Chemical evolution of the orion association. I. The oxygen abundance of main-sequence B stars

Oxygen abundances are presented from LTE and non-LTE analyses of O II lines in spectra of 18 B main-sequence stars of the four subgroups comprising the Orion association. The lowest abundances are found in the oldest subgroups (Ia and Ib). The youngest subgroup Id, the Trapezium cluster, and some stars of the slightly older Ic subgroup seem to have an abundance that is up to about 40% higher. It is suggested that this enrichment resulted from the mixing of supernovae ejecta from the Ic subgroup with those adjacent portions of the dense molecular cloud that subsequently collapsed to form the Trapezium cluster. These stellar analyses confirm published abundance analyses of the H II region's emission lines that show Orion to have an oxygen abundance that is less than that of the Sun: the mean LTE/NLTE abundances are log ∈(O) = 8.67/8.65 ± 0.12 for the 18 stars whereas the solar abundance is log ∈(O) = 8.93.