Stellar structure and rates of stellar precession

Abstract

We investigate the detectability and behaviour of spin–axis precession, applying classical formulae to twelve close binary systems (α CrB, AR Cas, FT Ori, QX Car, AI Hya, AG Per, V526 Sgr, V346 Cen, YZ Cas, MY Cyg, β Aur, RZ Cha), for which recent data have been published (Berkowitz, 2024). Using EZ-Web ZAMS models together with an extended version of the RADAU code, we computed internal structure parameters k₂, gyration radii β, and mechanical ellipticities εₘ from which corresponding theoretical precession ratios (P/U) can be derived. Comparison with long-term photometric inclination changes |Δi| reveals a clear physical trend: systems with high precessional rates and relatively large masses (e.g., QX Car, AR Cas, AI Hya) exhibit the largest inclination variations, whereas older and more evolved systems (such as FT Ori and α CrB) show significantly weaker signatures. In all cases, the observed |Δi| values lie systematically below the corresponding ZAMS predictions, consistent with increasing central concentration, reduced mechanical ellipticity, and progressive tidal damping as the stars evolve. Overall, the results show that spin–axis precession is not only detectable in young massive binaries, but its amplitude directly traces their internal structure, underscoring precession’s potential as a powerful diagnostic for probing stellar interiors and guiding future evolutionary modelling