The spatial distribution of the λ orionis pre-main-sequence population

The lambda Ori star-forming region presents a snapshot of a moderate-mass giant molecular cloud 1-2 Myr after cloud disruption by OB stars, with the OB stars, the low-mass stellar population, remnant molecular clouds, and the dispersed gas all still present. We have used optical photometry and multiobject spectroscopy for lithium absorption to identify 266 pre-main-sequence stars in 8 deg(2) of the region. We also present new photometry for the massive stars, from which we derive a distance of 450 pc and Stromgren a turnoff age of 6-7 Myr. Using these parameters and pre-main-sequence evolutionary models, we map the star formation history of the low-mass stars. We find that low-mass star formation started throughout the region at about the same time as the birth of the massive stars, and thereafter the birth rate accelerated. Within the last 1-2 Myr star formation ceased in the center of the star-forming region, near the concentration of OB stars, while it continues in dark clouds 20 pc away. We suggest that a supernova 1-2 Myr ago destroyed the molecular cloud core from which the OB stars formed, but it did not terminate star formation in more distant reaches of the giant molecular cloud. We find no secure evidence for triggered or sequential star formation in the outer molecular clouds. The global star formation of the lambda Ori region has generated the field initial mass function, but local star formation in subregions shows large deviations from the expected ratio of high- to low-mass stars.