2026ApJ...998..314G

The mixing of material in stellar envelopes critically determines surface elemental abundances. A well-established tension remains between theoretical models and observational data for stars near the main-sequence turn-off. Resolving this issue requires developing a more physically complete treatment of material mixing, likely arising from processes inadequately represented in standard stellar models. This paper discusses four mixing mechanisms, including convection, convective overshooting, radiative diffusivity, and atomic diffusion, with particular focus on their combined impact on the surface elemental abundances of stars in the open cluster NGC 2420 during stellar evolution. We find that changes in the surface elemental abundances depend on the initial mass of the star and its evolutionary stage. The addition of extra mixing (radiative diffusivity) can resolve the issue of pure hydrogen atmospheres that arises for stars with masses greater than 1.35 M_⊙. Our best-fitting model (Con+Ov+Diff) with a moderate overshoot parameter f_ov ∼ 0.0174 reproduces the observed abundance dip in NGC 2420, confirming atomic diffusion as the key mechanism and supporting a cluster age of ∼2.5 Gyr.