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

We present U, B, V, I broadband, 6200 Å TiO mediumband, and Hα 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 × 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 Hα 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 Hα photometry the photospheric continuum at its wavelength, deriving calibrated line excess for the full sample. This represents the largest Hα 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.