Constraining the initial conditions of NGC 2264 using ejected stars found in Gaia DR2

Abstract

Fast, ejected stars have been found around several young star-forming regions, such as the Orion Nebula Cluster (ONC). These ejected stars can be used to constrain the initial density, spatial and kinematic substructure when compared to predictions from 𝑁-body simulations. We search for runaway and slower walkaway stars using Gaia DR2 within 100 pc of NGC 2264, which contains subclustered regions around higher-mass OB-stars (S Mon, IRS 1 and IRS 2). We find five runaways and nine walkaways that trace back to S Mon and six runaways and five walkaways that trace back to IRS 1/2 based on their 3D-kinematics. We compare these numbers to a range of 𝑁-body simulations with different initial conditions. The number of runaways/walkaways is consistent with initial conditions with a high initial stellar density (∼10 000 M pc −3 ), a high initial amount of spatial substructure and either a subvirial or virialised ratio for all subclusters. We also confirm the trajectories of our ejected stars using the data from Gaia EDR3, which reduces the number of runaways from IRS 1/2 from six to four but leaves the number of runaways from S Mon unchanged. The reduction in runaways is due to smaller uncertainties in the proper motion and changes in the parallax/distance estimate for these stars in Gaia EDR3. We find further runaway/walkaway candidates based on proper motion alone in Gaia DR2, which could increase these numbers once radial velocities are available. We also expect further changes in the candidate list with upcoming Gaia data releases.

Constraints on star formation in NGC2264

“…In this paper, we follow-up the analysis of NGC 2264 in Schoettler et al (2021) to determine whether the spatial distribution of the stars in this region is also consistent with the analysis of the runaway and walkaway stars. This paper is organised as follows.…”

“…In this figure, we show the observed values for S Mon via the blue triangles, and for IRS 1/2 via the pink heptagons. In general, the simulation data are shown by grey lines/points, save for when the individual simulations also produce the same numbers of runaway and walkaway stars to the observed subclusters (Schoettler et al 2021). In that case, the simulations are shown by blue lines/points (S Mon) or pink lines/points (IRS 1/2).…”

“…Where information about the membership probability was given, we use stars with a probability of 50 per cent or above as part of our census. As noted in Schoettler et al (2021), while the members of NGC 2264 are fairly well constrained on-sky, they are far less constrained in their distance measurement. While the majority of the members are found around at a distance of around 700-800 pc, there are also identified members in several of the literature sources we use that show distances far above and below this value range.…”

“…Indeed, NGC 2264 is referred to as the 'Christmas Tree cluster', due to its unusual spatial distribution. NGC 2264 has had at least two temporally distinct episodes of star formation, but Schoettler et al (2021) was able to pinpoint the initial conditions of these two episodes and -like the ONC -found them to be consistent with dense (10 4 M ⊙ pc −3 ), substructured and subvirial star-formation.…”

“…In Schoettler et al (2021) we applied this analysis to NGC 2264, which unlike the ONC is not a smooth centrally concentrated cluster. Indeed, NGC 2264 is referred to as the 'Christmas Tree cluster', due to its unusual spatial distribution.…”