On the time variability of the star formation efficiency

TL;DR

This paper investigates whether the star formation efficiency per free fall time varies over the lifetime of giant molecular clouds and how this impacts models of turbulence and star formation rates.

Contribution

It challenges the assumption that H2 masses remain constant over GMC lifetimes and introduces a toy model demonstrating constant efficiency can explain observational data.

Findings

H2 mass evolution assumption is critical in interpreting data.

A simple model with constant efficiency fits observations.

Time variability of efficiency has significant implications for star formation theories.

Abstract

A star formation efficiency per free fall time that evolves over the life time of giant molecular clouds (GMCs) may have important implications for models of supersonic turbulence in molecular clouds or for the relation between star formation rate and H2 surface density. We discuss observational data that could be interpreted as evidence of such a time variability. In particular, we investigate a recent claim based on measurements of H2 and stellar masses in individual GMCs. We show that this claim depends crucially on the assumption that H2 masses do not evolve over the life times of GMCs. We exemplify our findings with a simple toy model that uses a constant star formation efficiency and, yet, is able to explain the observational data.