THE ORION OB1 ASSOCIATION .1. STELLAR CONTENT

Walraven photometry of established and probable members of the Orion O B1 association is presented. Effective temperature, surface gravity, l uminosity and mass are derived for all stars, using atmosphere models by Kurucz (1979). Absolute magnitudes are calculated using the Straizy s and Kuriliene (1981) tables. Distance moduli and visual extinctions are determined. A comparison of the visual extinctions to IRAS 100 mum data shows that the near edge of the Orion A and B clouds lies at a d istance of approximately 320 pc, while the far edge is at approximatel y 500 pc. A method for deriving the ages of the subgroups by comparing theoretical isochrones to the observations in the log g, log T(eff) p lane is presented. The derived ages suggest, contrary to earlier studi es, that subgroup 1b is younger than 1c, which can possibly be explain ed by past geometries of the system of stars and gas. The initial mass function for Orion OB1 is derived with the aid of the Kolmogorov-Smir noff test. Through extensive simulations, we show that it is very diff icult to derive accurately the IMF from the available data. To within somewhat weak limits the IMF is found to be of the form xi(log M) = AM -1.7+/-0.2 for all subgroups. The energy output of the subgroups in th e form of stellar winds and supernovae is calculated and compared to t he observed size and expansion velocity of the Orion-Eridanus bubble. It is shown that the energy output of the association can account for the morphology and kinematics of the ISM.