The evolution of abundance gradients in spiral galaxies
ABSTRACT The temporal evolution of abundance gradients in the disks of galaxies is computed assuming gas infall and radial gas flows. Analytical and numerical models show: The initial value of the gradient is determined by radial variations of the metal yield and the star formation timescale, and the dependence of the star formation rate on gas density. This gradient may be modified by the action of radial gas flows, if the infall occurs slower than the typical star formation timescale. Depending on the velocity field, the gradient can be steepened or flattened both by inflow or outflows. Comparison with observational data of the Galaxy shows: The existence of steep present abundance gradients (seen with H II regions and planetary nebulae) and the absence of metallicity gradients in old disk stars and disk globular clusters implies that the yield should not have a radial variation and the star formation should depend fairly linearly on the gas density. Radial gas flows towards the galactic centre can account for the presently available data, if they have a velocity in the solar vicinity of about 1 km/s, increasing with galactocentric distance.