We have made a link between the foreground population toward the well-known star formation benchmark ONC region and the stellar population of the poorly studied NGC 1980 cluster (or iota Ori cluster). Not only did we detect a well-populated main-sequence (from B-stars to M-stars), the foreground sources have 1) a defined spatial distribution that peaks near iota Ori, 2) a fainter luminosity function when compared to the extincted young population embedded inside the cloud, and 3) a low-velocity dispersion, typical of that of other young clusters.
Unlike the ONC, NGC 1980 is a slightly older cluster (\(4-5\) Myr), lacks an obvious H\(_{\rm{II}}\) region, and is comparatively free of dust extinction. Surprisingly, the radial velocity of NGC 1980 is currently indistinguishable from the radial velocity of the ONC embedded population or the radial velocity of Orion A cloud, suggesting that both clusters are genetically related. They are situated at about the same distance from Earth.
A general concern that this study raises is the risk of population mixing in star formation studies. It is unlikely that the ONC is atypical in this respect, and a dedicated multi-wavelength study to separate the different populations, together with a sensitive proper motion survey of the region, is urgently needed. The ONC is still the closer massive star formation region to Earth, and albeit more complicated than first assumed, it is still the one offering the best detailed view on the formation of massive stars and clusters.
To summarize, we used of the optical effects of dust extinction to block the background stellar population of the Orion A cloud, and found that there is a rich foreground stellar population in front of the cloud, in particular the ONC. This population contains a well populated main-sequence, from B-stars to M-stars.
The spatial distribution of the foreground population is not random but clusters strongly around NGC 1980 (iota Ori), has a fainter luminosity function, and a different velocity dispersion from the reddened population inside the Orion A cloud. This foreground population is, in all likelihood, the extended stellar content of the poorly studied NGC 1980 cluster.
We estimate the number of members of NGC 1980 to be about 1800-2000, which makes it one of the most massive clusters in the entire Orion complex, and estimate its age to be \(\sim 4-5\) Myr by comparing the median spectral energy distributions among known young populations and constraints from the age of the post-main-sequence star iota Ori.
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 what is currently taken as the ONC population (30-60% excluding the Trapezium cluster).
Our results suggest that what is normally considered as the ONC in the literature should be seen as a mix of several unrelated populations: 1) the youngest population, including the Trapezium cluster and ongoing star formation in the dense gas inside the nebula, 2) the young foreground population, dominated by the NGC 1980 cluster, and 3) the poorly constrained population of foreground and background Galactic field stars.
We re-determined the mean radial velocity for the Trapezium and NGC 1980 clusters to be \(25.4\pm3.0\) km/s and \(24.4\pm1.5\) km/s, respectively, or indistinguishable within the errors, and similar to the radial velocity of the Orion A cloud, suggestive of a genetical connection between the two.
We identified a hitherto unrecognized group of about 50 YSOs west of L1641N (L1641W) 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.
This work supports a scenario where the ONC and L1641N are not directly associated with NGC 1980, i.e., they are not the same population emerging from its parental cloud but are instead distinct overlapping populations. This calls for a revision of most of the observables in the benchmark ONC region (e.g., ages, age spread, mass function, disk frequency, etc.).
Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III web site is http://www.sdss3.org/.
SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, University of Cambridge, University of Florida, the French Participation Group, the German Participation Group, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University.
This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration.
Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.
This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency.
This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.