Ionised gas abundances in barred spiral galaxies
  This is the third paper of a series devoted to study the properties of bars
from long slit spectroscopy to understand their formation, evolution and their
influence on the evolution of disk galaxies. In this work we aim to determine
the gas metallicity distribution of a sample of 20 barred early-type galaxies.
We compare the nebular and stellar metallicity distributions to conclude about
the origin of the warm gas. We compare the results of nebular emission
metallicities using different semi-empirical methods. We carry out AGN
diagnostic diagrams along the radius to determine the radius of influence of
the AGN and the nuclei nature of the studied galaxies. We then derive the gas
metallicities along the bars and compare the results to the distribution of
stellar metallicities in the same regions. Most of the gas emission is
centrally concentrated, although 15 galaxies also show emission along the bar.
In the central regions, gas oxygen abundances are in the range 12+$\log$(O/H)=
8.4-9.1. The nebular metallicity gradients are very shallow in the bulge and
bar regions. For three galaxies (one of them a LINER), the gas metallicities
lie well below the stellar ones in the bulge region. These results do not
depend on the choice of the semi-empirical calibration used to calculate the
abundances. We see that the galaxies with the lowest abundances are those with
the largest rotational velocities. The presence of gas of significantly lower
metallicity than the stellar abundances in three of our galaxies, points to an
external origin as the source of the gas that fuels the present star formation
in the centre of some early-type barred galaxies. The fact that the bar/disk
nebular metallicities are higher than the central ones might be indicating that
the gas could be accreted via cooling flows instead of radial accretion from
gas sitting in the outer parts of the disk.
