Dynamical heating of newborn stars driven by accretion-induced orbital tightening

Abstract

In previous works, we have shown that stars in the Orion and the Lagoon Nebula Clusters, and simulations of collapsing clouds, exhibit constant velocity dispersion as a function of mass, a result described by Lynden-Bell 50 yr ago as an effect of a violent relaxation mechanism. In contrast, numerical simulations of turbulent clouds show that newborn massive stars experience stronger dynamical heating than low-mass stars. We analysed turbulent numerical simulations and found that this effect arises from the fact that, in clouds that are globally turbulence-supported against collapse, massive stars are formed within more massive and denser clumps and in more crowded environments compared to low-mass stars. This allows them to accrete more mass and interact with other stars simultaneously. As they become more massive, their orbits tighten, increasing their velocity dispersion. In contrast, low-mass stars are formed in the periphery of such cores, more separated, and at lower densities. Thus, their velocity dispersion remains lower because they do not accrete as vigorously as massive stars and tend to be more isolated. We call this mechanism 'accretion-induced orbital tightening'. Our results and previous findings about violent relaxation provide a key observational diagnostic of how to distinguish the dynamic state of star-forming molecular clouds through the kinematics of their newborn stars.