The radial abundance gradient of chlorine in the Milky Way
  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.
