Dynamical evolution of star-forming regions: III. Unbound stars and predictions for Gaia

Abstract

We use N-body simulations to probe the early phases of the dynamical evolution of star-forming regions and focus on mass and velocity distributions of unbound stars. In this parameter space study, we vary the initial virial ratio and degree of spatial and kinematic substructure and analyse the fraction of stars that become unbound in two different mass classes (above and below 8 M ⊙ ). We find that the fraction of unbound stars differs depending on the initial conditions. After 10 Myr, in initially highly subvirial, substructured simulations, the high-mass and lower-mass unbound fractions are similar at ∼23 per cent. In initially virialised, substructured simulations, we find only ∼16 per cent of all high-mass stars are unbound, whereas ∼37 per cent of all lower-mass stars are. The velocity distributions of unbound stars only show differences for extremely different initial conditions. The distributions are dominated by large numbers of lower-mass stars becoming unbound just above the escape velocity of ∼3 km s −1 with unbound high-mass stars moving faster on average than lower-mass unbound stars. We see no high-mass runaway stars (velocity > 30 km s −1 ) from any of our initial conditions and only an occasional lower-mass runaway star from initially subvirial/substructured simulations. In our simulations, we find a small number of lower-mass walkaway stars (with velocity 5-30 km s −1 ) from all of our initial conditions. These walkaway stars should be observable around many nearby star-forming regions with Gaia.

Runaway and walkaway stars from the ONC with Gaia DR2

“…While simulations show (Schoettler et al 2019) that we can expect to find RW stars where only one of their velocity components would identify them as a RW or WW star, there can be other explanations such as the possible binarity of the system. In this case, the difference in RV measurements from two surveys hint at this star being part of a binary, located in the ONC centre.…”

“…This large number of cluster stars and higher local stellar density can increase the likelihood of dynamical ejections (e.g. Oh & Kroupa 2016;Farias et al 2019;Schoettler et al 2019). Hillenbrand (1997) considered the projected size of the ONC in two dimensions to be 2.5 × 4.5 pc and Kroupa et al (2018) suggested the cluster to have a nominal radius of ∼2.5 pc.…”

“…To predict the number and velocities of RW and WW stars in the observational data, we have run a set of 20 N-body simulations with initial conditions similar to those the ONC is thought to have evolved from. Schoettler et al (2019) used N-body simulations to show that the number and velocity distribution of ejected stars can be used to constrain the initial spatial and kinematic substructure and we use this approach to provide predictions for our search.…”

“…We find another potential 3D RW star TYC 4774-868-1 (Gaia DR2 3209532135777678208). This star could be an example of a special case described in Schoettler et al (2019). These authors found stars that appear bound in proper mo- Kounkel et al (2018); Column 4: indication of 3D-candidate status; Column 5: minimum flight time since ejection (crossing of search boundary); Column 6: age from PARSEC isochrones (Bressan et al 2012); Column 7-9: from literature sources -1 Van Altena et al 1988 12.9 ±3.6 --2.9 14.0 +36.0…”

“…Only a few low-mass RW star detections have been made up to now (De la Fuente Marcos & de la Fuente Marcos 2018; Yeom et al 2019). Schoettler et al (2019) used N-body simulations to show that young star-forming regions eject RW and WW stars across a wide mass range (0.1-50 M ⊙ ). They suggested that we should be able to find a number of low/intermediatemass RWs/WWs in addition to massive ones around many nearby, young star-forming regions and that the number and velocity distributions of RW/WW stars could allow us to constrain the initial conditions of star formation and the dynamical histories of these regions.…”