When the tale comes true: multiple populations and wide binaries in the Orion Nebula Cluster

Abstract

Context. Recently published high-quality OmegaCAM photometry of the 3x3 deg around the Orion Nebula Cluster (ONC) in r, and i filters revealed three well-separated pre-main sequences in the color-magnitude diagram (CMD). The objects belonging to the individual sequences are concentrated toward the center of the ONC. The authors concluded that there are two competitive scenarios: a population of unresolved binaries and triples with an exotic mass ratio distribution, or three stellar populations with different ages (≈ 1 Myr age differences). Aims. We use Gaia DR2 in combination with the photometric OmegaCAM catalog to test and confirm the presence of the putative three stellar populations. We also study multiple stellar systems in the ONC for the first time using Gaia DR2. Methods. We selected ONC members based on parallaxes and proper motions and take advantage from OmegaCAM photometry that performs better than Gaia DR2 photometry in crowded regions. We identify two clearly separated sequences with a third suggested by the data. We used Pisa stellar isochrones to estimate ages of the stellar populations with absolute magnitudes computed using Gaia parallaxes on a star by star basis. Results. 1) We confirm that the second and third sequence members are more centrally concentrated toward the center of the ONC. In addition we find an indication that the parallax and proper motion distributions are different among the members of the stellar sequences. The age difference among stellar populations is estimated to be 1-2 Myr. 2) We use Gaia proper motions and other measures to identify and remove as many unresolved multiple system candidates as possible. Nevertheless we are still able to recover two well-separated sequences with evidence for the third one, supporting the existence of the three stellar populations. 3) Due to having ONC members with negligible fore-or background contamination we were able to identify a substantial number of wide binary objects (separation between 1000-3000 au) and with relative proper motions of the binary components consistent with zero. This challenges previously inferred values that suggested no wide binary stars exist in the ONC. Our inferred wide-binary fraction is ≈ 5%. Conclusions. We confirm the three populations correspond to three separated episodes of star formation. Based on this result, we conclude that star formation is not happening in a single burst in this region. In addition we identify 5% of wide-binary stars in the ONC that were thought not to be present.

A stellar relic filament in the Orion star-forming region

“…In this work we also used the photometric catalogue obtained via a set of deep multi-band images acquired with OmegaCAM (098.C-0850(A), PI: Beccari), a 1 deg 2 camera (Kuijken 2011) attached to the 2.6-m VLT Survey Telescope (VST) at ESO's Paranal Observatory. The catalogue was used in Jerabkova et al (2019) to study the stellar population in a 3 × 3 degree area around the centre of the ONC in the r and i filters. While the details of the data-reduction procedure are described in Jerabkova et al (2019), here we recap that the data were reduced and calibrated by the Cambridge Astronomy Survey Unit 2 , while a large number of stars in common with the AAVSO Photometric All-Sky Survey (APASS) were used to correct for any residuals in the photometric calibration.…”

“…The catalogue was used in Jerabkova et al (2019) to study the stellar population in a 3 × 3 degree area around the centre of the ONC in the r and i filters. While the details of the data-reduction procedure are described in Jerabkova et al (2019), here we recap that the data were reduced and calibrated by the Cambridge Astronomy Survey Unit 2 , while a large number of stars in common with the AAVSO Photometric All-Sky Survey (APASS) were used to correct for any residuals in the photometric calibration. The OmegaCAM catalogue includes 93,846 objects homogeneously sampled in the r and i bands down to r ≈ 21 − 22 mag (ABmag) over a 3 × 3 deg 2 area around the cluster's centre.…”

“…This can be mitigated by using the criterion above. For more details, we refer to Jerabkova et al (2019).…”

A multiple power-law distribution for initial mass functions

“…Moreover, young clusters like the ONC have mass distributions that are still forming. Furthermore, there is reason to believe that there is more than one distinct population in play Jerabkova et al (2019) The notion of nebula time is a step toward capturing more complex histories and transient features. But the nebula time machinery is far from comprehensive in this regard.…”

“…It also emphasizes the time-dependent nature of the mass function within any star-forming region. Recent work by Drass et al (2016) and Jerabkova et al (2019) shows that the Orion Nebula Cluster has had several episodes of star formation evidenced by distinct stellar populations of different ages. Section 2 of this paper presents the derivation of the new multi-power-law distribution, while Section 3 illustrates its various properties and limits.…”