The return of the proplyds - understanding the dynamics of ionization triggered stars
ABSTRACT Proplyds and stars inside HII-regions are a well studied phenomenon. It is possiblethat they were triggered by the expansion of the HII-region itself. Here, we presentcalculationsonthe dynamicsofHII-regions.We showthat the triggeredstarsthat formin the expanding shell of swept up material around the HII region rarelyreturn into theHII 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 proplydsare barelyobservedin other HII regions.Wepropose that the properties of young stellar objects in HII regions in general dependcritically 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 feedbackof the massive star. Further out are Class II protostars, where the ionization alreadyremoved the envelope. Even further away, one should ﬁnd Class I stars, which eitherhave been triggered by the ionizing radiation or pre-existed and have not lost theirenvelope yet. This radial sequence is not necessarily an age sequence but rather a resultof the dwindling importance of stellar winds and ionizing radiation with distance. Weinvestigate the observational signature of triggered star formation and ﬁnd that thestellar distribution for ionization triggered star formation shows a distinct feature, apeak at the current position of the ionization front. Therefore, it is generally possibleto tell triggered and in situ distributions of stars apart.Key words: (ISM:) H II regions, ISM: globules, ISM: kinematics and dynamics,stars: formation, stars: kinematics, methods: analytical