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Contribution from H\II. Regions to [Si\II.] 34.8 $\mu$m Emission

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Session 34 -- Airbourne Astronomy
Display presentation, Tuesday, 9:30-6:30, Pauley Room

[34.12] Contribution from H\II. Regions to [Si\II.] 34.8 $\mu$m Emission

R.H.Rubin (NASA/Ames), R.J.Dufour, D.K.Walter (Rice), M.R.Haas (NASA/Ames), S.D.Lord (IPAC/Caltech)

Soon after the discovery of the [Si\II.] 34.8 $\mu$m line with the facility Cryogenic Grating Spectrometer on the Kuiper Airborne Observatory (KAO) (Haas et~al. 1986, ApJ, 301, L57), it became apparent that the line was stronger than predicted in almost all objects observed. Theoretical expectations are based on predictions of the line intensity from photodissociation region (PDR) models (e.g., Tielens \& Hollenbach 1985, ApJ, 291, 722 and updates). In M82, for instance, a Si/H $\sim$1.3 times solar (Lord et~al.~1993) would be required $-$ a very high value for the ISM. One possible explanation is that there may be significant emission in the H\II. region abutting the PDRs. Generally, the best diagnostics of Si abundance in H\II. regions are Si\III.] 1883, 1892 \AA, because gaseous silicon is predominantly Si$^{++}$. From new and archival high-dispersion IUE spectra including these lnes and a detailed axisymmetric ``blister" model for Orion, we found Si/H~= $4.5 \times 10^{-6}$ (Rubin, Dufour, \& Walter 1993, ApJ, 413 in press). Compared to solar (Anders \& Grevesse 1989, Geo.Cos.Acta, 53, 197), this corresponds to a Si depletion of $\sim$8, suggesting that most Si resides in dust grains even in the ionized volume.

Our model predicts that only 8\% of the observed [Si\II.]~34.8~$\mu$m emission in Orion arises from the H\II. region. Assuming that the rest comes from the PDR, we find a gas-phase Si/H~= 2.4$\times 10^{-6}$ there $-$ or $\sim$1/2 the value in the ionized gas. A possible conclusion is that some of the dust originally in the molecular cloud is destroyed in the harsher environment of the H\II. region. From a grid of models (Rubin, Hollenbach, \& Lord in prep.), we expect that for other objects such as the nuclei of galaxies (e.g., M82 and the Galactic center) and edge-on H\II. regions significantly offset from their exciting star(s) (e.g., M17SW), there will be a larger fraction of the total [Si\II.]~34.8~$\mu$m coming from the ionized region. Primarily, this is due to a lower average ionizing radiation field compared with nucleon density (lower ionization parameter) than found in star-centered H\II. regions.

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