tokens: formation of multiple populations, as we are going to outline in the next. Figure 17. Left-hand panel: Difference between the mass-loss of 2Ge and 1G stars, , against the F 275 W − F 814 W colour extension of the HB. The groups of M3- and M13-like clusters are represented as orange and blue dots, as labelled. Right-hand panel: Present-day and initial cluster mass against the colour extension of the HB (top and bottom panels, respectively). 5.3 Extra-mass-loss in the 2Ge as a tracer of the multiple populations formation The discovery that 2Ge stars of all GCs lose more mass than the 1G (Fig. 11 , see also Tailo et al. 2015 , 2019b , a ) and, in particular, the correlations between the extra-mass-loss Δμ e and the extra-helium content of the 2Ge population δ Y max (Fig. 12 ) and the total (present-day and initial) mass of the clusters (Fig. 13 ) represent powerful constraints for the mechanisms of formation of multiple stellar populations. Since evolutionary effects alone cannot provide such increase (see Appendix D ) the need of an additional mechanism arises, allowing us to gain information on the environment where the distinct generations were born. Tailo et al. ( 2015 ) discuss, in the specific context of the blue hook morphology of ω Cen, how the formation environment of the different multiple populations can affect the stellar evolution during the RGB phase, mainly by affecting the initial stellar rotation. Here, we apply the same scenario to the less extreme environments encountered by the 2Ge of most clusters (see the schematic illustration in Fig. 18 ). Very young low-mass stars ( M /M ⊙ ∼ 0.1−1.0) behave as the T -Tauri stars in the Galactic field, pre-main sequence stars in the convective phase of contraction towards the main sequence. T -Tauri’s typically rotate with a period of 1–12 d, and their rotation rate is maintained constant in time by the magnetic coupling of the star with the accretion disc (magnetic disc-locking, see Armitage Livio 1996 ). Bouvier et al. ( 1997a ) and Bouvier, Forestini Allain ( 1997b ) developed a model to explain why main sequence stars in young clusters (such as α Persei) show a wide distribution of rotation velocities. They showed that, the earlier would pre-main sequence stars break the magnetic disc-locking, the faster would be their rotation when they reach the main sequence. Afterwards, during the main-sequence phase, the surface rotation rate (together with the rotation of the whole convective envelope) slows down thanks to the angular momentum loss associated with the stellar winds, but the core mostly preserves its fast rotation. Figure 18. Schematic interpretation of the different rotation of 1G and 2G stars. During the pre-main-sequence evolution, the rotation of the central contracting star is magnetically locked to the rotation of its residual accretion disc, having a diameter of 50–100 au. While the 1G formation occurs in an ambient of moderate density (yellow region), where the disc–disc
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