The OB associations of NGC 7217 and UGC 12732

We present the first results in an ongoing project to identify objectively OB associations in galaxies with varying luminosities, morphologies and environments. Our goal is to determine whether the physical characteristics of associations correlate with the properties or environment of the parent galaxy. As these associations are the primary sites of star formation in galaxies, knowledge of their characteristics is essential in understanding how stars form in galaxies. In the closest galaxies, OB associations can be identified as groups of blue stars. In more distant galaxies where individual stars can not be resolved, OB associations traditionally have been identified subjectively by eye, leading to subjective biases in the derived properties of the associations. For this reason we have developed an objective method to identify associations on UBVI images using the mathematical technique of cluster analysis. We have applied this method to NGC 7217, a luminous Sb galaxy, and UGC 12732, a less luminous irregular, and have measured the integrated magnitudes, colors and sizes of candidate OB associations in these galaxies. We identify 89 OB associations in NGC 7217 and 136 OB associations in UGC 12732. The mode of the size distribution in both galaxies agrees with the typical size of OB associations in nearby galaxies of 80-100 pc.