The Orion association, with its constituent nebulae and clusters, provides an important reference point for massive-star and nebular (H ii region) abundances. The NLTE results for B-type stars in the Orion OB1 association tend to agree well with those for nearby B-type stars and with each other. For example, K92 derive [O/H] = 8.60 ± 0.11 from 7 stars, while Cunha & Lambert (1992, 1994) derive 8.65 ± 0.12 and 8.61 ± 0.17 from 18 stars in Orion OB1. This agrees well with the oxygen abundance derived for the M42 nebula of 8.65 ± 0.03 by Esteban et al. (2004), so all seems well. However, the three B-type stars of the M42 ionizing (Trapezium) cluster have much higher [O/H] abundances of 8.92, 8.76 and 8.97 dex, very different from that of the nebula. The solution, as hinted at above, is that these stars lie at the hot end of the effective temperature range and their temperatures are overestimated (also resulting in surface gravities higher than expected for main-sequence stars). Simon-Diaz et al. (2006) have resolved this discrepancy in a very detailed analysis of the Trapezium cluster stars using NLTE models with line-blocking. They derive lower temperatures (and surface gravities) with resulting [O/H] abundances of 8.65, 8.59 and 8.64 dex, in good agreement with the nebular results and results for nearby B-type stars. This work is described further in Chapter 11 of these proceedings.
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