Apsidal Motion in O-Star Binaries: GENEC rotating binary models put to the k2-test

TL;DR

This paper tackles the $k_2$-discrepancy in massive stars by leveraging apsidal motion in the close binary HD 152248 as a diagnostic. Using GENEC stellar evolution models with two rotational-mixing schemes and explicit tidal physics, the authors test how $k_2$ at the observed stellar radius $R_\star$ responds to overshooting, metallicity, helium content, and wind properties. They find that reproducing the observed $k_2$ requires substantial convective boundary mixing (large $\alpha_{ov}$), while changes in $Z$ or wind mass loss have limited effect; binary interaction tends to worsen the discrepancy, and an unobserved high helium abundance may partially help but is not supported. The study concludes that a missing mechanism slowing stellar radius expansion is needed to reconcile models with the observed internal density structure, and it advocates expanding the analysis to a larger set of binaries to constrain this physics. This work reinforces the potential of apsidal-motion measurements as a stringent probe of massive-star interiors and the efficiency of internal mixing processes.

Abstract

Unveiling massive stars' internal structure and the physical origin and efficiency of the internal mixing processes? It is now possible using the apsidal motion rate in close eccentric binaries! The apsidal motion rate depends on the tidal interactions occurring between the stars and is proportional to k2, a measure of the star's inner density profile. Confronting standard stellar models with observations reveals the famous k2-discrepancy: models predict too high a k2 for the stars, that is to say, stars with too low a density contrast between their core and envelope. We built bespoke GENEC stellar evolution models including tidal mixing for the twin massive binary HD 152248. The models reveal the instabilities allowing to reproduce the stellar density profiles: advecto-diffusive models better reproduce k2 than magnetic models. A large overshooting is necessary to converge towards the observed k2, yet alone is not sufficient. While a change in metallicity or mass-loss rate has no significant impact on k2, a larger initial helium abundance allows us to better reproduce the k2. Yet, a super-solar helium abundance is not observationally supported. Our analyses highlight the need for a process in the stars that slows down the radial expansion.

Figures (4)

• Figure 1: Evolution of stellar parameters with radius for single (columns 1&2) and binary (columns 3&4) star models, with observational values and error bars (plain and dashed lines).
• Figure 2: Evolution of $\Omega_\text{surf}$ and $v_\text{surf}$ with time for single and binary models.
• Figure 3: Evolution of stellar parameters with $R$ for binary models with different $Z$ and $He$.
• Figure 4: Evolution of stellar parameters with $R$ for binary models with larger mass-loss rate.