On the Absolute Magnitudes of the O Stars

The conclusion published in 1992 by Garmany & Stencel from a study of northern OB associations, that the absolute magnitudes of the O stars show "a large scatter ... intrinsic to the classification system," is critically examined. It is found that the differences between their derived absolute magnitudes of O stars and this author's 1973 calibration exhibit large systematic effects in several associations, ranging from -0.74 to +1.02 mag with substantially smaller dispersions. Of course, when these results are combined, the scatter equals the full range of the systematic effects. To investigate the possibility of distance errors, the Garmany & Stencel B0–B2.5 stars in the same associations are subjected to the same analysis. The results for the B stars show no significant systematic differences, eliminating errors in the association distances derived by Garmany & Stencel from the B stars as the source of the differences found for the O stars. It is noteworthy that the dispersions in the absolute magnitudes of the B stars within a given association are similar to or larger than those of the O stars. An examination of the distribution on the sky of the stars shows that the O and B stars in the discrepant associations are generally not colocated; such was already known to be the case for the important Perseus OB1 association. It is suggested that despite their efforts to improve them, significant problems remain with the association memberships adopted by Garmany & Stencel; the relatively small dispersions of the O star absolute magnitudes even in the discrepant cases indicate that they belong to different, usually more distant associations near the lines of sight to the B associations with which they have been mistakenly connected. Several individual cases of unrecognized multiple systems and classification errors are also found in the Garmany & Stencel sample. It is concluded that the scatter in the absolute magnitudes of the O stars is not as large as found by Garmany & Stencel, and not larger than that of the B stars.