The Head of Orion or Lambda Orionis Star Forming Region is part of the Orion molecular cloud complex, but at δ ∼ +10 ◦, it is located about 15◦ north of the center of the complex. The star λ Ori is the brightest member of Collinder 69, a well-defined, compact open cluster at the center of the star forming region. Two other already identified and somewhat smaller clusters lie near Collinder 69 and are associated with the Barnard 30 and Barnard 35 molecular clouds. At mid-infrared wavelengths and 21 cm, λ Ori marks the center of a 4◦ diameter circular shell of gas and dust thought to be the result of winds from the high mass stars in the cluster or from a supernova whose progenitor was the highest mass member of the cluster. The properties of these three associations are quite diverse. Collinder 69 is quite rich, with one O star (λ Ori itself) and of order a dozen B stars (Duerr et al. 1982) and a well-populated sequence of low-mass stars and brown dwarfs whereas the Barnard 30 and Barnard 35 associations appear to be almost entirely composed of low-mass stars. Moreover, the fraction of Classical TTauri low-mass stars in the Barnard 30 and Barnard 35 clusters seems to be much higher than that of Collinder 69. Thus, we believe that the λ Ori star-forming region can serve as a valuable testbed for star formation studies.
It is the comparative properties of these three regions what leads the work pre- sented here. We have made use of a variety of ground- and space-based observations to better understand the different cluster populations in Collinder 69, Barnard 30 and Barnard 35. In addition, with the help of IR data from Spitzer Space Telescope, we have been able to produce comprehensive spectral energy distributions which allow to classify the cluster members in line with their formation stage, Class I (when the central protostar is surrounded by a disk and deeply embedded within a dust/gas envelope), to Class II (or Classical T Tauri stars, where the envelope has disappeared but the accretion disk remains), and Class III (weak line T Tauri stars, when only a fast rotating star remains). By comparing these results for the three associations, we aim to address the questions of the frequency and radial structure of circumstellar disks as a function of environment.
