Magnetic Fields
in the
Non-Masing ISM
Richard M. Crutcher, University of Illinois
Some Collaborators:
Tom Troland, University of Kentucky
Edith Falgarone, Ecole Normale Superieure
Paulo Cortes, University of Chile
Shih-Ping Lai, National Tsing Hua University
Ramprasad Rao, SubMillimeter Array
Derek Ward-Thompson, Cardiff University
Jason Kirk, Cardiff University
Doug Roberts, Northwestern University
Josep Girart, University of Barcelona
Carl Heiles, UC Berkeley
Crystal Brogan, NRAO
Anuj Sarma, DePaul University
Miller Goss, NRAO
Alyssa Goodman, Harvard University
Phil Myers, CfA
Why Magnetic Fields?
“Magnetic fields are to astrophysicists
what sex is to psychoanalysts.”
- H. C. van de Hulst
Zeeman Effect
Z = 
B
Z, Z 1 – 2 Hz/ G, (ZH I = 1.4 Hz/G)
V = LR (dI/d)(Zcos)
 line of sight B
Q or U (d2I/d2)(Zsin)2
 plane of sky B (not really)
What Drives (Triggers) Star Formation?
• Two (extreme case) paradigms:
1. magnetic support (turbulence unimportant)
-
self-gravitating clouds are magnetically supported
magnetic field only frozen into ions, not neutrals
gravity leads to contraction of neutrals through ions and magnetic
field: ambipolar diffusion
mass in core overwhelms core magnetic field, collapses
2. compressible turbulence (magnetic fields unimportant)
-
turbulence forms structure in the interstellar medium
dense clumps form, and usually dissipate
some clumps are self-gravitating and collapse
• Observations of magnetic fields in molecular clouds
can distinguish between these models
1. ratio of gravity to magnetic support: M/
2. scaling of magnetic field strength with density
Scaling of B with : B 
B 0
B 1
B 2/3
Ciolek & Mouschovias 1994
B 1/2
Magnetic support,
ambipolar diffusion
Mass-to-Flux Ratio: M/
mass/flux ratio gravitational collapse / magnetic support
Nakano & Nakamura 1978
N (H 2 )
M observed

observed
B
• definition
( M / ) observed

(M / ) critical
• Geometry correction
C observed / 2 Blos only
C observed / 3 Blos + disk morphology
supercritical
• Observing M/

subcritical
1
M 

 
2 G
 
critical
Ciolek & Mouschovias 1994
critical
• Uniform disk
Arecibo OH Dark Cloud Core Zeeman Survey
• ~800 hours total telescope time
• >400 hours actual on-source
integration time
• 33 dark cloud core positions
• 1665 & 1667 MHz OH lines
observed simultaneously
• Stokes I and V spectra
• integration times from ~2 to ~50
hours per position
• 10 detections of Blos > 2
Troland & Crutcher 2007
L1448 Result
Blos = 25 5 µG
Blos = 28 6 µG
n(H2) 1 104, N(H2) 5 1021, Blos = -26 µG, 1.6
Troland & Crutcher 2007
Mass-to-Flux Ratio (Dark Cloud Cores)
_
c 1.7
obs/2
obs/3
Troland & Crutcher 2007
critical to slightly supercritical, gravitationally bound
Mass-to-Flux Ratios (H I Clouds)
Arecibo “Millennium” Survey
B (G)
Heiles & Troland 2004
_
c 0.16
N (1020 H atoms/cm2)
(highly subcritical, but not gravitationally bound)
W3 (OH)
CN Zeeman, Blos =1.1 mG
Turner & Welch 1984
Falgarone, Crutcher, & Troland 2007
W3 (OH)
8-11 mG
Gusten et al. 1994
n(H2) 6 106, N(H2) 5 1023, Blos 3.1 mG, 1.5
Results for Field Strength
PRELIMINARY ANALYSIS ONLY
Results for Mass/Flux
PRELIMINARY ANALYSIS ONLY
Results for Field Strength
ISM Component
n(H)
Btotal (G)
C

(B n)
__________________________________________________
diffuse ionized medium
~0.1
7 3
-
(synchrotron equipartition, RMs)
H I clouds
~0
~50
6 2
~0.1
(H I Zeeman)
molecular clouds
(OH, CN Zeeman)
103–107 10–3,000+
~1
~0.5
Summary and Conclusions
• B invariant (B 6-10 G) over
4 orders of magnitude in
density (~ 10-1 to 103 cm-3)
B 0
1. GMC formation by accumulation along field lines
- turbulent accumulation
- Parker instability
- magneto-rotational instability
• B n1/2, n > 103 cm-3 (in fact, Btotal = n1/2 G)
• M/ ~ critical in molecular cloud cores
2. Magnetic fields are sufficiently strong to support
cores in star formation regions
The Future
Ciolek & Mouschovias 1994

Dib & Kim 2006
The Future
Measure differential M/between core and envelope:
[ M / ]core
[Tline V / Blos ]core

[M / ]envelope
[Tline V / Blos ]envelope