A quick start guide to BOND: Bayesian Oxygen and Nitrogen abundance Determinations in H II regions using strong and semistrong lines

Abstract

We present a quick-start guide to BOND, a statistical method to derive oxygen and nitrogen abundances in H II regions. BOND compares a set of carefully selected strong and semistrong emission lines to a grid photoionization models. The first novelty, in comparison to other statistical methods, is that BOND relies on the [Ar III]/[Ne III] emission line ratio to break the oxygen abundance bimodality. In doing so, we can measure oxygen and nitrogen abundances without assuming any a priori relation between N/O and O/H. The second novelty is that BOND takes into account changes in the hardness of the ionizing radiation field, which can come about due to the ageing of H II regions or the stochastically sampling of the IMF. We use the emission line ratio He I/Hb, in addition to commonly used strong lines, to constrain the hardness of the ionizing radiation field. Finally, we also stress the pragmatic considerations behind our Bayesian inference.