5.3. Individual Element Abundances and Ratios
The [ / Fe] ratio in the Magellanic Clouds, NGC6822, and WLM measured in the above studies is lower than the solar ratio. There are several possible reasons. This may be a consequence of the lower star formation rates in dIrrs or the possibility of a steeper initial mass function (see Tsujimoto et al. 1995; Pagel & Tautvaišiene 1998), i.e., a reduced contribution of Type II supernovae as compared to Type Ia supernovae (e.g., Hill et al. 2000; Smith et al. 2002), or the possibility of metal loss through selective winds (Pilyugin 1996). It seems that in the LMC, where element abundances (most notably oxygen) were measured in clusters of different ages, the evolution of the [ / Fe] ratio was fairly flat with time (Hill et al. 2000 and Hill 2004). Field red giants also show reduced [O/Fe] values (Smith et al. 2002).
The r-process (traced by, e.g., Eu), which is the dominant process in massive stars, appears to prevail at low metallicities or the early stages of the LMC's evolution. The s-process (traced by, e.g., Ba and La), which takes place in cool intermediate-mass giants such as asymptotic giant branch stars, dominates at higher metallicities (Hill 2004). Nitrogen appears to be close to primary and may come mainly from nonmassive stars (see also Maeder, Grebel, & Mermilliod 1999). Stellar and gaseous nitrogen abundances in the Magellanic Clouds show considerable variations. Nitrogen enhancement in supergiants and old red giants may be due to mixing of CN-enhanced material to their surface during the first dredge-up (e.g., Russell & Dopita 1992; Venn 1999; Dufton et al. 2000; Smith et al. 2002; not observed, however, in LMC B main sequence stars; Korn et al. 2002).