Hα imaging of early-type Sa-Sab spiral galaxies.: II.: Global properties

New results, based on one of the most comprehensive H alpha imaging surveys of nearby Sa - Sab spirals completed to date, reveals early-type spirals to be a diverse group of galaxies that span a wide range in massive star formation rates. While themajority of Sa - Sab galaxies in our sample are forming stars at a modest rate, a significant fraction ( similar to 29%) exhibit star formation rates greater than 1 M-. yr(-1), rivaling the most prolifically star-forming late-type spirals. A similar diversity is apparent in the star formation history of Sa - Sab spirals as measured by their H alpha equivalent widths. Consistent with our preliminary results presented in the first paper in this series, we find giant H II regions [L(H alpha) >= 10(39) ergs s(-1)] in the disks of similar to 37% of early-type spirals. We suspect that recent minor mergers or past interactions are responsible for the elevated levels of H alpha emission and, perhaps, for the presence of giant H II regions in these galaxies. Our results, however, are not in total agreement with the H alpha study of Kennicutt & Kent, who did not find any early-type spirals with H alpha equivalent widths > 14 angstrom. A close examination of the morphological classification of galaxies, however, suggests that systematic differences between the Revised Shapley-Ames Catalog and the Second Reference Catalogue may be responsible for the contrasting results.