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Session 106 - HII Regions and Ultracompact HII Regions.
Display session, Thursday, January 16
Metropolitan Ballroom,

[106.01] Extinction Corrections and the Decrement of the \ionHeI lines in the Orion Nebula

P. G. Martin (CITA), R. H. Rubin (NASA/Ames), G. J. Ferland (Kentucky), R. J. Dufour, C. R. O'Dell (Rice), J. A. Baldwin (CTIO), J. J. Hester (Arizona State), D. K. Walter (S.Carolina State)

We are engaged in a comprehensive program to find reliable elemental abundances in and to probe the physical structure of the Orion Nebula, the brightest and best-resolved \ionHII region. Our study makes use of emission-line diagnostics from HST imagery and spectra (FOS, GHRS), supplemented by KAO data and extensive ground-based observations.

All optical/ultraviolet observed fluxes are affected by extinction, mostly from the foreground ``neutral lid'' (O'Dell amp; Wen 1992), and must be corrected before further analysis and interpretation. We have reconsidered the differential wavelength dependence of the infrared to optical extinction using Balmer and Paschen lines for several positions (including data from HST positions we term 1SW, x2, and others). We improve on the usual extrapolation into the ultraviolet by examining the stellar extinction of the Trapezium stars, and we have validated the use of this stellar-based extinction curve for correcting the nebular emission by using HST observations of the important common-upper-level pair [\ionOII] 2471 and 7325 ÅAn analytic expression for the wavelength dependence of the extinction is developed which provides a solid basis for extinction corrections when normalized at a given position using, for example, our present WFPC2 imagery of \hbeta/\halpha\ or the Balmer decrement from spectra.

In the course of this work we examined the decrement of the \ionHeI lines. Our FOS observations have revealed a very surprising decrement within the 2^3S -- n^3P series, not in accord with theory (Smits 1996). It appears that the anomalous \ionHeI decrement in the ultraviolet cannot be explained by extinction effects.

Supported by NSERC and by NASA/AURA/STScI grants related to GO-4385, GO-5748, amp; GO-6056.

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