Evolution of the analytical expression for metallicity Z according to constant St (Eq. (32)), indicated by dashed lines, and constant St_χ (Eq. (36)), indicated by solid lines. For both models, we apply the initial condition Z₀ = 0.01 at 0.2 Myr and therefore the initial lines overlap. Then, until the metallicity drops to ~10% at approximately 1 Myr, both expressions yield a similar evolution. However, the metallicity decreases faster for the constant St case in the following Myrs. The employed fiducial values are listed in Table 1.