Abstract
Recent observations have suggested that circumstellar disks may commonly form around young stellar objects. Although the formation of circumstellar disks can be a natural result of the conservation of angular momentum in the parent cloud, theoretical studies instead show disk formation to be difficult from dense molecular cores magnetized to a realistic level, owing to efficient magnetic braking that transports a large fraction of the angular momentum away from the circumstellar region. We review recent progress in the formation and early evolution of disks around young stellar objects of both low-mass and high-mass, with an emphasis on mechanisms that may bridge the gap between observation and theory, including non-ideal MHD effects and asymmetric perturbations in the collapsing core (e.g., magnetic field misalignment and turbulence). We also address the associated processes of outflow launching and the formation of multiple systems, and discuss possible implications in properties of protoplanetary disks.
Highlights
In the process of star formation, circumstellar disks are common structures that form as a result of the conservation of angular momentum of the parent molecular cloud
Krasnopolsky and Königl (2002) found that magnetic braking is enhanced behind the AD-shock, which further suppresses disk formation. This result is later confirmed by the 2D simulation of Mellon and Li (2009), who adopted a simple parametrized ambipolar diffusivity with relatively large ion density coefficient (Shu 1991) and suggested that disk formation only becomes possible in dense cores with weak magnetic field and/or low cosmic ray ionization rate
With respect to the questions of angular momentum transport from clouds to disks, disk formation and disk growth, a variety of MHD simulations have been performed (Banerjee and Pudritz 2007; Hennebelle et al 2011; Peters et al 2011; Seifried et al 2011; Commerçon et al 2011; Myers et al 2013; Matsushita et al 2017; Kölligan and Kuiper 2018), the majority of those focus on the impact of magnetic fields on cloud collapse and cloud fragmentation. The conclusion of those investigations is in line with the results reported for the low-mass star formation case, namely that ideal MHD simulations yield the formation of magnetic-pressure-dominated pseudo-disks (Hennebelle et al 2011; Peters et al 2011; Commerçon et al 2011)
Summary
In the process of star formation, circumstellar disks are common structures that form as a result of the conservation of angular momentum of the parent molecular cloud. 3. Magnetic braking (Mestel and Spitzer 1956), being a plausible solution for the angular momentum problem, can instead transport away too much angular momentum so that disk formation is completely suppressed, given a relatively strong coupling between the magnetic field and the neutral matter. Magnetic braking (Mestel and Spitzer 1956), being a plausible solution for the angular momentum problem, can instead transport away too much angular momentum so that disk formation is completely suppressed, given a relatively strong coupling between the magnetic field and the neutral matter
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