Evolution of the 4  M_⊙ model star with a metallicity Z = 3 × 10⁻⁵ as a function of the time, counted from the beginning of the TP-AGB phase. The top left panels reports the variation in luminosity (solid black track, scale on the left) and of the temperature at the base of the convective envelope (red lines, scale on the right), whereas the top right panel shows the evolution of the stellar radius (scale on the left) and of the mass-loss rate (red, scale on the right). The variation in CNO surface abundances is shown in the bottom left panels, where the different lines refer to the mass fractions of ¹²C (red), ¹⁴N (black), and ¹⁶O (blue). The bottom right panel reports the variation in surface mass fractions of various elements involved in the Ne-Na-Mg-Al-Si nucleosynthesis. The long-dashed and dotted-dashed green lines in the left panels refer to the evolution of the luminosity (top, left panel) and surface ¹⁴N (bottom, left) of a model star calculated by assuming a Blöcker (1995) mass-loss rate, with the free parameter entering the formula taken as η_R = 0.005 and η_R = 0.02, respectively. The dashed cyan lines in the same panels refer to the luminosity and the surface ¹⁴N of a model star calculated by doubling the OS.