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Orion revisited I. The massive cluster in front of the Orion nebula cluster

Article published in A&A in 2012

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, TEST 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. Our results support 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. The nearest massive star formation region and the template for massive star- and cluster formation models is then substantially contaminated by the foreground stellar population of the massive NGC 1980 cluster, formed about 4–5 Myr ago in a different, but perhaps related, event in the larger Orion star formation complex. This result calls for a revision of most of the observables in the benchmark ONC region (e.g., ages, age spread, cluster size, mass function, disk frequency, etc.). (Nordström 2003)

Introduction

The Orion nebula is one of the most often studied objects in the sky, with observational records dating back to about 400 years, coinciding with the early developments of the telescope (Muench et al., 2008). It is an object of critical importance for astrophysics as it contains the nearest (400 pc) massive star formation region to Earth, the Orion nebula cluster (ONC) (e.g. Johnson, 1965; Walker, 1972), which is the benchmark region for massive star- and cluster formation studies. Recent distance estimates to the Orion nebula using parallax put this object at about 400 pc from Earth (389$$^{+24}_{-21}$$ pc (Sandstrom et al., 2007), 414$$\pm$$7 pc (Menten et al., 2007), 437$$\pm$$19 pc (Hirota et al., 2007), and 419$$\pm$$6 pc (Kim et al., 2008)). Some of the most basic observables of the star formation process, such as, 1) star formation rates (Lada et al., 1995; Lada et al., 2010), 2) star formation history (Hillenbrand, 1997), 3) age spreads (Jeffries et al., 2011; Reggiani et al., 2011), 4) the initial mass function to the substellar regime (Hillenbrand et al., 2000; Muench et al., 2002; Da Rio et al., 2012; Hsu et al., 2012), 5) the fraction, size distribution, and lifetime of circumstellar disks (Hillenbrand et al., 1998; Lada et al., 2000; Muench et al., 2001; Vicente et al., 2005), 6) their interplay with massive stars (O’dell et al., 1993), binarity (Petr et al., 1998; Köhler et al., 2006), rotation (Herbst et al., 2002), magnetic fields (Feigelson et al., 2003), and 7) young cluster dynamics (Hillenbrand et al., 1998a; Furész et al., 2008; Tobin et al., 2009), have all been derived from this benchmark region (see the meticulous reviews of Bally, 2008; Muench et al., 2008; O’Dell et al., 2008). Naturally, the ONC is also the benchmark region for theoretical and numerical models of massive and clustered star formation (Palla et al., 1999; Klessen et al., 2000; Clarke et al., 2000; Bonnell et al., 2001; Bate et al., 2003; Tan et al., 2006; Huff et al., 2006; Krumholz et al., 2011; Allison et al., 2011)