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

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8. Reference abundances

The total abundances implied by the data will be compared to the Solar System abundances, since these are those known with the highest precision and for the greatest number of elements. Both the solar and meteoritic values have been taken from the compilation of Grevesse et al. (1996).

On the other hand, several recent results on the abundances of elements like O, C, N and Kr relative to H in the interstellar medium and in B stars of the solar neighbourhood, are being interpreted as evidence that these abundance ratios are only about [FORMULA] of the solar values (Snow & Witt 1996; Cardelli & Meyer 1997; Sofia et al. 1997 and references therein). Therefore, the resulting abundances will be also compared with those characteristic of B stars. The values used here have been taken from Snow & Witt (1996) and represent the mean value of the abundances derived by several authors for stars in the solar neighbourhood, mainly B-type stars.

The elements C and O - the most abundant after H and He - may preferentially condense into dust grains; but still most of their atoms remain in the gaseous phase. Their abundances have been measured towards several lines of sight in the solar neighbourhood, and show similar values, implying that their degree of incorporation into dust grains is quite constant, at least outside the densest molecular clouds (Cardelli et al. 1996and references therein). These interstellar values will be compared to the nebular abundances.

In general, the resulting nebular abundances will not be compared with the results obtained by other authors for the HII regions studied here, since the different determinations of the ionic abundances lead to similar results (at least for nebular areas characterized by similar ionization conditions), whereas the values of the total abundances depend on the ionization-correction factors assumed in each case.

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999