Effects of the radial flows on the chemical evolution of the Milky Way disk

Context. The majority of chemical evolution models assume that the Galactic disk forms by means of infall of gas and divide the disk into several independent rings without exchange of matter between them. However, if gas infall is important, radial gas flows should be taken into account as a dynamical consequence of infall. Aims. We test the effects of radial gas flows on detailed chemical evolution models (one-infall and two-infall) of the Milky Way disk with different prescriptions for the infall law and star formation rate. Methods. We modified the equation of chemical evolution to include radial gas flows according to the method described in Portinari & Chiosi (2000, A&A, 355, 929). Results. We found that with a gas radial inflow of constant speed the metallicity gradient tends to steepen. Taking into account a constant timescale for the infall rate along the Galaxy disk and radial flows with a constant speed, we obtained too flat a gradient, at variance with data, implying that an inside-out formation and/or a variable gas flow speed are required. To explain the observed gradients, the gas flow should increase in modulus with the galactocentric distance, in both the one-infall and two-infall models. However, the inside-out disk formation coupled with a threshold in the gas density (only in the two-infall model) for star formation and/or a variable efficiency of star formation with galactocentric distance can also reproduce the observed gradients without radial flows. Conclusions. We show that the radial flows can be the most important process in reproducing abundance gradients but only with a variable gas speed. Finally, one should consider that uncertainties in the data concerning gradients prevent us from drawing firm conclusions. Future more detailed data will help us to ascertain whether the radial flows are a necessary ingredient in the formation and evolution of the Galactic disk and disks in general.