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.
