SILICON AND CARBON ABUNDANCES IN THE ORION NEBULA

We assess the gas-phase abundances of Si and C from our recent measurements of Si++, C++, and C+ in the Orion Nebula by expanding on our earlier '' blister '' models. The C and Si abundances are derived from new IUE high-dispersion spectra of the C++ 1907, 1909 angstrom and Si++ 1883, 1892 angstrom lines and archival I UE data. Gas-phase Si/C = 0.016 in the Orion ionized volume and is particularly insensitive to uncertainties in extinction and temperature structure. The solar value is 0.098. Gas-phase C/H = 2.8 x 10(-4) and Si/H = 4.5 x 10(-6). Compared to solar, Si is depleted by a factor of approximately 8 in the ionized region, while C is much less depleted (factor of 1.3), if depleted at all. This suggests that most Si resides in dust grains even in the ionized volume. Thus, most of the observed [Si II] 34.8 mum emission in Orion does not arise in the H II region.