A tale of three cities

Abstract

As part of the accretion discs in Hα with OmegaCAM (ADHOC) survey, we imaged a region of 12×8 square degrees around the Orion Nebula Cluster in r, i and Hα. Thanks to the high-quality photometry obtained, we discovered three well-separated pre-main sequences in the color-magnitude diagram. The populations are all concentrated towards the cluster's center. Although several explanations can be invoked to explain these sequences, we are left with two competitive but intriguing scenarios: a population of unresolved binaries with an exotic mass ratio distribution, or three populations with different ages. Independent high-resolution spectroscopy supports the presence of discrete episodes of star formation, each separated by about a million years. The stars from the two putative youngest populations rotate faster than the older ones, in agreement with the evolution of stellar rotation observed in pre-main sequence stars younger than 4 Myr in several star forming regions. Whatever the final explanation, our results prompt a revised look at the formation mode and early evolution of stars in clusters.

A solution to the proplyd lifetime problem

“…Our findings indicate a clear empirical test in terms the stellar population definitions in Beccari et al (2017) by correlating stellar age with PPD properties. Additionally, the shallow relationship found by E18 is at odds with that found in other regions (e.g.…”

“…Secondly, the recent sub-mm survey of disc masses and radii by Eisner et al (2018, hereafter E18) offers further observational constraints on a successful model of PPD evolution, and the host-mass dependent disc properties permit a test of theoretical predictions. Thirdly, Beccari et al (2017, see also Jerabkova et al 2019) recently found evidence of three stellar populations with distinct ages in the ONC. This has multiple consequences for models of a PPD population, most obviously that a subset of discs has evolved for a shorter period than the oldest stars in the region.…”

“…Most recently, the OmegaCAM survey by Beccari et al (2017, see also Jerabkova et al 2019 found three distinct populations of stars in the distribution of r −i colours, which cannot be attributed to variable extinction or projected distances. The authors claim the finding could either be due to three populations of different age or a previously undiscovered population of tight binaries which are skewed towards an unusually high mass ratio and make up ∼ 50% of the population.…”

APOGEE Net: Improving the Derived Spectral Parameters for Young Stars through Deep Learning

“…In the ONC, Beccari et al (2017) have observed three distinct populations with the mean ages of 1.2, 1.9 and 2.9 Myr, and that these populations, while overlapping, cover different volume in the sky, with the youngest one being the most centrally concentrated, and the oldest one being the most diffuse. However, they could not rule out that these populations are not due to an unresolved binary and tertiary sequences.…”

“…Right: Kernel Density Estimation of the log g with the kernel size of 0.1 dex. Top panels show the three populations identified by Beccari et al (2017). Middle panels highlight spectroscopic binaries from Kounkel et al (2019).…”