Do all O stars form in star clusters?

TL;DR

This paper reviews the debate on whether all O stars form exclusively in star clusters, considering observational challenges and physical processes affecting their formation and detection.

Contribution

It provides a comprehensive overview of the current arguments for and against the idea that all O stars form in clusters, highlighting key challenges.

Findings

Summarizes observational limitations like dust obscuration and resolution.

Discusses physical processes such as stellar dynamics and gas expulsion.

Evaluates evidence supporting both formation scenarios.

Abstract

The question whether or not massive stars can form in isolation or only in star clusters is of great importance for the theory of (massive) star-formation as well as for the stellar initial mass function of whole galaxies (IGIMF-theory). While a seemingly easy question it is rather difficult to answer. Several physical processes (e.g. star-loss due to stellar dynamics or gas expulsion) and observational limitations (e.g. dust obscuration of young clusters, resolution) pose severe challenges to answer this question. In this contribution we will present the current arguments in favour and against the idea that all O stars form in clusters.