The N/O Abundance Ratio in the Orion Nebula From UV Lines

Session 108 - HII Regions.
Display session, Thursday, January 18
North Banquet Hall, Convention Center

[108.03] The N/O Abundance Ratio in the Orion Nebula From UV Lines

R. H. Rubin (NASA/Ames), G. J. Ferland (U KY), R. J. Dufour, C. R. O'Dell, Z. Wen (Rice), J. A. Baldwin (CTIO), J. J. Hester (ASU), P. G. Martin (Toronto), D. K. Walter (SCSU)

We address the N/O ratio in the Orion Nebula using HST FOS UV--optical spectra obtained in our Cycle 3 program. These permitted the first measurement of the N\sc ii] 2142 Å\ line in an H\sc ii region (actually a doublet transition from ^5S_2 to ^3P_2 \lambda_vac = 2143.45 Åand to ^3P_1 2139.68 Åcollectively called 2142 ÅThe intensity of this line compared to [O\sc ii] 2471 Å\ permits a new assessment of the important N/O abundance in Orion. The N\sc ii] 2142/[O\sc ii] 2471 ratio provides a direct measure of N^+/O^+ that is less sensitive to uncertainties in electron temperature (T_e), T_e fluctuations, or extinction, compared with previous methods based on [N\sc ii] 6583 ÅO\sc ii] 3727 ÅUnfortunately, in the 1993 UV FOS spectra, the N\sc iii] 1747-54 Å\ and O\sc iii] 1661-66 Å\ multiplets have poor signal-to-noise, precluding derivation of N/O from N^+2/O^+2. However, additional FOS spectra recently taken in Cycle 5 may permit use of this technique (plus possible new measurements of N\sc ii] 2142 ÅThese results, along with GHRS spectra, will be presented also.

We have ``retrofitted" two independent models of Orion (Baldwin et al. 1991; Rubin et al. 1991) to predict the N\sc ii] 2142 line strength and compare these with the measured value. We have also updated our N^+ atomic data and use the same input ionizing spectrum (from Kurucz LTE stellar atmospheres). For our separate models, we then prorate input nitrogen abundance to force the predicted line ratio to match the extinction-corrected observed N\sc ii] 2142/[O\sc ii] 2471 value, yielding (N/O)_uv. We also predict and compare the [N\sc ii] 6583/[O\sc ii] 3727 ratio, which provides (N/O)_opt, as well as the observed IR [N\sc iii] 57\mum/[O\sc iii]52\mum ratio, yielding (N/O)_ir. To more fully test the robustness of these results, it is important to use more realistic non-LTE stellar atmosphere models that are now becoming available.

Supported by AURA/STScI grants related to GO-4385 amp; GO-6056.

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