The nebular properties ( electronic density , extinction , age , O/H abundances ) of H ii 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 H ii 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 ( \langle N _ { e } \rangle \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 H ii regions ( \sim 250 Å ) 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 .