Radial abundance gradient for oxygen. The black solid line refers to the one-infall model without threshold and with inside-out formation (S1I). The red short-dashed line represents the model with a threshold of 4 M_⊙ pc^-2 (S1IT4), the blue long-dashed line the model with a threshold of 4 M_⊙ pc^-2 combined with a radial inflow of 0.1 km s^-1 (R1T01), and the green dotted line the model with a threshold of 4 M_⊙ pc^-2 combined inflow of 0.5 km s^-1 (R1T05). The data are the same as in Fig. 2.