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2000ApJ...539..687O - Astrophys. J., 539, 687-705 (2000/August-3)

Calibration of nebular emission-line diagnostics. II. Abundances.


Abstract (from CDS):

We examine standard methods of measuring nebular chemical abundances, including estimates based on direct Te measurements and also emission-line diagnostics. We use observations of the LMC H II regions DEM L199, DEM L243, DEM L301, and DEM L323, the ionizing stars of which have classifications ranging from O7 to WN3. Following common practice, we assume a two-zone Te structure given by T(O++) and T(O+) to compute ionic abundances. We compare with photoionization models tailored to the observed properties of the individual objects, and we emphasize the importance of correctly relating Te in the two zones, which can otherwise cause errors of ∼0.2 dex in abundance estimates. The data show no spatial variations or local metallicity enhancements to within 0.1-0.15 dex in any of the objects, notably including DEM L199, which hosts three Wolf-Rayet stars.

Our data agree well with both the modeled R23 and S23 abundance diagnostics for O and S. We present the first theoretical tracks for S23, which are in excellent agreement with a larger available data set. However, contrary to earlier suggestions, S23 is much more sensitive to the ionization parameter (U) than is R23. This occurs because S23 does not sample S IV, which is often a significant population. We therefore introduce S234≡([S II]+[S III]+[S IV])/Hβ and demonstrate that it is virtually independent of U. Predicted and observed spatial variations in S234 are thus dramatically decreased in contrast to S23. The intensity of [S IV] 10.5 µm can be easily estimated from the simple correspondence between [S IV]/[S III] and [O III]/[O II]. Using this method to estimate S234 for data in the literature yields excellent agreement with our model tracks, hence we give a theoretical calibration for S234. Our models show that the double-valued structure of S23 and S234 remains an important problem as for R23, and, at present, we consider calibrations of these S diagnostics reliable only at Z≲0.5Z. However, the slightly larger dynamic range and excellent compatibility with theoretical predictions suggest the S parameters to be more effective abundance diagnostics than R23.

Abstract Copyright:

Journal keyword(s): Galaxies: Abundances - Galaxies: ISM - ISM: H II Regions - ISM: Abundances - Galaxies: Magellanic Clouds - ISM: Supernova Remnants

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