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] -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.