inferred: k̄2 = 0.00173 ± 0.00052. For the Clés models to reproduce the k2-value of the primary star, a significantly enhanced mixing is required, notably through the turbulent mixing, but at the cost that other stellar parameters cannot be reproduced simultaneously. Conclusions. The difficulty to reproduce the k2-value simultaneously with the stellar parameters as well as the incompatibility between the age estimates of the primary and secondary stars are indications that some physics of the stellar interior are still not completely understood.