Kippenhahn diagram for the 1.25 M_⊙ star computed with thermohaline mixing (no rotation-induced processes). Here we focus on the evolution phase around the RGB luminosity bump (located at a total stellar luminosity of  ~ 45 L_⊙ as indicated by the left arrow). Hatched area is the convective envelope. Green dotted lines delimit the hydrogen-burning shell above the degenerate helium core. The zone where the thermohaline instability develops is shown in hatched blue. The right arrow indicates the total stellar luminosity (L_c,th ~ 94   L_⊙) at which the thermohaline region extends up to the convective envelope and thus when the surface chemical composition starts changing.