  We explore a simple semi-analytic model for what happens when an O star (or
cluster of O stars) forms in an isolated filamentary cloud. The model is
characterised by three configuration parameters: the radius of the filament,
R_FIL, the mean density of H_2 in the filament, n_FIL, and the rate at which
the O star emits ionising photons, Ndot_LyC. We show that for a wide range of
these configuration parameters, ionising radiation from the O star rapidly
erodes the filament, and the ionised gas from the filament disperses into the
surroundings. Under these circumstances the distance from the O star to the
ionisation front (IF) is given approximately by L ~ 5.2 pc [R_FIL/0.2pc]^-1/6
[n_FIL/10^4cm^-3]^-1/3 [Ndot_LyC/10^49s^-1]^1/6 [t/Myr]^2/3, and we derive
similar simple power-law expressions for other quantities, for example the rate
at which ionised gas boils off the filament, and the mass of the
shock-compressed layer (SCL) that is swept up behind the IF. We show that a
very small fraction of the ionising radiation is expended locally, and a rather
small amount of molecular gas is ionised and dispersed. We discuss some
features of more realistic models, and the extent to which they might modify or
invalidate the predictions of this idealised model. In particular we show that,
for very large R_FIL and/or large n_FIL and/or low Ndot_LyC, continuing
accretion onto the filament might trap the ionising radiation from the O star,
slowing the erosion of the filament even further.
