Deep 3.8 micron observations of the Trapezium cluster

We present deep 3.8 mum L' imaging observations of the Trapezium cluster in Orion obtained with the ESO VLT. We use these observations to ( 1) search for infrared excess emission and evidence for protoplanetary disks associated with the faint, substellar population of this young cluster and ( 2) investigate the nature and extent of a recently discovered population of deeply embedded sources located in dense molecular gas behind the cluster. We detected 38 L' sources with substellar luminosities. In addition, we detected 24 L' sources that were spectroscopically classified as substellar objects in previous studies. Examining the infrared colors of all these sources, we determine an infrared excess fraction of 50% +/- 20% from the JHK(s)L' colors for both the luminosity-selected and spectroscopically selected substellar samples. This finding confirms the presence of infrared excess, likely due to circumstellar disks, around a significant fraction of the cluster's substellar population, consistent with the indications of earlier observations obtained at shorter (JHK(s)) wavelengths. Our deep L' imaging survey also provides new information concerning the deeply embedded population of young objects located in the molecular cloud behind the cluster and revealed in an earlier L-band imaging survey of the region. In particular, our present L' survey doubles the number of sources in the cluster region known to possess extremely red K-L colors. These objects exhibit K-L' colors indicative of deeply buried, possibly protostellar, objects that likely mark the site of the most recent and ongoing star formation in the region. We find the surface density distribution of the deeply embedded population to follow that of the background molecular ridge and to be highly structured, consisting of a string of at least five significant subclusters. These subclusters may represent the primordial building blocks out of which the cluster was and perhaps still is being assembled. These observations may thus provide insights into the early stages of cluster formation and appear consistent with recent simulations that suggest that the Trapezium cluster may have formed from numerous but small primordial subclusters.