Massive star feedback in clusters: variation of the FUV interstellar
  radiation field in time and space
  We investigate radiative feedback from a 34 M$_\odot$ star in a $10^4$
M$_\odot$ turbulent cloud using three-dimensional radiation-hydrodynamics (RHD)
models. We use Monte Carlo radiative transfer to accurately compute
photoionization equilibrium and radiation pressure, with multiple atomic
species and silicate dust grains. We include the diffuse radiation field, dust
absorption/re-emission, and scattering. The cloud is efficiently dispersed,
with 75 per cent of the mass leaving the (32.3 pc)$^3$ grid within 4.3 Myr (1.1
$t_{ff}$). This compares to all mass exiting within 1.6 Myr (0.74 $t_{ff}$) in
our previously published $10^3$ M$_\odot$ cloud. At most 20 per cent of the
mass is ionized, compared to 40 per cent in the lower mass model, despite the
ionized volume fraction being 80 per cent in both, implying the higher mass
cloud is more resilient to feedback. The total Jeans-unstable mass increases
linearly up to 1500 M$_\odot$ before plateauing after 2 Myr, corresponding to a
core formation efficiency of 15 per cent. We also measure the time-variation of
the far-ultraviolet (FUV) radiation field, $G_0$, impinging on other cluster
members, taking into account for the first time how this changes in a dynamic
cluster environment with intervening opacity sources and stellar motions. Many
objects remain shielded in the first 0.5 Myr whilst the massive star is
embedded, after which $G_0$ increases by orders of magnitude. Gas motions later
on cause comparable drops which happen instantaneously and last for $\sim$ 1
Myr before being restored. This highly variable UV field will influence the
photoevaporation of protoplanetary discs near massive stars.
