The Role of Magnetic Fields in Protostellar Outflows and Star Formation

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 Atacama Large Millimeter Array (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.

The JCMT BISTRO Survey: Magnetic Fields Associated with a Network of Filaments in NGC 1333

“…They found that the correlation is best explained by random orientations of outflows and B-fields. On the other hand,Galametz et al (2018) observed a sample of 12 low-mass Class 0 envelopes in nearby clouds using the SMA and pointed out that the envelope-scale B-field is preferentially either aligned with or perpendicular to the outflow direction (e.g.,Bally 2016;Lee et al 2017;Pudritz & Ray 2019). Bipolar outflows are launched by the rotating accretion disk of the protostar and thus could be used to infer the orientation of the rotation axis (e.g., Bally 2016;Lee et al 2017;Baug et al 2020;Pudritz & Ray 2019)…”

Analytic Model for the Time-dependent Electromagnetic Field of an Astrophysical Jet

“…Astrophysical jets were first observed over a century ago by Curtis (1918), but their magnetic character was only realized more recently; a review of recent observational, theoretical, and computational advances is given in Pudritz & Ray (2019). The essential observed features of astrophysical jets are axisymmetry, association with an accretion disk (presumed to be the energy source), bidirectionality, collimation, and a finite length that it is increasing with time.…”

Super-rotating protostellar jets

“…The ratio of the Alfvén radius to the jet launching radius λ is as large as λ ≡ rA/r0 = 25.8 − 33.9. Thus, the long magnetic lever arm efficiently transports the angular momentum from the circumstellar region for the super-rotating jet case 29 .…”

“…As a result, the 7 fluid elements receive the angular momentum and are expelled from 8 the circumstellar region by magnetic effect 30 . On the other hand, a par-9 cel of gas in the circumstellar disk near the protostar loses the angular 10 momentum and falls onto the protostar 29 . Therefore, the excess angular momentum is ejected from the circumstellar disk by the rotating jet, and the gas whose angular momentum has been removed by the jet falls 13 onto the protostar and promotes the protostellar growth.…”

“…(a) The mean velocity map. R1-R9 indicate slices of the position-velocity diagram at13,19,25,29, 33, 38, 42, 46, and 49 arcsec from the center of the 1.3 mm continuum emission. (b)-(e) Position-velocity diagrams for (b) R1, (c) R3, (d) R7, and (e) R9 are shown, respectively.…”