Signatures of mass segregation from competitive accretion and monolithic collapse

TL;DR

This study compares the observable signatures of massive star formation theories, competitive accretion and monolithic collapse, using simulations to determine if spatial distributions can distinguish the mechanisms.

Contribution

The paper demonstrates that spatial distribution alone cannot reliably differentiate between competitive accretion and monolithic collapse in massive star formation.

Findings

Massive stars sit in deeper potential wells only without feedback.

Feedback reduces the correlation between mass and potential well depth.

Spatial distribution is not a robust diagnostic for formation mechanism.

Abstract

The two main competing theories proposed to explain the formation of massive ($>10$M$_\odot$) stars -- competitive accretion and monolithic core collapse -- make different observable predictions for the environment of the massive stars during, and immediately after, their formation. Proponents of competitive accretion have long predicted that the most massive stars should have a different spatial distribution to lower-mass stars, either through the stars being mass segregated, or being in areas of higher relative densities, or sitting deeper in gravitational potential wells. We test these predictions by analysing a suite of SPH simulations where star clusters form massive stars via competitive accretion with and without feedback. We find that the most massive stars have higher relative densities, and sit in deeper potential wells, only in simulations in which feedback is not present. When feedback is included, only half of the simulations have the massive stars residing in deeper potential wells, and there are no other distinguishing signals in their spatial distributions. Intriguingly, in our simple models for monolithic core collapse, the massive stars may also end up in deeper potential wells, because if massive cores fragment the stars are still massive, and dominate their local environs. We find no robust diagnostic test in the spatial distributions of massive stars that can distinguish their formation mechanisms, and so other predictions for distinguishing between competitive accretion and monolithic collapse are required.