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

Abstract

Large numbers of small opaque dust clouds -termed 'globulettes' by Gahm et al -have been observed in the H ii 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 2.2 ) 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 H ii 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 H ii 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 ISM. Once there the ambient pressure drops and they disperse on timescales around 30-300 kyr and should be observable using ALMA out to distances of order a parsec. Since we find that globulettes are stable, the only route via which they might still form brown dwarfs or planets is during their collision with the shell or some other violent perturbative event.

Survival Rates of Planets in Open Clusters: the Pleiades, Hyades, and Praesepe clusters

“…The results obtained by microlensing surveys toward the Galactic Bulge suggest that the frequency of freefloating Jovian planets are 0.25 planets per main-sequence star (Mróz et al 2017). Several dynamical processes have been proposed in order to explain the origin of free-floating planets (FFPs) in our Galaxy: planet-planet scattering (Veras & Raymond 2012), direct formation in grobulettes (Gahm et al 2007;Haworth et al 2015), and the aftermath of evolved stars such as supernova explosions (Veras & Tout 2012).…”

A Candidate Planetary-Mass Object With a Photoevaporating Disk in Orion

“…Haworth et al (2015) doubt the possibility that the external pressure in the form of radiation or ram pressure can drive globulettes to form brown dwarfs or free-floating planets, and instead they propose the collapse of the globulettes might be triggered by their collisions with the shells that border the expanding H II regions. If Proplyd133-353 was formed in such a globulette near θ 1 Ori C due to either mechanism, we might explain both its younger age (with respect to the one of the ONC) and the lifetime of its disk since its parental cloud can protect it from direct photoevaporation for a timescale of 10 5 -10 6 years (Gahm et al 2007;Haworth et al 2015).…”

Illuminating the Tadpole's metamorphosis I: MUSE observations of a small globule in a sea of ionizing photons

“…While the model assumes equilibrium, it is unstable to collapse if sufficiently compressed. Haworth et al (2015) used pressure-bounded BE spheres to model the stability and evolution of small globulettes in Carina cataloged by Grenman & Gahm (2014), including the Tadpole. We revisit the parameters of the Tadpole assuming it is well described by a BE sphere but using the physical parameters we measure with the MUSE data.…”

“…If even a small fraction of these globulettes undergo gravitational collapse, they may contribute significantly to the low-mass end of the IMF. However, Haworth et al (2015) showed that these low-mass globulettes are stable and require an additional perturbation to stimulate collapse. Estimated photoevaporation rates (e.g., Smith et al 2004;Reiter et al 2015a) suggest that such small, low-mass objects will be rapidly ablated.…”