New calibrations for estimating the N/O ratio in HII regions

Abstract

We use a sample of 536 \hii\ regions located in nearby spirals, with an homogeneous determination of their $T_e$-based abundances, to obtain new empirical calibrations of the N2O2, N2S2, O3N2, and N2 strong-line indices to estimate the nitrogen-to-oxygen abundance ratio when auroral lines are not detected. All indices are strongly correlated with the $T_e$-based $\log$(N/O) for our \hii\ region sample, even more strongly than with $12+\log$(O/H). N2O2 is the most strongly correlated index, and the best fit to the $\log$(N/O)-N2O2 relation is obtained with a second-order polynomial. The derived relation has a low dispersion ({\em rms}$<$0.09~dex), being valid in the range $-1.74<$ N2O2 $<0.62$ (or $-1.81<$ $\log$(N/O) $<-0.13$). We have compared our calibration with previous ones and have discussed the differences between them in terms of the nature of the objects used as calibrators.

14 pages, 11 figures, accepted for publication in MNRAS

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