Christina Schoettler
Theory predicts that we should find fast, ejected (runaway) stars of all masses around dense, young star-forming regions. Simulations show that the number and distribution of these ejected stars could be used to constrain the initial conditions of the regions. We search for runaway and slower walkaway stars within 100 pc of the Orion Nebula Cluster (ONC) using Gaia DR2 astrometry and photometry. We compare our findings to predictions for the number and velocity distributions of runaway stars from simulations that we run for 4 Myr with initial conditions tailored to the ONC. In Gaia DR2, we find nine 3D-runaways and 24 3D-walkaways from the ONC, all of which are low/intermediate-mass (<8 solar masses). Our simulations show that the number of runaways within 100 pc decreases the older a region is (as they quickly travel beyond this boundary), whereas the number of walkaways increases up to 3 Myr. We find fewer walkaways in Gaia DR2 than the maximum suggested from our simulations, which may be due to observational incompleteness. However, the number of Gaia DR2 runaways agrees with the number from our simulations with a maximum age of ~2.4 Myr, allowing us to confirm existing age estimates for the ONC (and potentially other star-forming regions) using runaway stars.
Most stars form in clustered/grouped environments and we observe star-forming regions at different evolutionary stages in the night sky. Stars are often found on the periphery of star clusters, but it is unclear if they originate from the cluster or are background/foreground stars in the galaxy.
The next generation of telescopes will enable us to distinguish stars that have been ejected from these clusters and such ejected stars can be used to test models of star formation. Simulating the evolution of star clusters with different initial conditions and then comparing the number and velocities of ejected stars with observations will allow us to place constraints on the initial density, spatial and velocity structure of star-forming regions.
Runaways (and slower walkaways) are stars that are ejected from a cluster after a dynamical interaction with another stellar system or when one component of a binary star system explodes as a supernova (Blaauw 1961, Poveda+ 1967). These ejected stars travel with velocities faster than the cluster’s escape velocity and can reach velocities of several hundred km/s. During their life, they can travel large distances (we show a plot illustrating that stars can travel distances of ~50 pc during a simulation covering 10 million years (Myr)). As a result, runaways are often observed on the outskirts of star-forming regions.
We use N-body simulations to study the gravitational interactions of stars in dense, dynamical systems such as young star clusters, over a defined time period. Different initial conditions are run using specialised software producing data on the position, velocity, and close encounters for every star (Hut 2003).
Running simulations with initial conditions like those expected for the ONC, we see that the maximum number of runaway stars within 100 pc of the simulated cluster decreases the older the cluster is. We find 10 runaways at an age of 2 Myr, this decreases to only 5 runaways after 4 Myr. The number of slower walkaway stars increases up to 32 after 3 Myr, then decreases to 30 after 4 Myr as stars leave the 100 pc region around the cluster.
We use high-resolution astrometry (three-dimensional spatial and velocity information) and photometry from the space-based telescope Gaia (Data Release 2, Gaia Collaboration 2018). Using observations of proper motion and locations of stars within 100 pc of the ONC, we trace back ejected stars to find runaways and walkaways from this cluster. However not all stars that are traced back originate in the ONC. Some stars are older visitors that have passed through the cluster. We plot a Colour-Magnitude Diagram to estimate ages for our trace-back stars and use a 4 Myr isochrone as the upper age limit for the ONC - stars above it are young enough to have been born in the cluster.
We find nine runaways and 24 walkaway stars that are young enough in the Gaia DR2 observations. Comparing these numbers to our simulations shows that the ONC is likely younger than 3 Myr, as there should only be 5 RWs within 100 pc at this age. The number of runaways is consistent with those at an upper age of ~2.4 Myr and we show that ejected stars can be used to estimate/confirm ages of young star clusters.