Revisiting the radial abundance gradients of nitrogen and oxygen of the Milky Way

TL;DR

This study provides new insights into the radial abundance gradients of nitrogen and oxygen in the Milky Way's disc, revealing a nearly flat N/O ratio and potential flattening of oxygen abundance in the inner Galaxy, based on spectroscopic data of HII regions.

Contribution

It introduces a new, ionisation correction factor-independent measurement of nitrogen abundance gradients and reassesses oxygen gradients with a larger, homogeneous dataset.

Findings

Nitrogen abundance gradient is independent of ionisation correction factors.

The N/O ratio remains nearly flat across the Galactic disc.

Possible flattening or drop of oxygen abundance in the inner Galaxy.

Abstract

We present spectra obtained with the 10.4 m Gran Telescopio Canarias telescope of 13 Galactic HII regions, most of them of very low ionisation degree. The objects are located along the Galactic disc, with R_G from 5.7 to 16.1 kpc. We determine T_e([NII]) for all of them. We obtain - for the first time - a radial abundance gradient of N that is independent on the ionisation correction factor. The radial distribution of the N/O ratio is almost flat, indicating that the bulk of N is not formed by standard secondary processes. We have made a reassessment of the radial O abundance gradient combining our results with previous similar ones by Esteban et al. (2017); producing a homogeneous dataset of 35 HII regions with direct determinations of the electron temperature. We report the possible presence of a flattening or drop of the O abundance in the inner part of the Galactic disc. This result confirms previous findings from metallicity distributions based on Cepheids and red giants. Finally, we find that the scatter of the N and O abundances of HII regions with respect to the gradient fittings is not substantially larger than the observational uncertainties, indicating that both chemical elements seem to be well mixed in the interstellar gas at a given distance along the Galactic disc