The Spatial Distribution of the $\lambda$ Orionis
		     Pre-Main-Sequence Population

	      Christopher J. Dolan and Robert D. Mathieu
      Department of Astronomy, University of Wisconsin--Madison,
	     475 North Charter Street, Madison, WI 53706
	     dolan@astro.wisc.edu; mathieu@astro.wisc.edu


ABSTRACT

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 multi-object spectroscopy for lithium
absorption to identify 266 PMS stars in 8 degrees$^2$ of the
region.  We also present new Str\"omgren photometry for the massive
stars, from which we derive a distance of 450 pc and 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 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 IMF, but local star
formation in sub-regions shows large deviations from the expected
ratio of high- to low-mass stars.