A WIDE-FIELD SURVEY OF THE ORION NEBULA CLUSTER IN THE NEAR-INFRARED

We present J, H, and K-S photometry of the Orion Nebula Cluster (ONC) obtained at the CTIO/Blanco 4 m telescope at Cerro Tololo with the Infrared Side Port Imager camera. From the observations we have assembled a catalog of about similar to 7800 sources distributed over an area of approximately 30' x 40', the largest of any survey deeper than the Two Micron All Sky Survey (2MASS) in this region. The catalog provides absolute coordinates accurate to about 0.15 arcsec and 3 sigma photometry in the 2MASS system down to J similar or equal to 19.5 mag, H similar or equal to 18.0 mag, K-S similar or equal to 18.5 mag, enough to detect planetary size objects 1 Myr old under A(V) similar or equal to 10 mag of extinction at the distance of the Orion Nebula. We present a preliminary analysis of the catalog, done by comparing the (J-H, H-K-S) color-color diagram, the (H, J-H) and (K-S, H-K-S) color-magnitude diagrams, and the JHK(S) luminosity functions (LFs) of three regions at an increasing projected distance from the Trapezium. Sources in the inner region typically show IR colors compatible with reddened T Tauri stars, whereas the outer fields are dominated by field stars seen through an amount of extinction which decreases with the distance from the center. The color-magnitude diagrams make it possible to clearly distinguish between the main ONC population, spread across the full field, and background sources. The LFs of the inner region, corrected for completeness, remain relatively flat in the substellar regime regardless of the strategy adopted to remove background contamination.