  Proplyds and stars inside HII-regions are a well studied phenomenon. It is
possible that they were triggered by the expansion of the HII-region itself.
Here, we present calculations on the dynamics of HII-regions. We show that the
triggered stars that form in the expanding shell of swept up material around
the HII region rarely return into the HII regions on timescales that are
inferred for the proplyds and observed young stars. However, in very dense
environments like Orion, the triggered stars return in time. Thus, our model
can explain why proplyds are barely observed in other HII regions. We propose
that the properties of young stellar objects in HII regions in general depend
critically on the distance from the massive, ionizing central star cluster.
Closest in, there are proplyds, where the disk of a young star interacts
directly with the feedback of the massive star. Further out are Class II
protostars, where the ionization already removed the envelope. Even further
away, one should find Class I stars, which either have been triggered by the
ionizing radiation or pre-existed and have not lost their envelope yet. This
radial sequence is not necessarily an age sequence but rather a result of the
dwindling importance of stellar winds and ionizing radiation with distance. We
investigate the observational signature of triggered star formation and find
that the stellar distribution for ionization triggered star formation shows a
distinct feature, a peak at the current position of the ionization front.
Therefore, it is generally possible to tell triggered and in situ distributions
of stars apart.
