Star formation in bar environments - II. Physical properties, age and abundances of HII regions

The nebular properties (electronic density, extinction, age, O/H abundances ) of H II regions found along the bars of the sample of barred spiral galax ies studied by Martin & Friedli (1997) are examined. From line ratio diagno stic diagrams, it is showed that regions located along the major axis of th e bars have a normal photoionization spectrum, that is, line ratios reprodu ctible from nebular conditions and ionizing star radiation field normally e ncountered in extragalactic H II regions. There is an indication, however, that their degree of ionization might be somewhat different. Another ioniza tion mechanism (high-velocity shocks or hard UV radiation) is clearly prese nt for regions found nearby the centers of the galaxies. The electronic den sity of the regions along the bars is very close to that of disc regions ([ N-e] similar to 80 cm(-3)). On average, bar and disc regions have a similar visual extinction (A(V) similar to 1 mag) with exceptions for some regions located near the bar dust lanes of the earlier types of galaxies in our sa mple. Although the average H alpha equivalent width of bar H II regions (si milar to 250 Angstrom) is half that of disc regions, this disparity could b e due to uncertainties in the galactic continuum and does not translate int o a significant age difference. The oxygen abundance distribution was also investigated in the bar of these galaxies. The O/H scatter was found to be very small (<0.1 dex) indicating that mixing of the chemical composition by gas flows is very efficient in a barred structure.