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Astron. Astrophys. 351, 1075-1086 (1999)

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1. Introduction

One of the main sources of uncertainty in the determination of chemical abundances in HII regions is the correction for unseen ionization states. Two ionization correction schemes are usually used, the first one relies on empirically deduced formulae based on the similarity of ionization potentials for two distinct ions, whereas the second approach relies on the predictions of single ionization models that are assumed to reproduce the main characteristics of the observed object. The accuracy attained by these two approaches to derive total abundances varies from element to element and is difficult to estimate, thereby hampering a meaningful comparison of the chemical abundances determined for different objects, and consequently preventing an assessment of the accuracy of the derived Galactic abundance gradients.

The analysis presented here is based on CCD spectra obtained for seven of the brightest Galactic HII regions: M42, M43, M8, M16, M17, M20 and NGC 7635. This analysis centres on the emission lines not belonging to the Fe ions; the results related to the [FeII ] and [FeIII ] lines and the Fe abundance are presented elsewhere (Rodríguez 1996, 1999, and a further paper currently in preparation). The spectra cover a relatively large wavelength range ([FORMULA]) and allow the calculation of a homogeneous set of physical conditions and ionic abundances for several positions in HII regions with fairly different characteristics. This set of data is shown to imply that the seven HII regions share similar abundances within the expected errors. This result is inferred from the relative trends followed by the ionic abundances in all the positions; therefore it does not depend on any assumption for the contribution of the unobserved ions. Since the deduced common values for the chemical abundances in all the objects imply that the ionic abundances measured in each object are proportional to the ionization fraction of the corresponding ion, the observational results are compared to the predictions of grids of ionization models to assess their reliability.

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999