The star formation history and the evolution of the radial distribution of the elements in disk galaxies are yet not well understood. Theoretical models of galactic chemical evolution make contrasting predictions about the time evolution of the metallicity gradients.

We use the Local Group galaxy M33 to test chemical evolution models and trace the star formation and accretion history of disk galaxies. We analyze O/H and S/H abundances in planetary nebulae, HII regions (including a new sample of HII regions), and young stars, together with known [Fe/H] abundances in the old stellar population of M33. We model the time-evolution of gas (diffuse and condensed in clouds), stars, and chemical abundances in the galactic disk, which is accreting gas from an external reservoir.

With our model we are able to reproduce the available observational constraints on the gas and stellar mass distribution in M33 and to predict the time evolution of several chemical abundances. In particular, we find that a model characterized by a continuous infall of gas on the disk, at a rate almost constant with time, can also account for the relatively high rate of star formation and for the flat metallicity gradients. Supported by a large sample of high resolution observations for this nearby galaxy, we conclude that the metallicity in the disk of M33 has increased with time at all radii, with a continuous flattening of the gradient over the last ~8Gyr.