Stellar associations in M81

A catalog of 180 stellar associations in M81 has been compiled using two plates taken with the 2-m Rozhen telescope. The catalog gives the positions of associations, their mean angular sizes, and H II identifications. It is found that the frequency distribution of associations in M81 is similar to that in M33, and their characteristic length is ≃ 80 pc. These small dense groups are real OB associations because their stellar contents are the same as those in the Galaxy. Most of the associations selected in the present work coincide with H II regions. The stellar associations form groups of two or more members, called star complexes, with a length-scale of about 400 pc.