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

TL;DR

This paper introduces 'accretion-induced orbital tightening' as a mechanism explaining why massive newborn stars in turbulent clouds exhibit higher velocity dispersions than low-mass stars, linking star formation environment to stellar dynamics.

Contribution

The study identifies a new mechanism where accretion causes orbital tightening in massive stars, explaining their increased velocity dispersion in turbulent star-forming regions.

Findings

Massive stars form in denser, more crowded regions.

Orbital tightening increases velocity dispersion of massive stars.

Low-mass stars remain more isolated with lower velocity dispersion.

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 years 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 analyzed 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.