The extinction map of the OMC-1 molecular cloud behind the Orion nebula⋆

Context. The Orion nebula and its associated young stellar cluster are located at the front-side of the optically thick OMC-1 molecular cloud. In order to disentangle the cluster members from background contamination, it is important to know the extinction provided by the OMC-1, which is poorly known, the available measurements yielding contradictory results. Aims. Our main goal is to derive a new extinction map of the OMC-1, obtaining information about the structure of the OMC-1 and the Orion nebula cluster. Methods. The most recent near-infrared catalog of stars is used to study the distribution of reddening across a 0.3 deg area covering the Orion nebula cluster. On the basis of the observed (H, H − KS) diagram, we establish a criterion for disentangling contaminants from bona-fide cluster members. For contaminant stars, interstellar reddenings are estimated by comparison with a synthetic galactic model. A statistical analysis is then performed to consistently account for local extinction, reddening and star-counts analysis. Results. We derive the extinction map of the OMC-1 with angular resolution <5′. We also assemble a sample of candidate cluster members, for which we measure the extinction provided by the nebular environment. These extinction measurements are analyzed similarly to the contaminant sample, and an extinction map of the Orion nebula is derived. Conclusions. The extinction provided by the OMC-1 is variable on spatial scales of a few arcminutes, while showing a general increase from the outskirts (AV ∼ 6) to the direction of the Trapezium asterism (AV 30). The Orion nebula extinction map is more irregular and optically thinner, with AV of the order of a few magnitudes. Both maps are consistent with the optical morphology, in particular the Dark Bay to the north-east of the Trapezium. Both maps also show the presence of a north-south high-density ridge, which confirms the filamentary structure of the Orion molecular complex inside which star formation is still taking place.