A wiyn lithium survey for young stars in the lambda Orionis star-forming region

We have found 72 pre-main-sequence (PMS) stars near the center of the lambd a Orionis star-forming region by spectroscopically testing a magnitude-limi ted sample for the presence of lithium lambda 6708 absorption, a diagnostic of youth. All of these stars show large lithium equivalent widths and radi al velocities consistent with Orion membership, but only two were discovere d 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 in itiation of high-mass star formation 5-7 Myr ago. However, the subsequent d etailed star formation history is model dependent. Baraffe et al. isochrone s imply that high- and low-mass stars began to form together 5-7 Myr ago, w ith the low-mass stellar formation ceasing abruptly 1 Myr ago. On the other hand, D'Antona & Mazzitelli isochrones indicate a narrow spread of PMS age s, which suggests a burst of low-mass star formation 1-2 Myr ago. Furthermo re, kinematic arguments require that the parent molecular cloud gravitation ally bound the stars together until recently, but at present the requisite gas mass is not visible. This leads us to conjecture that both the high- an d low-mass stars were in a tightly bound cluster until a supernova blast ab out 1 Myr ago disrupted the parent cloud. This supernova also impacted on t he PMS formation process by either (1) ceasing formation through removal of the gas supply or (2) triggering star births via cloud compression, depend ing on choice of stellar evolution models. Finally, we find that despite th eir youth, only four 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.