Early studies of PMS stars focused on T associations, especially Taurus-Auriga. However, one might expect low mass star formation to be different in OB associations. It is possible for O stars winds to effect the environs. Further, the initial mass functions have been reported to be different in OB association when compared with regions which only form low mass stars (Miller & Scalo 1979). The Orion OB association is among the most interesting of these, owing to its relative closeness and complexity. It contains a large molecular cloud and four sub-complexes of OB stars with different ages, starting with the very youngest of stars and extending beyond 10 million years.
Blaauw (1964) found that OB associations often break down into substructures of different ages. He identified Orion OB1a as the stars northwest of the belt. Orion OB1b is composed of the stars near the belt, including the belt stars and Orionis. Stars south of the belt are in Orion OB1c and the stars in the immediate vicinity of M42 are in Orion OB1d. Warren & Hesser (1977, 1978) carried out one of the largest surveys of the whole association using photometry. They divided the Orion OB1b into 3 sub--subgroups arguing that distance increases from west to east.
Ages for the stars in this association have generally been determined by fitting the main sequence turn--off of the O and B stars. One of the most recent studies of this region was performed by Brown (1996). He finds no perceptible distance change within the Orion OB1b association and finds the distance modulus to be 7.8 and 7.9 for Orion OB1a and OB1b respectively. These distances are about 5% closer than the distances found by Warren & Hesser (1977, 1978). The ages derived for the different sub-associations vary. Brown publishes ages of 11.4, 1.7, 4.6 and <1 Myr for OB1a through OB1d respectively. Warren & Hesser found ages of 7.9, 5.1 3.7 and < 1 Myr, which is somewhat discrepant with Brown in terms of relative ages. Blaauw (1991) revisited these sub--associations and found 12 and 7 Myr for Orion OB1a and OB1b respectively. For the remainder of the dissertation, I will use the most recent values (Brown 1996) when referring to the various sub-associations with the caveat that the correct values are not obvious. The observations which I present will concentrate on Orion OB1a and OB1b.