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

The lambda Ori star-forming region presents a snapshot of a moderate-mass g iant molecular cloud 1-2 Myr after cloud disruption by OB stars, with the O B stars, the low-mass stellar population, remnant molecular clouds, and the dispersed gas all still present. We have used optical photometry and multi object spectroscopy for lithium absorption to identify 266 pre-main-sequenc e stars in 8 deg(2) of the region. We also present new photometry for the m assive stars, from which we derive a distance of 450 pc and Stromgren a tur noff age of 6-7 Myr. Using these parameters and pre-main-sequence evolution ary models, we map the star formation history of the low-mass stars. We fin d that low-mass star formation started throughout the region at about the s ame time as the birth of the massive stars, and thereafter the birth rate a ccelerated. 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 conti nues in dark clouds 20 pc away. We suggest that a supernova 1-2 Myr ago des troyed 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 formati on in the outer molecular clouds. The global star formation of the lambda O ri region has generated the field initial mass function, but local star for mation in subregions shows large deviations from the expected ratio of high - to low-mass stars.