Outside-in stellar formation in the spiral galaxy M33?
  We present and discuss results from chemical evolution models for M33. For our models we adopt a galactic formation with an inside-out scenario. The models are built to reproduce three observational constraints of the M33 disk: the radial distributions of the total baryonic mass, the gas mass, and the O/H abundance. From observations, we find that the total baryonic mass profile in M33 has a double exponential behavior, decreasing exponentially for r<= 6 kpc, and increasing lightly for r > 6 kpc due to the increase of the gas mass surface density. To adopt a concordant set of stellar and H II regions O/H values, we had to correct the latter for the effect of temperature variations and O dust depletion. Our best model shows a good agreement with the observed the radial distributions of: the SFR, the stellar mass, C/H, N/H, Ne/H, Mg/H, Si/H, P/H, S/H, Ar/H, Fe/H,and Z. According to our model, the star formation efficiency is constant in time and space for r <= 6 kpc, but the SFR efficiency decreases with time and galactocentric distance for r > 6 kpc. The reduction of the SFR efficiency occurs earlier at higher r. While the galaxy follows the inside-out formation scenario for all r, the stars follow the inside-out scenario only up to r = 6 kpc, but for r > 6 kpc the stars follow an outside-in formation. The stellar formation histories inferred for each r imply that the average age of the stars for r > 6 increases with r. 
F. Robles-Valdez, L. Carigi and M. Peimbert
astro-ph.CO astro-ph.GA