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

TL;DR

This study uses Gaia data to identify ejected stars around NGC 2264, constraining its initial conditions and substructure through comparison with N-body simulations, revealing high initial density and substructure.

Contribution

It provides the first detailed analysis of ejected stars in NGC 2264, linking observed kinematics to initial cluster conditions using Gaia data and simulations.

Findings

Identified 14 ejected stars tracing back to key subclusters.

Results are consistent with high initial stellar density (~10,000 M$_{ ext{sun}}$ pc$^{-3}$).

Gaia EDR3 data refines ejected star trajectories and candidate list.

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 ($\sim$10 000 M$_{\odot}$ 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.