5.5. Other Elements
Few other elements have been measured systematically in extragalactic H II regions, over a wide range of O/H.
Silicon can be measured in H II regions through the UV Si III] doublet at 1883, 1892 Å. Figure 21 shows Si/O measurements from Garnett et al. (1995b) for a small sample of metal-poor galaxies, along with data for several samples of B stars and the Orion nebula. Si/O appears to be roughly constant but smaller than the average for the Sun and the solar neighborhood B stars. Silicon is certainly depleted onto grains in the ISM, and the results in Figure 21 are consistent with a Si depletion of about -0.2 to -0.4 dex in the H II region sample. This is probably appropriate for not very dense, ionized gas (Sofia, Cardelli, & Savage 1994).
Figure 21. Si/O abundance ratios in irregular galaxies, from Garnett et al. 1995b (filled circles). The open squares represent two different measurements for the Orion nebula, while the stars show averages for four samples of Galactic B stars (see Garnett et al. 1995b for details).
Iron has a variety of emission lines from [Fe II] and [Fe III] in the optical spectrum. The [Fe III] 4658 Å is often observed in extragalactic H II regions. Izotov & Thuan (1999) measured [Fe III] in several metal-poor emission-line galaxies and derived Fe abundances. They obtained Fe/O ratios that were similar to the values found for metal-poor stars in the Galactic halo, and used this to argue that the their emission-line galaxies were very young. However, the ionization corrections for Fe are very uncertain, and Fe is highly depleted onto grains in the ISM. If Si is depleted by 0.2 to 0.4 dex, one can use the depletion analysis of Sofia et al. (1994) to estimate that Fe is depleted by about 0.7 to 1.1 dex. It is improbable that O/Fe ratios in H II regions can be used to interpret the enrichment history of the ISM without better understanding of the depletion factors.