Apsidal motion in HD 152248 reveals challenges in modeling massive star interiors

Star system: HD 152248 A | Distance from the Sun: 4715 light years

HD 152248 is a massive twin binary system used to study the internal structure of stars more than twenty times the mass of the Sun. By measuring the apsidal motion—the slow rotation of the orbit’s long axis—astronomers can probe how mass is distributed inside these stars. This study uses detailed stellar models to compare predictions with observations, focusing on the internal structure constant k2, which is sensitive to the density contrast between the core and outer layers. The models include different ways of mixing material inside the stars and account for effects like tidal locking and magnetic fields.

The results show that all models predict k2 values that are systematically higher than observed, a problem known as the k2-discrepancy. This means the models produce stars with less dense cores than reality. Both hydrodynamic and magnetic models require a large amount of convective boundary mixing, called overshooting, to match the observed properties, but even this does not fully resolve the discrepancy. Other factors, such as metallicity, helium content, and mass-loss rates, have little effect on the outcome. The study confirms that apsidal motion is a powerful tool for testing and improving models of massive star interiors.

Key findings
- HD 152248’s apsidal motion provides strong constraints on internal stellar structure.
- All models show a k2-discrepancy, predicting less dense cores than observed.
- Large convective boundary mixing (overshooting) is needed in models.
- Other parameters like metallicity and mass-loss have little impact on k2.
- Apsidal motion remains a robust method for probing massive star interiors.

Scientific study

The Beauty of k2: Probing Stellar Interiors Using Apsidal Motion. I. The Benchmark Massive Binary HD 152248 (external link)