A MULTI-COLOR OPTICAL SURVEY OF THE ORION NEBULA CLUSTER. I. THE CATALOG

We present U, B, V, I broadband, 6200 angstrom TiO mediumband, and H alpha narrowband photometry of the Orion Nebula Cluster (ONC) obtained with the WFI imager at the ESO/MPI 2.2 telescope at La Silla Observatory. The nearly simultaneous observations cover the entire ONC in a field of about 34 x 34 arcmin. They enable us to determine stellar colors avoiding the additional scatter in the photometry induced by stellar variability typical of pre-main-sequence stars. We identify 2612 point-like sources in the I band; 58%, 43%, and 17% of them are also detected in V, B, and U, respectively. 1040 sources are identified in the H alpha band. In this paper we present the observations, the calibration techniques adopted, and the resulting catalog. We show the derived color-magnitude diagram of the population and discuss the completeness of our photometry. We define a spectrophotometric TiO index that takes into account the fluxes in the V, I, and TiO bands. Comparing it with spectral types of ONC members in the literature, we find a correlation between the index and the spectral type valid for M-type stars, which is accurate to better than 1 spectral subclass for M3-M6 types and better than 2 spectral subclasses for M0-M2 types.. This allows us to newly classify 217 stars. In a similar way, we subtract from our H alpha photometry the photospheric continuum at its wavelength, deriving calibrated line excess for the full sample. This represents the largest H alpha star catalog obtained to date on the ONC. This data set enables a full re-analysis of the properties of the pre-main-sequence population in the Orion Nebula Cluster to be presented in an accompanying paper.