How do bound star clusters form?

TL;DR

This paper investigates various formation scenarios for bound star clusters in the Milky Way, using Bayesian analysis to compare models against observational data, and finds that a 'conveyor belt' formation mode with low star formation efficiency best explains the observations.

Contribution

It introduces a Bayesian framework to test different cluster formation models against multiple observational constraints, favoring the 'conveyor belt' mode with low star formation efficiency.

Findings

Models with large-scale collapse or increasing efficiency fail to match observations.

Conveyor belt formation with low star formation rate per free-fall time fits data.

Bound cluster formation likely involves simultaneous gas accretion and star formation.

Abstract

Gravitationally-bound clusters that survive gas removal represent an unusual mode of star formation in the Milky Way and similar spiral galaxies. While forming, they can be distinguished observationally from unbound star formation by their high densities, virialised velocity structures, and star formation histories that accelerate toward the present, but extend multiple free-fall times into the past. In this paper we examine several proposed scenarios for how such structures might form and evolve, and carry out a Bayesian analysis to test these models against observed distributions of protostellar age, counts of young stellar objects relative to gas, and the overall star formation rate of the Milky Way. We show that models in which the acceleration of star formation is due either to a large-scale collapse or a time-dependent increase in star formation efficiency are unable to satisfy the combined set of observational constraints. In contrast, models in which clusters form in a "conveyor belt" mode where gas accretion and star formation occur simultaneously, but the star formation rate per free-fall time is low, can match the observations.