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2.4. Nitrogen Abundances

A final result from galactic abundance studies is that nitrogen can also be selectively enhanced at moderate to high metallicities due to ``secondary'' CNO nucleosynthesis. There is growing evidence for a substantial ``primary'' N contribution at low Z in some objects, based on a plateau in [N/O] at roughly -0.7 for [O/H] ltapprox -0.7 in galactic HII regions (2). But at higher metallicities (in the regime relevant to QSOs, see below), secondary production dominates and [N/O] grows roughly in proportion to [O/H] (HF93a, Vila-Costas & Edmunds 1993, Van Zee et al. 1998, Izotov & Thuan 1999). Shields (1976) noted that this special behavior should make nitrogen a particularly valuable probe of the chemical evolution in QSOs.

2 I use the notation for logarithmic abundances relative to solar, [a / b] = log(a / b) - log(b / a)smsun.