A Photometric Study of the Young Stellar Population throughout the λ Orionis Star-forming Region

We present VRI photometry of 320,917 stars with 11 ≲ R ≲ 18 throughout the λ Ori star-forming region. Using the more spatially limited spectroscopic surveys of Dolan & Mathieu to define the color-magnitude domain of young low-mass members of the association and removing statistically the field stars in this domain, we use our photometry to identify a representative pre–main-sequence (PMS) population throughout the interior of the molecular ring. The spatial distribution of this population shows a concentration of PMS stars around λ Ori and in front of the B35 dark cloud. However, few PMS stars are found outside these pockets of high stellar density, suggesting that star formation was concentrated in an elongated cloud extending from B35 through λ Ori to the B30 cloud. We find a lower limit for the global stellar mass of about 500 M⊙. We find that the global ratio of low- to high-mass stars is similar to that predicted by the field initial mass function, but this ratio varies strongly as a function of position in the star-forming region. Locally, the star formation process does not produce a universal initial mass function. Using our derived stellar ages across the region, we construct a history of the star-forming complex. This history incorporates a recent supernova to explain the distribution of stars and gas today. We infer that most of the present molecular ring was formed by ejecta from the center driven by the supernova blast about 1 Myr ago. However, we suggest that the B30 and B35 clouds were primordial, and massive enough to be mostly little disturbed by the shock. The stars that we see today trace the former extent of the cloud complex. Given the kinematics of the stellar population, we predict that the association will disperse into the field within a few tens of megayears.