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[submitted] BOND: Bayesian Oxygen and Nitrogen abundance Determinations in H II regions using strong and semistrong lines

BOND infers oxygen and nitrogen abundances in giant H II regions by comparison with a large grid of photoionization models using strong and semi-strong emission line ratios. Our grid spans a wide range in O/H, N/O and ionization parameter U, and covers different starburst ages and nebular geometries. 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/Hβ, in addition to commonly used strong lines, to constrain the hardness of the ionizing radiation field. In a nutshell, our Bayesian inference code relies on the emission line ratios [O III]/Hβ, [O II]/Hβ and [N II]/Hβ, [Ar III]/Hβ, [Ne III]/Hβ, He I/Hβ as its input parameters, while its output values are the measurements and uncertainties for O/H and N/O.

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