The last 5 Gyr of Galactic chemical evolution based on H II region abundances derived from a temperature independent method

TL;DR

This study compares two chemical evolution models of the Milky Way, based on different methods of measuring oxygen abundance in HII regions, finding that the temperature independent method yields a model that better fits observational data and explains galactic features.

Contribution

It introduces two models of galactic chemical evolution using different abundance determination methods and demonstrates the superiority of the temperature independent method in matching observations.

Findings

The TIM-based model fits stellar constraints well.

The DM-based model fails to reproduce stellar constraints.

The TIM model explains the galactic abundance gradient flattening.

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 HII region gradients. Part of this dispersion arises from two different methods used to determine O/H in HII 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 HII 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.