Resolve the origin of the missing blue loop in BPASS models

Determine why the BPASS evolutionary comparison lacks the blue-loop phase associated with intermediate-mass helium-core burning in the primary star.

Background

The paper compares the disc-mediated binary calculations with BPASS and POSYDON evolutionary tracks. The BPASS primary diverges from the other models by not displaying the blue loop expected during intermediate-mass helium-core burning. The authors cannot identify the cause and suggest that approximations in helium-flash modelling may be responsible.

References

It is unclear why it is absent, but it might relate to helium flash modelling approximations in the underlying stars code (see \citealt*{Walmswell2015} for a detailed investigation of the blue loop phenomenon).

— Disc-mediated angular momentum transport resolves the mass-gain problem in rotating binaries  (2609.30230 - Ljung et al., 24 Sep 2026) in Section 3.3, Comparison with published binary grids