  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.
