The Formation of Free-Floating Brown Dwarves and Planetary-Mass Objects by Photo-Erosion of Prestellar Cores

We explore the possibility that, in the vicinity of an OB star, a prestellar core which would otherwise have formed an intermediate or low-mass star may form a free-floating brown dwarf or planetary-mass object, because the outer layers of the core are eroded by the ionizing radiation from the OB star before they can accrete onto the protostar at the centre of the core. The masses of objects formed in this way are given approximately by $\sim 0.010 M_\odot (a_{\rm I} / 0.3 {\rm km} {\rm s}^{-1})^6 (\dot{\cal N}_{\rm Lyc} / 10^{50} {\rm s}^{-1})^{-1/3} (n_{\rm 0} / 10^3 {\rm cm}^{-3})^{-1/3} $, where $a_{\rm I}$ is the isothermal sound speed in the neutral gas of the core, $\dot{\cal N}_{\rm Lyc}$ is the rate of emission of Lyman continuum photons from the OB star (or stars), and $n_{\rm 0}$ is the number-density of protons in the HII region surrounding the core. We conclude that the formation of low-mass objects by this mechanism should be quite routine, because the mechanism operates over a wide range of conditions ($10^{50} {\rm s}^{-1} \la \dot{\cal N}_{\rm Lyc} \la 10^{52} {\rm s}^{-1}$, $10 {\rm cm}^{-3} \la n_0 \la 10^5 {\rm cm}^{-3}$, $0.2 {\rm km} {\rm s}^{-1} \la a_{\rm I} \la 0.6 {\rm km} {\rm s}^{-1}$) and is very effective. However, it is also a rather wasteful way of forming low-mass objects, in the sense that it requires a relatively massive initial core to form a single low-mass object. The effectiveness of photo-erosion also implies that that any intermediate-mass protostars which have formed in the vicinity of a group of OB stars must already have been well on the way to formation before the OB stars switched on their ionizing radiation; otherwise these protostars would have been stripped down to extremely low mass.