The theory of globulettes: Candidate precursors of brown dwarfs and free-floating planets in H II regions

Large numbers of small opaque dust clouds - termed 'globulettes' by Gahm et al. - have been observed in the HII regions surrounding young stellar clusters. With masses typically in the planetary (or low-mass brown dwarf) regime, these objects are so numerous in some regions (e.g. the Rosette) that, if only a small fraction of them could ultimately collapse, then they would be a very significant source of free-floating planets. Here, we review the properties of globulettes and present a theoretical framework for their structure and evolution. We demonstrate that their interior structure is well described by a pressure-confined isothermal Bonnor-Ebert sphere and that the observed mass-radius relation (M ∝ R) is a systematic consequence of a column density threshold below which components of the globulette are not identified. We also find that globulettes with this interior structure are very stable against collapse within HII regions. We follow Gahm et al. in assuming that globulettes are detached from the tips of pillars protruding in from the swept-up shell that borders the expanding HII region and produce a model for their dynamics, finding that globulettes will eventually impact the shell. We derive an expression for the time it takes to do so and show that dissipation of energy via dust cooling allows all globulettes to survive this encounter and escape into the wider interstellar medium. Once there, the ambient pressure drops and they disperse on time-scales around 30-300 kyr and should be observable using ALMA out to distances of the order of a parsec.