Publication Date:
2016-06-09
Description:
We present the Bayesian oxygen and nitrogen abundance determinations ( bond ) method. bond is a Bayesian code (available at: http://bond.ufsc.br ) to simultaneously derive oxygen and nitrogen abundances in giant H ii regions. It compares observed emission lines to a grid of photoionization models without assuming any relation between O/H and N/O. Our grid spans a wide range in O/H, N/O and ionization parameter U , and covers different starburst ages and nebular geometries. Varying starburst ages accounts for variations in the ionizing radiation field hardness, which arise due to the ageing of H ii regions or the stochastic sampling of the initial mass function. All previous approaches assume a strict relation between the ionizing field and metallicity. The other novelty is extracting information on the nebular physics from semistrong emission lines. While strong lines ratios alone ([O iii ]/Hβ, [O ii ]/Hβ and [N ii ]/Hβ) lead to multiple O/H solutions, the simultaneous use of [Ar iii ]/[Ne iii ] allows one to decide whether an H ii region is of high or low metallicity. Adding He i /Hβ pins down the hardness of the radiation field. We apply our method to H ii regions and blue compact dwarf galaxies, and find that the resulting N/O versus O/H relation is as scattered as the one obtained from the temperature-based method. As in previous strong-line methods calibrated on photoionization models, the bond O/H values are generally higher than temperature-based ones, which might indicate the presence of temperature fluctuations or kappa distributions in real nebulae, or a too soft ionizing radiation field in the models.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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