Deep 3.8 micron Observations of the Trapezium Cluster
  We present deep 3.8 micron observations of the Trapezium cluster in Orion obtained with the ESO VLT. We use these data to search for infrared excess emission and evidence for protoplanetary disks associated with the faint, substellar population of this young cluster, and to investigate the nature and extent of a recently discovered population of deeply embedded sources located in dense molecular gas behind the cluster. Examining the infrared colors of 38 luminosity and 24 spectroscopically selected substellar candidates we determine an infrared excess fraction of 50+/-20% from the JHKsLp colors for both samples. This finding confirms the presence of infrared excess, likely due to circumstellar disks, around a significant fraction of the cluster's substellar population. Our deep Lp imaging survey also provides new information concerning the deeply embedded population of young objects located in the molecular cloud behind the cluster; in particular, it doubles the number of sources in the cluster region known to possess extremely red K-L colors. These objects exhibit K-Lp 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 5 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. 
C.J. Lada (1), A. A. Muench (1), E. A. Lada (2), J. F. Alves (3) ((1)   Harvard-Smithsonian CfA, (2) University of Florida, ...