Time variation of radial gradients in the galactic disk: electron temperatures and abundances

Aims. We investigate the electron temperature gradient in the galactic disk as measured by young HII regions on the basis of radio recombination lines and the corresponding gradient in planetary nebulae (PN) based on [OIII] electron temperatures. The main goal is to investigate the time evolution of the electron temperature gradient and of the radial abundance gradient, which is essentially a mirror image of the temperature gradient. Methods. The recently derived electron temperature gradient from radio recombination lines in HII regions is compared with a new determination of the corresponding gradient from planetary nebulae for which the progenitor star ages have been determined. Results. The newly derived electron temperature gradient for PN with progenitor stars with ages in the 4-5 Gyr range is much steeper than the corresponding gradient for HII regions. These electron temperature gradients are converted into O/H gradients in order to make comparisons with previous estimates of the flattening rate of the abundance gradient. Conclusions. It is concluded that the O/H gradient has flattened out in the past 5 Gyr at a rate of about 0.0094 dex kpc Gyr, in good agreement with our previous estimates.