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

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

Dec 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995aas...18710803r&link_type=abstract

American Astronomical Society, 187th AAS Meeting, #108.03; Bulletin of the American Astronomical Society, Vol. 27, p.1439

Astronomy and Astrophysics

Astronomy

Scientific paper

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 Nii] 2142 Angstroms line in an Hii region (actually a doublet transition from (5S_2) to (3P_2) lambda vac = 2143.45 Angstroms and to (3P_1) 2139.68 Angstroms - collectively called 2142 Angstroms). The intensity of this line compared to [Oii] 2471 Angstroms permits a new assessment of the important N/O abundance in Orion. The Nii] 2142/[Oii] 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 [Nii] 6583 Angstroms /[Oii] 3727 Angstroms. Unfortunately, in the 1993 UV FOS spectra, the Niii] 1747-54 Angstroms and Oiii] 1661-66 Angstroms 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 Nii] 2142 Angstroms). These 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 Nii] 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 Nii] 2142/[Oii] 2471 value, yielding (N/O)uv. We also predict and compare the [Nii] 6583/[Oii] 3727 ratio, which provides (N/O)opt, as well as the observed IR [Niii] 57mu m/[Oiii]52mu m 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 & GO-6056.

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