The radial abundance gradient of chlorine in the Milky Way

TL;DR

This study measures the radial abundance gradient of chlorine in the Milky Way using a novel method that sums ionic abundances without ionization correction factors, revealing a consistent gradient with oxygen and proposing an empirical correction scheme.

Contribution

First direct determination of Cl/H ratios in the Milky Way without using ionization correction factors, updating atomic data and methodology for more accurate abundance measurements.

Findings

Cl/H ratios are lower than those obtained with ICF schemes.

The radial gradients of Cl and O are identical within uncertainties.

The mean Cl/O ratio across the Galactic disc is log(Cl/O) = -3.42 +/- 0.06.

Abstract

We determine the radial abundance gradient of Cl in the Milky Way from HII regions spectra. For the first time, the Cl/H ratios are computed by simply adding ionic abundances and not using an ionization correction factor (ICF). We use a collection of published very deep spectra of Galactic HII regions. We have re-calculated the physical conditions, ionic and total abundances of Cl and O using the same methodology and updated atomic data for all the objects. We find that the slopes of the radial gradients of Cl and O are identical within the uncertainties: -0.043 dex/kpc. This is consistent with a lockstep evolution of both elements. We obtain that the mean value of the Cl/O ratio across the Galactic disc is log(Cl/O) = -3.42 +/- 0.06. We compare our Cl/H ratios with those determined from Cl++ abundances and using some available ICF schemes of the literature. We find that our total Cl abundances are always lower than the values determined using ICFs, indicating that those correction schemes systematically overestimate the contribution of Cl+ and Cl+++ species to the total Cl abundance. Finally, we propose an empirical ICF(Cl++) to estimate the Cl/H ratio in HII regions.