Using ejected stars to constrain the initial conditions of young star-forming regions

Star-forming regions have fascinated observers for many centuries. These regions are the locations where most stars form and are considered to be a fundamental unit of star formation. However, what we observe of these regions now is not necessarily how they formed as dynamical evolution can quickly change the initial density, spatial and kinematic substructure. Knowledge about these initial conditions is essential to be able to constrain star formation theories.
Star-forming regions and their likely initial conditions have been studied in the past with different methods, all of which focused on stars that are considered to still be members. In this thesis, I investigate if we can use ejected stars that are often found on the outskirts of young star-forming regions to infer their initial properties. These stars are commonly known as runaway stars, having “run away” from their birth region.
These fast-moving stars have been studied since their discovery in the 1940s using simulations and observations. With improvements in computing capabilities, simulations have made huge advances in the past 50 years, allowing us to make our models of the formation and evolution of these star-forming regions increasingly realistic. For many decades, runaway star observations were limited to bright massive stars, but this changed with the launch of the Gaia telescope. This mission has already provided accurate
measurements of the positions and velocities for over 1.5 billion stars in our Galaxy. With this data, discoveries of countless new runaways have been made and we now know that they occur across all stellar masses.
In this work, I use a combination of simulations and Gaia observations and show that ejected stars can be used to constrain the initial conditions of the ONC and NGC 2264. Both regions appear to have formed from high stellar density, substructured and subvirial initial conditions.