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Abstract
The role of outflows in the formation of stars and the protostellar disks that generate them is a central question in astrophysics. Outflows are associated with star formation across the entire stellar mass spectrum. In this review, we describe the observational, theoretical, and computational advances on magnetized outflows, and their role in the formation of disks and stars of all masses in turbulent, magnetized clouds. The ability of torques exerted on disks by magnetized winds to efficiently extract and transport disk angular momentum was developed in early theoretical models and confirmed by a variety of numerical simulations. The recent high resolution ALMA observations of disks and outflows now confirm several key aspects of these ideas, e.g. that jets rotate and originate from large regions of their underlying disks. New insights on accretion disk physics show that magneto-rotational instability (MRI) turbulence is strongly damped, leaving magnetized disk winds as the dominant mechanism for transporting disk angular momentum. This has major consequences for star formation, as well as planet formation. Outflows also play an important role in feedback processes particularly in the birth of low mass stars and cluster formation. Despite being almost certainly fundamental to their production and focusing, magnetic fields in outflows in protostellar systems, and even in the disks, are notoriously difficult to measure. Most methods are indirect and lack precision, as for example, when using optical/near-infrared line ratios. Moreover, in those rare cases where direct measurements are possible - where synchrotron radiation is observed, one has to be very careful in interpreting derived values. Here we also explore what is known about magnetic fields from observations, and take a forward look to the time when facilities such as SPIRou and the SKA are in routine operation.
Highlights
Of the many important roles that magnetic fields play in the formation of stars, perhaps none is more dramatic nor as full of consequence as is the launch and collimation of powerful outflows
We focus on recent rapid progress in the Atacama Large Millimeter Array (ALMA) era that has been made in the physics and observations of outflows and their role in star formation
How might we do this? One powerful method is hinted at by the fact that a small number of jets/outflows have non-thermal radio spectra as demonstrated in the case of more massive young stars such as Herbig-Haro 80/81 (Carrasco-González et al, 2010) and in their lower mass counterparts (Ainsworth et al, 2014). Such radiation appears to come from high-energy electrons and, as discussed below, it may be common at very weak flux levels in “standard” low luminosity outflows but detectable with the new suite of radio interferometers such as e-Multi-Element Radio Linked Interferometer (MERLIN), the Jansky Very Large Array (VLA) (JVLA) and at low frequencies
Summary
Of the many important roles that magnetic fields play in the formation of stars, perhaps none is more dramatic nor as full of consequence as is the launch and collimation of powerful outflows. Magnetized outflows carry significant amounts of angular momentum and thrust, and are powered by the gravitational potential energy released during the collapse In this way, outflows can act as an important, and even dominant form of protostellar feedback during star formation. On physical scales beyond the molecular core radius (typically ≈ 0.04 pc), protostellar outflows could stir up the surrounding molecular cloud and drive turbulence This would, to some degree, stave off the formation of too much dense, star forming gas as first suggested in the pioneering paper of Norman and Silk (1980), and is in agreement with many current MHD simulations (Federrath, 2016). The reader may consult reviews of earlier material in Ray et al (2007), Pudritz et al (2007), Frank et al (2014), Li et al (2014b), and Bally (2016)
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