Radial abundance gradient for oxygen. The black solid line refers to the one infall model without any threshold and with the inside-out formation (S1I). The red short dashed line represents the model with a radial inflow of 1 km s^-1 (R1I1) and the blue long-dashed line the best-fit model using a variable velocity for the radial inflow (R1IL). The data are the same as in Fig. 2.