A WIYN Lithium Survey for Young Stars in the $\lambda$ Orionis
			 Star-Forming Region

	      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

We have found 72 pre-main sequence (PMS) stars near the center of the
$\lambda$~Orionis star-forming region by spectroscopically testing a
magnitude-limited sample for the presence of lithium
$\lambda$6708~{\AA} absorption, a diagnostic of youth.  All of these
stars show large lithium equivalent widths and radial velocities
consistent with Orion membership, yet only two were discovered
previously via H$\alpha$ or X-ray surveys.  Comparison with PMS
evolutionary tracks show that the low-mass star formation did not
begin prior to the initiation of high-mass star formation 5-7 Myr ago.
However, the subsequent detailed star-formation history is
model-dependent.  Baraffe et~al.\ (1998) isochrones imply that high-
and low-mass stars began to form together 5-7 Myr ago, with the
low-mass stellar formation ceasing abruptly 1 Myr ago.  On the other
hand, D'Antona \& Mazzitelli (1998) isochrones indicate a narrow
spread of PMS ages which suggest a burst of low-mass star formation
1-2 Myr ago.  Furthermore, kinematic arguments require that the parent
molecular cloud gravitationally bound the stars together until
recently, yet at present the requisite gas mass is not visible.  This
leads us to conjecture that both the high- and low-mass stars were in
a tightly bound cluster until a supernova blast about 1 Myr ago
disrupted the parent cloud.  This supernova also impacted on the PMS
formation process by either (1) ceasing formation through removal of
the gas supply or (2) triggering star births via cloud compression,
depending on choice of stellar evolution models.  Finally, we find
that despite their youth only 4 of the 72 PMS stars have T Tauri-like
H$\alpha$ emission, suggesting the absence of accretion disks.  We
conjecture that this may be the result of photoevaporation of the
disks while the low-mass stars were in much closer proximity to the OB
stars prior to becoming gravitationally unbound.