TY - JOUR
T1 - Review of Zeeman Effect Observations of Regions of Star Formation
AU - Crutcher, Richard M.
AU - Kemball, Athol J.
N1 - Funding Information:
RC receives support from NSF AST 18-15987.
Publisher Copyright:
© Copyright © 2019 Crutcher and Kemball.
Copyright:
Copyright 2020 Elsevier B.V., All rights reserved.
PY - 2019/10/17
Y1 - 2019/10/17
N2 - The Zeeman effect is the only observational technique available to measure directly the strength of magnetic fields in regions of star formation. This chapter reviews the physics of the Zeeman effect and its practical use in both extended gas and in masers. We discuss observational results for the five species for which the Zeeman effect has been detected in the interstellar medium—H I, OH, and CN in extended gas and OH, CH
3OH, and H
2O in masers. These species cover a wide range in density, from ~10 cm
−3 to ~10
10 cm
−3, which allows magnetic fields to be measured over the full range of cloud densities. However, there are significant limitations, including that only the line-of-sight component of the magnetic field strength can usually be measured and that there are often significant uncertainties about the physical conditions being sampled, particularly for masers. We discuss statistical methods to partially overcome these limitations. The results of Zeeman observations are that the mass to magnetic flux ratio, which measures the relative importance of gravity to magnetic support, is subcritical (gravity dominates magnetic support) at lower densities but supercritical for (Formula presented.) cm
−2. Above n
H ~ 300 cm
−3, which is roughly the density at which clouds typically become self-gravitating, the strength of magnetic fields increases approximately as B ∝ n
2/3, which suggest that magnetic fields do not provide significant support at high densities. This is consistent with high-density clouds being supercritical. However, magnetic fields have a large range in strengths at any given density, so the role of magnetic fields should differ significantly from one cloud to another. And for maser regions the dependence of field strength on density may have a slightly lower slope. Turbulent reconnection theory seems to best match the Zeeman observational results.
AB - The Zeeman effect is the only observational technique available to measure directly the strength of magnetic fields in regions of star formation. This chapter reviews the physics of the Zeeman effect and its practical use in both extended gas and in masers. We discuss observational results for the five species for which the Zeeman effect has been detected in the interstellar medium—H I, OH, and CN in extended gas and OH, CH
3OH, and H
2O in masers. These species cover a wide range in density, from ~10 cm
−3 to ~10
10 cm
−3, which allows magnetic fields to be measured over the full range of cloud densities. However, there are significant limitations, including that only the line-of-sight component of the magnetic field strength can usually be measured and that there are often significant uncertainties about the physical conditions being sampled, particularly for masers. We discuss statistical methods to partially overcome these limitations. The results of Zeeman observations are that the mass to magnetic flux ratio, which measures the relative importance of gravity to magnetic support, is subcritical (gravity dominates magnetic support) at lower densities but supercritical for (Formula presented.) cm
−2. Above n
H ~ 300 cm
−3, which is roughly the density at which clouds typically become self-gravitating, the strength of magnetic fields increases approximately as B ∝ n
2/3, which suggest that magnetic fields do not provide significant support at high densities. This is consistent with high-density clouds being supercritical. However, magnetic fields have a large range in strengths at any given density, so the role of magnetic fields should differ significantly from one cloud to another. And for maser regions the dependence of field strength on density may have a slightly lower slope. Turbulent reconnection theory seems to best match the Zeeman observational results.
KW - Zeeman effect
KW - magnetic fields
KW - masers
KW - mass/flux ratio
KW - molecular clouds
KW - star formation
UR - http://www.scopus.com/inward/record.url?scp=85079127916&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85079127916&partnerID=8YFLogxK
U2 - 10.3389/fspas.2019.00066
DO - 10.3389/fspas.2019.00066
M3 - Review article
SN - 2296-987X
VL - 6
SP - 66
JO - Frontiers in Astronomy and Space Sciences
JF - Frontiers in Astronomy and Space Sciences
M1 - 66
ER -