Ionization--induced star formation V: Triggering in partially unbound clusters

TL;DR

This study uses SPH simulations to analyze how ionizing feedback from massive stars influences the structure and star formation processes in partially unbound molecular clouds, revealing complex interactions and challenges in identifying triggered star formation.

Contribution

It provides detailed simulation results on ionization effects in unbound clouds, highlighting the nuanced relationship between feedback and star formation, including triggered and spontaneous processes.

Findings

Ionizing feedback creates large bubble structures and pillars.

Star formation efficiencies are reduced by feedback.

Triggered star formation correlates with pillars and bubble walls.

Abstract

We present the fourth in a series of papers detailing our SPH study of the effects of ionizing feedback from O--type stars on turbulent star forming clouds. Here, we study the effects of photoionization on a series of initially partially unbound clouds with masses ranging from $10^{4}$--$10^{6}$M$_{\odot}$ and initial sizes from 2.5-45pc. We find that ionizing feedback profoundly affects the structure of the gas in most of our model clouds, creating large and often well-cleared bubble structures and pillars. However, changes in the structures of the embedded clusters produced are much weaker and not well correlated to the evolution of the gas. We find that in all cases, star formation efficiencies and rates are reduced by feedback and numbers of objects increased, relative to control simulations. We find that local triggered star formation does occur and that there is a good correlation between triggered objects and pillars or bubble walls, but that triggered objects are often spatially-mixed with those formed spontaneously. Some triggered objects acquire large enough masses to become ionizing sources themselves, lending support to the concept of propagating star formation. We find scant evidence for spatial age gradients in most simulations, and where we do see them, they are not a good indicator of triggering, as they apply equally to spontaneously-formed objects as triggered ones. Overall, we conclude that inferring the global or local effects of feedback on stellar populations from observing a system at a single epoch is very problematic.