Star formation in the Lambda Orionis region. I - The distribution of young objects

Abstract

We have made a well-sampled and deep survey for H..cap alpha.. emission objects in a region about 100 square degrees in area (about 5000 pc/sup 2/), centered on the lambda Orionis OB association. We detected 83 objects, of which 62 were previously known from earlier, less extensive surveys. The majority of the H..cap alpha.. emission objects in this region are not projected against dark cloud material, an many appear to be within the central regions of the extended (i.e., about 6/sup 0/ in diameter) low density H II region excited by lambda/sup 1/ Orionis. The vast majority of the H..cap alpha.. emission objects form an elongated, barlike structure 2/sup 0/ x 8/sup 0/ in extent. These dimensions are similar to those of giant molecular cloud complexes which spawn OB and T associations. We suggest that the spatial distribution of H..cap alpha.. emission objects and OB association members in this region provides a fossil record of a preexistent giant molecular cloud complex. The gas from the original complex has been recently removed from the vicinity of these stars by the action of the lambda Orionis H II region and is presently detected as a massive shell surrounding the H II region.more » Estimates of the mass of the shell and of the mass of the H..cap alpha.. emission objects and OB stars in the complex enable a determination to be made of the total star-formation efficiency: which we find to be 0.2%: 0.3%. Such low efficiency may be characteristic of giant molecular cloud complexes. The low star-formation efficiency we find suggests that the vast majority of the original binding mass of the lambda Orionis complex has been dissipated, leaving recently formed stars behind. Consequently, we expect this association to be unbound and expanding.« less