THE EVOLUTION OF ABUNDANCE GRADIENTS IN SPIRAL GALAXIES

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 timesc ale, and the dependence of the star formation rate on gas density. Thi s gradient may be modified by the action of radial gas flows, if the i nfall occurs slower than the typical star formation timescale. Dependi ng on the velocity field, the gradient can be steepened or flattened b oth by inflow or outflows. Comparison with observational data of the G alaxy shows: The existence of steep present abundance gradients (seen with HII regions and planetary nebulae) and the absence of metallicity gradients in old disk stars and disk globular clusters implies that t he yield should not have a radial variation and the star formation sho uld depend fairly linearly on the gas density. Radial gas flows toward s the galactic centre can account for the presently available data, if they have a velocity in the solar vicinity of about 1 km/s, increasin g with galactocentric distance.