Joao Alves edited Abstract.tex  over 10 years ago

Commit id: 349778cf9453b07b8ced7268a3d2702b266269bd

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\abstract  The aim of this work is to characterize the stellar population between Earth and the Orion A molecular cloud where the well-known star formation benchmark Orion nebula cluster (ONC) is embedded.  We used the denser regions the Orion A cloud to block optical background light, effectively isolating the stellar population in front of it. We then used a multi-wavelength observational approach to characterize the cloud's foreground stellar population.  We find that there is a rich stellar population in front of the Orion A cloud, from B-stars to M-stars, with a distinct 1) spatial distribution, 2) luminosity function, and 3) velocity dispersion from the reddened population inside the Orion A cloud. The spatial distribution of this population peaks strongly around NGC 1980 (iota Ori) and is, in all likelihood, the extended stellar content of this poorly studied cluster. We infer an age of $\sim 4-5$ Myr for NGC 1980 and estimate a cluster population of about 2000 stars, which makes it one of the most massive clusters in the entire Orion complex. This newly found population overlaps significantly with what is currently assumed to be the ONC and the L1641N populations, and can make up for more than 10-20\% of the ONC population (30-60\% if the Trapezium cluster is excluded from consideration). What is currently taken in the literature as the ONC is then a mix of several intrinsically different populations, namely 1) the youngest population, including the Trapezium cluster and ongoing star formation in the dense gas inside the nebula, 2) the foreground population, dominated by the NGC 1980 cluster, and 3) the poorly constrained population of foreground and background Galactic field stars. %We identify a hitherto unrecognized group of about 50 YSOs towards the West of L1641N that we speculate is either a foreground group ramming into the Orion A cloud, or a slightly older sibling of NGC 1641N, leaving the cloud.