On The Nature of Variations in the Measured Star Formation Efficiency of Molecular Clouds

TL;DR

This study uses magnetohydrodynamic simulations to investigate the variations in star formation efficiency of molecular clouds, revealing observational artifacts, the importance of feedback, and the stages of star formation.

Contribution

It demonstrates how feedback mechanisms influence observed SFE and clarifies the relationship between true and measured efficiencies in GMCs.

Findings

Simulations with feedback match observed SFEs

Without feedback, SFEs are 20 times larger

Feedback, especially radiative, is crucial in regulating SFE

Abstract

Measurements of the star formation efficiency (SFE) of giant molecular clouds (GMCs) in the Milky Way generally show a large scatter, which could be intrinsic or observational. We use magnetohydrodynamic simulations of GMCs (including feedback) to forward-model the relationship between the true GMC SFE and observational proxies. We show that individual GMCs trace broad ranges of observed SFE throughout collapse, star formation, and disruption. Low measured SFEs (<<1%) are "real" but correspond to early stages, the true "per-freefall" SFE where most stars actually form can be much larger. Very high (>>10%) values are often artificially enhanced by rapid gas dispersal. Simulations including stellar feedback reproduce observed GMC-scale SFEs, but simulations without feedback produce 20x larger SFEs. Radiative feedback dominates among mechanisms simulated. An anticorrelation of SFE with cloud mass is shown to be an observational artifact. We also explore individual dense "clumps" within GMCs and show that (with feedback) their bulk properties agree well with observations. Predicted SFEs within the dense clumps are ~2x larger than observed, possibly indicating physics other than feedback from massive (main sequence) stars is needed to regulate their collapse.