Star formation in M33 and in our galaxy

It is assumed that in the disks of M33 and our Galaxy, star formation results from a galactic spiral density wave plus successive generations of stars born via sequential star formation. For M33, orbit calculations and the observed distributions of giant H II regions and OB associations are used to calculate the reproductivity y for sequential star formation and the percentage of O stars in the annulus where the classical density wave operates. In both M33 and our Galaxy, the radial distribution of young stars agrees with the radial gradient predicted for a spiral density wave. Each of these galaxies can be characterized by an empirical linear relation between y and tau/sub seq/ , the time interval between the sequential generations. For both galaxies, the larger the values of tau/sub seq/ , the smaller the value of y and the smaller the percentage of O stars formed by sequential star formation. In M33, following values of y are obtained for plausible extreme values of tau/sub seq/ : (i) if the typical value of tau/sub seq/ is 3 x 106 yr, then y equals 0.94 and only 6% of the O stars are formed by the spiral density wave; (ii) if the typical value of tau/sub seq/ is 35 x 106 yr, then y equals 0.51 and 49% of the O stars are formed by the density wave. Comparison of the surface number densities of giant H II regions in the two galaxies indicates that tau/sub seq/ should be smaller in M33 than in our Galaxy. Therefore the reproductivity y is a factor of 1.1--3.1 larger in M33 than in our Galaxy, and the percentage of O stars that owe their origin to a spiral density wave is a factor of 1.4--11.7 larger in our Galaxy than in M33. The higher surface densities of gas and smaller drift speeds in M33 may account for the larger values of the reproductivity in that galaxy