Apsidal motion in massive eccentric binaries: The case of CPD-41$^\circ$7742, and HD 152218 revisited

TL;DR

This study analyzes apsidal motion in the massive eccentric binary CPD-41°7742, reconciling observational discrepancies and providing insights into stellar internal structure and evolution, especially regarding convective core size.

Contribution

It offers a comprehensive analysis of apsidal motion in CPD-41°7742, explicitly accounting for orbital dynamics and stellar properties, and highlights the need for enhanced mixing in stellar models.

Findings

Apsidal motion rate of 15.38° per year for CPD-41°7742

Reconciliation of previous periastron longitude discrepancies

Confirmation of larger convective cores in stellar evolution models

Abstract

This paper is part of a study of the apsidal motion in close eccentric massive binary systems, which aims to constrain the internal structure of the stars. We focus on the binary CPD-41{\deg}7742 and revisit HD152218. Independent studies of CPD-41{\deg}7742 in the past showed large discrepancies in the longitude of periastron of the orbit, hinting at the presence of apsidal motion. We perform a consistent analysis of all observational data, explicitly accounting for the apsidal motion. We use spectroscopic and photometric observations of CPD-41{\deg}7742 to infer values for the fundamental stellar and binary parameters. We apply a disentangling method to the spectra to simultaneously derive the RVs at the times of observation and reconstruct the individual stellar spectra. These are analysed by means of CMFGEN to determine the stellar properties. We determine the apsidal motion rate in two ways: We complement our RVs with those reported in the literature and we use the phase shifts between the primary and secondary eclipses. The light curves are analysed by means of Nightfall to constrain the orbital inclination and stellar masses. Stellar structure and evolution models are constructed with Cles. Different prescriptions for the mixing are adopted. Photometric data of HD152218 are analysed and stellar evolution models are built. The binary CPD-41{\deg}7742 displays apsidal motion at a rate of 15.38+0.42-0.51{\deg} yr-1. Our analysis of the observational data of CPD-41{\deg}7742 explicitly accounting for the apsidal motion allows us to explain the discrepancy in periastron longitudes. The age estimates are in good agreement with estimates obtained for other massive binaries in NGC 6231. This study confirms the need for enhanced mixing in the stellar evolution models to reproduce the observational stellar properties; this points towards larger convective cores than usually considered.