Ionization--induced star formation V: Triggering in partially unbound
  clusters
  We present the fourth in a series of papers detailing our SPH study of the
effects of ionizing feedback from O--type stars on turbulent star forming
clouds. Here, we study the effects of photoionization on a series of initially
partially unbound clouds with masses ranging from $10^{4}$--$10^{6}$M$_{\odot}$
and initial sizes from 2.5-45pc. We find that ionizing feedback profoundly
affects the structure of the gas in most of our model clouds, creating large
and often well-cleared bubble structures and pillars. However, changes in the
structures of the embedded clusters produced are much weaker and not well
correlated to the evolution of the gas. We find that in all cases, star
formation efficiencies and rates are reduced by feedback and numbers of objects
increased, relative to control simulations. We find that local triggered star
formation does occur and that there is a good correlation between triggered
objects and pillars or bubble walls, but that triggered objects are often
spatially-mixed with those formed spontaneously. Some triggered objects acquire
large enough masses to become ionizing sources themselves, lending support to
the concept of propagating star formation. We find scant evidence for spatial
age gradients in most simulations, and where we do see them, they are not a
good indicator of triggering, as they apply equally to spontaneously-formed
objects as triggered ones. Overall, we conclude that inferring the global or
local effects of feedback on stellar populations from observing a system at a
single epoch is very problematic.
