A photometric study of the Orion OB 1 association. III - Subgroup analyses

The four principal subgroups of the association are examined in detail using individual distances and reddening values determined for their B-type members from uvbyfl data presented in the previous papers of this series. Subgroup la appears not to show a spread in age as suggested by a previous study; nor does it show a systematic distance increase with right ascension when fainter members are considered, although such a trend in declination indicates a possible connection of this subgroup with the A Orionis clustering to the north. An eastwardly increase in distance is found for subgroup ib, in agreement with previous studies, but the reddening law for the east Belt appears normal. Tile small subclusterings in the vicinity of the Orion Nebula appear not to differ in evolutionary state, but their ages are considerably greater than those of stars in the nebula and its associated cluster. Both uniform differences and large individual discrepancies in the m0 index, similar to those found in a study of the Upper Scorpius region of the association Sco OB 2, have been found for the young stars surrounding the Orion Nebula. They appear to be a result of several effects connected with line and/or continuum anomalies and mechanisms associated with local absorption. Several pre-main-sequence candidates are found among the late B- and A-type stars in the nebula region, and the lower main-sequence turnup occurs at approximately spectral type A0, beyond which the A- and F-type stars define a uniform sequence running parallel to the main sequence, but above it by about 1.3 mag. A clear gap in the F-star sequence is found in the range 0.21 < (b - Y)o < 0.26, indicating possible implications for the theory of convection in stellar envelopes of pre-main-sequence stars. Tile distribution of reddened B stars in the northern and southern sections of the association is indicative of absorption by circumstellar remnant dust for stars near the pre-main-sequence turnup. New age estimates for the association subgroups, made using intrinsic colors of their bluest main-sequence stars, are in substantial agreement with earlier determinations, but the accuracy can be improved by correcting for rotational-velocity effects if and when rotating models for higher mass stars become available. Subject headings: clusters: associations - nebulae: Orion Nebula - photometry - stars: early-type - stars: evolution - stars: pre-main-sequence