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Paper: Triggered Collapse, Magnetic Fields, and Very Low Mass Star Formation (Invited Review)
Volume: 287, Galactic Star Formation Across the Stellar Mass Spectrum
Page: 281
Authors: Boss, A. P.
Abstract: Protostellar collapse calculations traditionally ignore the effects of nearby stars and of magnetic fields. Including these effects can help answer several questions, such as the origin of the Solar System and of free-floating planetary mass objects. Shock waves derived from massive stars can trigger the collapse of an otherwise stable dense cloud core, leading to the formation of single or binary low mass stars. Suitable sources of shock waves include distant supernovae, AGB winds, and nearby protostellar outflows. Short-lived radioisotopes produced by stellar nucleosynthesis or spallation can be injected into the collapsing cloud through Rayleigh-Taylor fingers, a process that may explain evidence for certain live radioisotopes in the early solar nebula. Magnetic fields are known to be dynamically important for dense cloud cores, yet have seldom been included in protostellar collapse and fragmentation calculations. The inclusion of magnetic field effects in a crude manner in these calculations suggests that, contrary to intuition, magnetic fields do not appear to inhibit fragmentation of a collapsing cloud into a binary or multiple protostar system. Rather, the ability of magnetic field tension effects to prevent the formation of a central density singularity seems to encourage fragmentation. Decompressional cooling during a magnetically-assisted rebound lowers the Jeans mass toward planetary values, suggesting that sub-brown dwarf stars could form from the collapse of solar mass dense clouds, provided that these very low mass protostars are dynamically ejected prior to accreting significantly more mass.
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