The Last 5 Gyr of Galactic Chemical Evolution Based on H ii Region Abundances Derived from a Temperature Independent Method

Abstract

Abstract Most of the chemical evolution models are not very reliable for the last 5 Gyr of galactic evolution; this is mainly because abundance gradients found in the literature show a big dispersion for young objects; a big culprit of this is the dispersion found in H ii region gradients. Part of this dispersion arises from two different methods used to determine O/H in H ii regions: the direct method (DM), based on forbidden lines; and the temperature independent method (TIM), based on permitted lines; the differences between these two methods are about 0.25 dex. We present two chemical evolution models of our galaxy to fit the O/H gradients of H ii regions, one obtained from the DM and the other obtained from the TIM. We find that the model based on the TIM produces an excellent fit to the observational stellar constraints (B-stars, Cepheids, and the Sun), while the model based on the DM fails to reproduce them. Moreover the TIM model reproduces the flattening observed in the 3–6 kpc galactocentric range; this flattening is attained with an inside-out star formation quenching in the inner disk starting ∼9 Gyr ago.