On the determination of N and O abundances in low-metallicity systems

We show that in order to minimize the uncertainties in the N and O abundances of low-mass, low-metallicity (O/H <= 1/5 solar) emission-line galaxies, it is necessary to employ separate parameterizations for inferring T-e(N+) and T-e(O+) from T-e(O+2). In addition, we show that for the above systems, the ionization correction factor (ICF) for obtaining N/O from N+/O+, where the latter is derived from optical emission-line flux ratios, is < ICF > = 1.08 +/- 0.09. These findings are based on state-of-the-art single-star H II region simulations, employing our own modeled stellar spectra as input. Our models offer the advantage of having matching stellar and nebular abundances. In addition, they have O/H as low as 1/50 solar (lower than any past work), as well as log (N/O) and log (C/O) fixed at characteristic values of -1.46 and -0.7, respectively. The above results were used to rederive N and O abundances for a sample of 68 systems with 12 + log (O/H) <= 8.1, whose dereddened emission-line strengths were collected from the literature. The analysis of the log (N/O) versus 12 + log (O/H) diagram of the above systems shows that ( 1) the largest group of objects forms the well-known N/O plateau with a value for the mean (and its statistical error) of -1.43(-0.0085)(+0.0084) (2) the objects are distributed within a range in log (N/O) of -1.54 to -1.27 in Gaussian fashion around the mean with a standard deviation of sigma = (+0.071)(-0.084), and (3) a chi(2) analysis suggests that only a small amount of the observed scatter in log (N/O) is intrinsic.