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 HII regions found along the bars of the sample of barred spiral galaxies studied by Martin & Friedli (1997) are examined. From line ratio diagnostic diagrams, it is showed that regions located along the major axis of the bars have a normal photoionization spectrum, that is, line ratios reproductible from nebular conditions and ionizing star radiation field normally encountered in extragalactic HII regions. There is an indication, however, that their degree of ionization might be somewhat different. Another ionization mechanism (high-velocity shocks or hard UV radiation) is clearly present for regions found nearby the centers of the galaxies. The electronic density of the regions along the bars is very close to that of disc regions ($<N_e > \sim 80 cm^{-3}$). On average, bar and disc regions have a similar visual extinction ($A_V \sim 1$ mag) with exceptions for some regions located near the bar dust lanes of the earlier types of galaxies in our sample. Although the average H$\alpha$ equivalent width of bar HII regions ($\sim$250 {\AA}) is half that of disc regions, this disparity could be due to uncertainties in the galactic continuum and does not translate into 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.