Study of the stellar line-strength indices and kinematics along bars - I. Bar age and metallicity gradients

Aims. This is the first paper of a series that aims to understand the formation and evolution of bars in early-type spirals and their influence in the evolution of the galaxy. Methods. Optical long-slit spectra along the bar major-axis of a sample of 20 galaxies are analyzed. Velocity and velocity dispersion profiles along the bar are presented. Line-strength indices in the bar region are also measured to derive stellar mean-age and metallicity distributions along the bars using stellar population models. Results. We obtain mean ages, metallicities and chemical abundances along the bar of 20 galaxies with morphological types from SB0 to SBbc. The main result is that we find a large variation in age and metallicity along the bar in 45% of our sample. We find three different types of bars according to their metallicity and age distribution along the radius: 1) Bars with negative metallicity gradients. They show a mean young/intermediate population ( Conclusions. The results from the metallicity and age gradients indicate that most galaxies with high central stellar velocity dispersion host bars that could have been formed more than 3 Gyr ago, while galaxies with lower central velocity dispersions show a wider distribution in their population and age gradients. A few bars show characteristics compatible with having been formed less than