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 tha
t 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.
In the widely adopted LambdaCDM scenario for galaxy formation, dwarf galaxies are the building blocks of larger galaxies. Since they formed at relatively early epochs when the background density was relatively high, they are expected to retain their
integrity as satellite galaxies when they merge to form larger entities. Although many dwarf spheroidal galaxies (dSphs) are found in the galactic halo around the Milky Way, their phase space density (or velocity dispersion) appears to be significantly smaller than that expected for satellite dwarf galaxies in the LambdaCDM scenario. In order to account for this discrepancy, we consider the possibility that they may have lost a significant fraction of their baryonic matter content during the first infall at the Hubble expansion turnaround. Such mass loss arises naturally due to the feedback by relatively massive stars which formed in their centers briefly before the maximum contraction. Through a series of N-body simulations, we show that the timely loss of a significant fraction of the dSphs initial baryonic matter content can have profound effects on their asymptotic half-mass radius, velocity dispersion, phase-space density, and the mass fraction between residual baryonic and dark matter.
We propose that the Pipe Nebula is an HII region shell swept up by the B2 IV beta Cephei star theta Ophiuchi. After reviewing the morphological evidence by recent observations, we perform a series of analytical calculations. We use realistic HII regi
on parameters derived with the radiative transfer code Cloudy from observed stellar parameters. We are able to show that the current size, mass and pressure of the region can be explained in this scenario. We investigate the configuration today and come to the conclusion that the Pipe Nebula can be best described by a three phase medium in pressure equilibrium. The pressure support is provided by the ionized gas and mediated by an atomic component to confine the cores at the observed current pressure. In the future, star formation in these cores is likely to be either triggered by feedback of the most massive, gravitationally bound cores as soon as they collapse or by the supernova explosion of theta Ophiuchi itself.
