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 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.