Are molecular outflows around high-mass stars driven by ionization feedback?

TL;DR

This study compares ionization-driven models of molecular outflows around high-mass stars with observations, concluding that such models only produce weak outflows and that outflows are more likely driven by collective effects of lower-mass stars.

Contribution

The paper evaluates the effectiveness of ionization-driven outflow models against observational data, suggesting a different dominant mechanism for observed outflows.

Findings

Ionization-driven models produce only weak outflows.

Expanding H II regions are unlikely the main driver of observed outflows.

Outflows are probably driven by collective action of lower-mass stars.

Abstract

The formation of massive stars exceeding 10 solar masses usually results in large-scale molecular outflows. Numerical simulations, including ionization, of the formation of such stars show evidence for ionization-driven molecular outflows. We here examine whether the outflows seen in these models reproduce the observations. We compute synthetic ALMA and CARMA maps of CO emission lines of the outflows, and compare their signatures to existing single-dish and interferometric data. We find that the ionization-driven models can only reproduce weak outflows around high-mass star-forming regions. We argue that expanding H II regions probably do not represent the dominant mechanism for driving observed outflows. We suggest instead that observed outflows are driven by the collective action of the outflows from the many lower-mass stars that inevitably form around young massive stars in a cluster.