Chemical evolution of the Orion association. II. The carbon, nitrogen, oxygen, silicon, and iron abundances of main-sequence B stars

Carbon, nitrogen, oxygen, and silicon abundances are presented from LTE and non-LTE analyses of C II, N II, and Si III lines in the spectra of 18 main-sequence B stars from the four subgroups comprising the Orion association. Iron LTE abundances from Fe III lines are also presented. The C, N, and Fe abundances show no significant variations across the subgroups, but the O and Si abundances are found to be higher for some of the youngest stars that are collocated on the sky and at a common distance. The O and Si abundances are correlated. Although such a correlation may in part reflect measurement errors, it is suggested that the enrichment of young stars in O and Si arose because they were formed from regions of the molecular cloud enriched with the ejecta of Type II supernovae, which are predicted to be rich in O and Si but not in C and N. With the exception of one star, we see no evidence for CN-cycled material on the stars' surfaces. The stellar abundances agree, within the expected uncertainties, with published nebular analyses that show Orion to be slightly underabundant in C, N, and O relative to the Sun.