Ralph E. Pudritz
Tom P. Ray- 1Department of Physics and Astronomy, McMaster University, Hamilton, ON, Canada
- 2School of Cosmic Physics, Dublin Institute for Advanced Studies, Dublin, Ireland
- star formation pic added by Chuck
TABLE OF CONTENTS
Abstract
1. Introduction
2. Observational Overview
3. Theoretical Overview
4. Conclusions
Author Contributions
Funding
Conflict of Interest Statement
Acknowledgments
Footnotes
References
EDITED BY, Christopher F. McKee, University of California, Berkeley, United States
REVIEWED BY, Zhi-Yun Li, University of Virginia, United States.
John Bally, University of Colorado Boulder, United States
The editor and reviewers' affiliations are the latest provided on their Loop research profiles and may not reflect their situation at the time of review.
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.
1. Introduction
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. It is now nearly 70 years since the first evidence for energetic outflows in star formation regions was discovered (Herbig, 1951; Haro, 1952) although not realized as such. In fact it would be many years later before these nebulous clouds (now known as Herbig-Haro objects) were correctly identified as radiative shocks driven by an outflow from a young star (Schwartz, 1977). The availability of new technologies and telescopes over the subsequent decades has yielded discoveries of bipolar molecular outflows at millimeter wavelengths, high speed (hundreds of km s−1) optical jets in permitted and forbidden line emission, and more recently even synchrotron emitting jets associated with low mass star formation. This extensive body of work shows that bulk flows ranging from several to hundreds of km s−1 are observed in these different tracers. Although first discovered to be associated with the birth of low mass T-Tauri stars, observations over the last two decades have shown outflows to be linked with the formation of objects across the entire mass spectrum: from brown dwarfs (Whelan et al., 2005) on up to O stars (Caratti o Garatti et al., 2017). These observations have made it clear that outflows are an essential component of star formation.
The first clues to the origin of protostellar jets emerged from studies of outflows from low mass stars, whose radiation pressures fail by orders of magnitude to drive them at the observed thrusts (see for example Wu et al., 2004; Vaidya et al., 2011 on the decollimating effects of radiation in the case of massive young stars). It was therefore natural to consider the possibility that T Tauri jets and outflows could be magnetized winds from rotating bodies. Two kinds of magnetized rotors are possible—rapidly spinning, magnetized protostars or magnetized accretions disks out of which all stars and their planetary systems form. While magnetic field measurements needed to confirm such a picture have been long in coming, direct observational evidence is now available using several approaches: the spectro-polarimetry of jet sources (Donati et al., 2010), synchrotron emission in outflows from some young stars and most recently the detection of polarization of the SiO line in a jet from a low mass star arising from the Goldreich-Kylafis effect (Lee et al., 2018). The observed fields range in strength from kilogauss close to the star to a fraction of a milligauss in distant parts for the outflow. Such values suggest that while magnetic forces dominate in the vicinity of the young stellar object (YSO), they are no longer dynamically important further out. This scenario is in agreement with numerical simulations (Hartigan et al., 2007).
The basic physics of how magnetized rotating stars drive winds and undergo magnetic braking as a consequence was developed by Chandrasekhar (1956) and Mestel (1961) before being applied by Weber and Davis (1967) who showed that the magnetized solar wind would spin down the Sun.
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