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]
Van Zee et al. 1998,
Izotov & Thuan 1999).
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).