The Evolution and Star Formation History of M33
  We construct a parameterized model to explore the main properties of the star
formation history of M33. We assume that the disk originates and grows by the
primordial gas infall and adopt the simple form of gas accretion rate with one
free parameter, the infall time-scale. We also include the contribution of gas
outflow process. A major update of the model is that we adopt a molecular
hydrogen correlated star formation law and calculate the evolution of the
atomic and molecular gas separately. Comparisons between the model predictions
and the observational data show that the model predictions are very sensitive
to the adopted infall time-scale, while the gas outflow process mainly
influences the metallicity profile. The model adopting a moderate outflow rate
and an inside-out formation scenario can be in good agreement with most of
observed constraints of M33 disk. We also compare the model predictions based
on the molecular hydrogen correlated star formation law and that based on the
Kennicutt star formation law. Our results imply that the molecular hydrogen
correlated star formation law should be preferred to describe the evolution of
the M33 disk, especially the radial distributions of both the cold gas and the
stellar population.
