Understanding our Origins : Star Formation in H ii Region Environments

Recent analysis of the decay products of short-lived radiounclides (SLRs) in meteorites, in particular the confirmation of the presence of live Fe in the early Solar System, provides unambiguous evidence that the Sun and Solar System formed near a massive star. We consider the question of the formation of low-mass stars in the environments of massive stars, presenting a scenario for the evolution of a star and its disk as it is overrun by the ionization front at the edge of an expanding H ii region. The stages in this scenario include: (1) compression of molecular gas around the edge of the H ii region; (2) induced low-mass star formation in this compressed gas; (3) an “EGG” phase when a dense star-forming clump is overrun by the ionization front; (4) a “proplyd” phase during which the disk is truncated by photoevaporation; (5) a long-lasting phase during which a young star and its truncated disk evolve in the hot, tenuous interior of an H ii region; and (6) a phase when the ejecta from one or more nearby supernova explosions overruns the disk, injecting SLRs including Al and Fe. Most of these stages can be observed directly. The exceptions are stage (2), which must be inferred from the localization of low-mass protostars in compressed molecular gas near ionization fronts, and stage (6), which is an unavoidable consequence of the presence of low-mass protostars seen near massive stars that will go supernova within a few million years. (This differs from models in which the same supernova is responsible for both triggering the formation of a star and injecting SLRs.) This mode of star formation may be more characteristic of how most low-mass stars form than is the mode of star formation seen in regions such as the Taurus-Auriga molecular cloud. We discuss the implications of this scenario for our understanding of star formation, including the possible role of photoionization in limiting the masses of stars. We also discuss the implications of the young Sun’s astrophysical environment for our understanding of the formation and evolution of the Solar System. These include the effects of intense UV radiation from nearby massive stars on the structure and chemistry of the disk, dynamical effects due to close encounters of the Solar System with other cluster members, and the role of the decay of SLRs in the evolution of the Solar System. We conclude that low-mass stars and their accompanying disks form and evolve differently near massive stars than they do in regions like Taurus-Auriga, and that these differences have profound implications for our understanding of our origins.