Apsidal Motion in O-Star Binaries - Constraining the internal structure of the stars

Stars more massive than about ten solar masses play a key role in many processes in the Universe, notably through their winds and powerful supernova explosions that contribute to the chemical enrichment of the Universe. More than 70% of them are bound by gravitational attraction to a companion star. These binary stars orbit around each other on an elliptic orbit. The tidal interactions occurring between the two stars give rise to the slow precession of the orbit, called the apsidal motion. The rate of this motion is directly related to the internal structure constant of each star, which is a measure of the mass distribution between the core and the external layers of the star. Measuring the apsidal motion rate hence provides a diagnostic of the otherwise di cult to constrain internal structure of stars and o ers a test of our understanding of stellar structure and evolution.
We analyse four binary systems located in the young open cluster NGC 6231, namely HD 152248, HD 152219, CPD-41° 7742, and HD 152218. We make use of the extensive set of spectroscopic and photometric observations as well as radial velocity data of these systems to determine their funda- mental stellar and orbital parameters, including the apsidal motion rate. We build stellar evolution models assuming di erent prescriptions for the internal mixing inside the stars. Whenever possible, we identify the models that best reproduce the observationally determined present-day properties of the stars, as well as their internal structure constants, and the apsidal motion rate of the system. We analyse the impact on the results of some poorly constrained input parameters in the models, including overshooting, turbulent di usion, metallicity, rotation, and binarity. We discuss some e ects that could bias our interpretation of the apsidal motion in terms of the internal structure constant.
We report the first measurement of apsidal motion in the binary system CPD-41° 7742 and we solve the discrepancy in the longitudes of periastron found in several past studies. The analysis of the stellar models reveals that, in the cases of HD 152248 and HD 152219, reproducing the internal stellar structure constant value and rate of apsidal motion simultaneously with the other stellar parameters requires a significant amount of internal mixing (either turbulent di usion or overshoot- ing) or enhanced mass-loss. We suggest that, qualitatively, the high turbulent di usion required to reproduce the observations could be partly attributed to stellar rotation. For all binaries except HD 152219, we derive an age estimate compatible with the cluster age.
A better understanding of the stellar structure and evolution of these massive binary stars will allow us to predict their remnant masses after supernovae explosion and their fate as either single or binary systems. Understanding the earliest phases of these systems is of primary importance in the context of the recent detection of gravitational wave events with the LIGO and VIRGO experiments as the underlying coalescences of two stellar-mass black holes or neutron stars have massive binary systems as progenitors.