Early evolution of the solar interior. The total mass of the accreting proto-Sun is indicated by the dashed line. The surface CZ is depicted as a cloudy region (see Kippenhahn & Weigert 1990) delimited by a radiative zone that grows outward from the center of the Sun. Three evolution models are shown corresponding to different values of the accretion efficiency parameter ξ: a classical evolution model with ξ = 0.5 (red line), a cold accretion model with ξ = 0.1 (green line) that is a lower limit of the ξ value in the observations of young clusters (Kunitomo et al. 2017), and a model with ξ = 0 (gray line) corresponding to a theoretical proto-Sun formed in the absence of any accretion heat. The right-hand y-axis provides the radius corresponding to the mass on the left-hand y-axis of the present-day Sun (which is 4.567 Gyr old). Bottom panel: highlights the key physical processes that occur during the growth of the proto-Sun: the presence of a circumstellar gas disk (during the first million years); an increase in the metallicity Z_acc of the accreted gas due to a pebble wave; the concomitant formation of planetesimals and planets; and a sudden decrease in Z_acc.