OB ASSOCIATIONS IN THE INNER 2 KILOPARSECS OF M33

UBV CCD photometry for the inner 7 x 10' of the Local Group spiral galaxy M33 is used to identify the OB associations and to study their properties. The association boundaries are identified using a "friends of friends" grouping algorithm, in which the major remaining source of subjectivity is the search radius chosen. We find 41 associations in this region of M33, each containing at least ten blue stars. These associations have mean radii of 40 pc, masses of 600M., and ages of 8 x 10(6) yr, where the age is estimated from the V magnitudes of the three brightest main-sequence stars. One-third of the Wolf-Rayet stars in this region of the galaxy lie more than 75 pc away from the nearest OB association. Thus, in this galaxy, the most massive stars do not form exclusively in large associations. A comparison of the masses of the associations with the masses of molecular clouds in M33 implies a star formation efficiency of approximately 10%. The OB associations in the southern spiral arm show a weak age gradient perpendicular to the arm, consistent with the presence of a spiral density wave. The associations in the northern arm lack any stars more massive than approximately 20 M. while the southern arm contains several stars with masses of 60M.. Either a recent cessation of star formation in the inner part of the northern arm or a difference in the upper mass cutoff to the initial mass function between the arms could explain this difference.