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5.3. Relative Abundance Patterns

5.3.1. N/O

Figure 19 shows the variation of log N/O with radius in NGC 2403 for our HII region sample. The weighted least-squares fit to the data yields a gradient of -0.032 ± 0.005 dex/kpc (to be compared with the O/H gradient of -0.201 ± 0.009 dex/kpc). This is very similar to the gradient found by Víchez et al. (1988) for N/O in M33.

Figure 19

Figure 19. N/O gradient in NGC 2403. (From Garnett et al. 1997b).

When discussing relative abundance gradients in spiral galaxies, one has to be careful to distinguish between gradients in terms of galactic radii, and gradients relative to metallicity baselines. I will try to follow my own advice. The rather shallow gradient in N/O is at odds with the notion that N is a purely secondary element: in such a situation N/O should vary linearly with O/H and the N/O radial gradient should have the same slope as the O/H radial gradient. A shallower slope is expected if N has a primary nucleosynthesis component (Matteucci & Tosi 1985; Díaz & Tosi 1986), as predicted in some models for the third dredge-up on the asymptotic giant branch of intermediate mass stars (e.g., Renzini & Voli 1981). Such a primary component has been invoked to explain the nearly constant N/O vs. O/H observed in dwarf irregular galaxies (Section 4.3.2). The N/O gradient can be explained in this picture if there is a primary N component at a level log N/O = -1.4, similar to the mean value observed in the dwarf irregulars.

However, there is a possible systematic uncertainty in the gradient due to the poorly known correction for N+2, which is predicted to be the dominant ionization state of N in many cases (e.g., Garnett 1990). Far-infrared spectroscopic measurements of [N III] and [O III] in Galactic HII regions (e.g., Lester et al. 1987) suggest that higher N/O ratios are obtained from these lines than from the optical [N II] and [O II] lines. Until the optical/IR discrepancies for nitrogen are resolved, the true gradient for N in NGC 2403 (and in spiral galaxies in general) remains uncertain.

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